Chapter 5 my_ppt
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Transcript of Chapter 5 my_ppt
UNIT -5 The Link layer and Local Area Networks
Link Layer 5-2
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-3
Link layer introductionterminology hosts and routers nodes communication channels
that connect adjacent nodes along communication path links wired links wireless links LANs
layer-2 packet frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
global ISP
Link Layer 5-4
Link layer context
datagram transferred by different link protocols over different links eg Ethernet on
first link frame relay on intermediate links 80211 on last link
each link protocol provides different services eg may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne
tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
Link Layer 5-5
Link layer services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
reliable delivery between adjacent nodes seldom used on low bit-error link (fiber
some twisted pair) wireless links high error rates
Link Layer 5-6
flow control pacing between adjacent sending and receiving
nodes error detection
errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame error correction
receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
Link layer services (more)
Link Layer 5-7
Where is the link layer implemented in each and every host link layer implemented
in ldquoadaptorrdquo (aka network interface card NIC) or on a chip Ethernet card 80211
card Ethernet chipset implements link
physical layer attaches into hostrsquos
system buses combination of
hardware software firmware
Link Layer 5-8
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Figure 9-1
Single-bit error
Figure 9-2
Multiple-bit errorFigure 9-3
Burst errorFigure 9-4
13
Error Detection Data transmission can contain errors
Single-bit Burst errors of length n
(n distance between the first and last errors in data block)
How to detect errors If only data is transmitted errors cannot be
detected Send more information with data that satisfies a special relationship Add redundancy
RedundancyFigure 9-5
Figure 9-6
16
Error Detection Methods Vertical Redundancy Check (VRC)
Append a single bit at the end of data block such that the number of ones is even Even Parity (odd parity is similar)0110011 011001100110001 01100011
VRC is also known as Parity Check Performance
bull Detects all odd-number errors in a data block
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
17
Error Detection Methods
Longitudinal Redundancy Check (LRC) Organize data into a table and create a
parity for each column11100111 11011101 00111001 10101001
11100111110111010011100110101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-2
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-3
Link layer introductionterminology hosts and routers nodes communication channels
that connect adjacent nodes along communication path links wired links wireless links LANs
layer-2 packet frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
global ISP
Link Layer 5-4
Link layer context
datagram transferred by different link protocols over different links eg Ethernet on
first link frame relay on intermediate links 80211 on last link
each link protocol provides different services eg may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne
tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
Link Layer 5-5
Link layer services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
reliable delivery between adjacent nodes seldom used on low bit-error link (fiber
some twisted pair) wireless links high error rates
Link Layer 5-6
flow control pacing between adjacent sending and receiving
nodes error detection
errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame error correction
receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
Link layer services (more)
Link Layer 5-7
Where is the link layer implemented in each and every host link layer implemented
in ldquoadaptorrdquo (aka network interface card NIC) or on a chip Ethernet card 80211
card Ethernet chipset implements link
physical layer attaches into hostrsquos
system buses combination of
hardware software firmware
Link Layer 5-8
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Figure 9-1
Single-bit error
Figure 9-2
Multiple-bit errorFigure 9-3
Burst errorFigure 9-4
13
Error Detection Data transmission can contain errors
Single-bit Burst errors of length n
(n distance between the first and last errors in data block)
How to detect errors If only data is transmitted errors cannot be
detected Send more information with data that satisfies a special relationship Add redundancy
RedundancyFigure 9-5
Figure 9-6
16
Error Detection Methods Vertical Redundancy Check (VRC)
Append a single bit at the end of data block such that the number of ones is even Even Parity (odd parity is similar)0110011 011001100110001 01100011
VRC is also known as Parity Check Performance
bull Detects all odd-number errors in a data block
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
17
Error Detection Methods
Longitudinal Redundancy Check (LRC) Organize data into a table and create a
parity for each column11100111 11011101 00111001 10101001
11100111110111010011100110101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-3
Link layer introductionterminology hosts and routers nodes communication channels
that connect adjacent nodes along communication path links wired links wireless links LANs
layer-2 packet frame encapsulates datagram
data-link layer has responsibility of transferring datagram from one node to physically adjacent node over a link
global ISP
Link Layer 5-4
Link layer context
datagram transferred by different link protocols over different links eg Ethernet on
first link frame relay on intermediate links 80211 on last link
each link protocol provides different services eg may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne
tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
Link Layer 5-5
Link layer services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
reliable delivery between adjacent nodes seldom used on low bit-error link (fiber
some twisted pair) wireless links high error rates
Link Layer 5-6
flow control pacing between adjacent sending and receiving
nodes error detection
errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame error correction
receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
Link layer services (more)
Link Layer 5-7
Where is the link layer implemented in each and every host link layer implemented
in ldquoadaptorrdquo (aka network interface card NIC) or on a chip Ethernet card 80211
card Ethernet chipset implements link
physical layer attaches into hostrsquos
system buses combination of
hardware software firmware
Link Layer 5-8
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Figure 9-1
Single-bit error
Figure 9-2
Multiple-bit errorFigure 9-3
Burst errorFigure 9-4
13
Error Detection Data transmission can contain errors
Single-bit Burst errors of length n
(n distance between the first and last errors in data block)
How to detect errors If only data is transmitted errors cannot be
detected Send more information with data that satisfies a special relationship Add redundancy
RedundancyFigure 9-5
Figure 9-6
16
Error Detection Methods Vertical Redundancy Check (VRC)
Append a single bit at the end of data block such that the number of ones is even Even Parity (odd parity is similar)0110011 011001100110001 01100011
VRC is also known as Parity Check Performance
bull Detects all odd-number errors in a data block
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
17
Error Detection Methods
Longitudinal Redundancy Check (LRC) Organize data into a table and create a
parity for each column11100111 11011101 00111001 10101001
11100111110111010011100110101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-4
Link layer context
datagram transferred by different link protocols over different links eg Ethernet on
first link frame relay on intermediate links 80211 on last link
each link protocol provides different services eg may or may not
provide rdt over link
transportation analogy trip from Princeton to
Lausanne limo Princeton to JFK plane JFK to Geneva train Geneva to Lausanne
tourist = datagram transport segment =
communication link transportation mode =
link layer protocol travel agent = routing
algorithm
Link Layer 5-5
Link layer services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
reliable delivery between adjacent nodes seldom used on low bit-error link (fiber
some twisted pair) wireless links high error rates
Link Layer 5-6
flow control pacing between adjacent sending and receiving
nodes error detection
errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame error correction
receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
Link layer services (more)
Link Layer 5-7
Where is the link layer implemented in each and every host link layer implemented
in ldquoadaptorrdquo (aka network interface card NIC) or on a chip Ethernet card 80211
card Ethernet chipset implements link
physical layer attaches into hostrsquos
system buses combination of
hardware software firmware
Link Layer 5-8
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Figure 9-1
Single-bit error
Figure 9-2
Multiple-bit errorFigure 9-3
Burst errorFigure 9-4
13
Error Detection Data transmission can contain errors
Single-bit Burst errors of length n
(n distance between the first and last errors in data block)
How to detect errors If only data is transmitted errors cannot be
detected Send more information with data that satisfies a special relationship Add redundancy
RedundancyFigure 9-5
Figure 9-6
16
Error Detection Methods Vertical Redundancy Check (VRC)
Append a single bit at the end of data block such that the number of ones is even Even Parity (odd parity is similar)0110011 011001100110001 01100011
VRC is also known as Parity Check Performance
bull Detects all odd-number errors in a data block
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
17
Error Detection Methods
Longitudinal Redundancy Check (LRC) Organize data into a table and create a
parity for each column11100111 11011101 00111001 10101001
11100111110111010011100110101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-5
Link layer services framing link access
encapsulate datagram into frame adding header trailer
channel access if shared medium ldquoMACrdquo addresses used in frame headers to
identify source dest bull different from IP address
reliable delivery between adjacent nodes seldom used on low bit-error link (fiber
some twisted pair) wireless links high error rates
Link Layer 5-6
flow control pacing between adjacent sending and receiving
nodes error detection
errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame error correction
receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
Link layer services (more)
Link Layer 5-7
Where is the link layer implemented in each and every host link layer implemented
in ldquoadaptorrdquo (aka network interface card NIC) or on a chip Ethernet card 80211
card Ethernet chipset implements link
physical layer attaches into hostrsquos
system buses combination of
hardware software firmware
Link Layer 5-8
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Figure 9-1
Single-bit error
Figure 9-2
Multiple-bit errorFigure 9-3
Burst errorFigure 9-4
13
Error Detection Data transmission can contain errors
Single-bit Burst errors of length n
(n distance between the first and last errors in data block)
How to detect errors If only data is transmitted errors cannot be
detected Send more information with data that satisfies a special relationship Add redundancy
RedundancyFigure 9-5
Figure 9-6
16
Error Detection Methods Vertical Redundancy Check (VRC)
Append a single bit at the end of data block such that the number of ones is even Even Parity (odd parity is similar)0110011 011001100110001 01100011
VRC is also known as Parity Check Performance
bull Detects all odd-number errors in a data block
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
17
Error Detection Methods
Longitudinal Redundancy Check (LRC) Organize data into a table and create a
parity for each column11100111 11011101 00111001 10101001
11100111110111010011100110101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-6
flow control pacing between adjacent sending and receiving
nodes error detection
errors caused by signal attenuation noise receiver detects presence of errors
bull signals sender for retransmission or drops frame error correction
receiver identifies and corrects bit error(s) without resorting to retransmission
half-duplex and full-duplex with half duplex nodes at both ends of link can
transmit but not at same time
Link layer services (more)
Link Layer 5-7
Where is the link layer implemented in each and every host link layer implemented
in ldquoadaptorrdquo (aka network interface card NIC) or on a chip Ethernet card 80211
card Ethernet chipset implements link
physical layer attaches into hostrsquos
system buses combination of
hardware software firmware
Link Layer 5-8
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Figure 9-1
Single-bit error
Figure 9-2
Multiple-bit errorFigure 9-3
Burst errorFigure 9-4
13
Error Detection Data transmission can contain errors
Single-bit Burst errors of length n
(n distance between the first and last errors in data block)
How to detect errors If only data is transmitted errors cannot be
detected Send more information with data that satisfies a special relationship Add redundancy
RedundancyFigure 9-5
Figure 9-6
16
Error Detection Methods Vertical Redundancy Check (VRC)
Append a single bit at the end of data block such that the number of ones is even Even Parity (odd parity is similar)0110011 011001100110001 01100011
VRC is also known as Parity Check Performance
bull Detects all odd-number errors in a data block
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
17
Error Detection Methods
Longitudinal Redundancy Check (LRC) Organize data into a table and create a
parity for each column11100111 11011101 00111001 10101001
11100111110111010011100110101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-7
Where is the link layer implemented in each and every host link layer implemented
in ldquoadaptorrdquo (aka network interface card NIC) or on a chip Ethernet card 80211
card Ethernet chipset implements link
physical layer attaches into hostrsquos
system buses combination of
hardware software firmware
Link Layer 5-8
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Figure 9-1
Single-bit error
Figure 9-2
Multiple-bit errorFigure 9-3
Burst errorFigure 9-4
13
Error Detection Data transmission can contain errors
Single-bit Burst errors of length n
(n distance between the first and last errors in data block)
How to detect errors If only data is transmitted errors cannot be
detected Send more information with data that satisfies a special relationship Add redundancy
RedundancyFigure 9-5
Figure 9-6
16
Error Detection Methods Vertical Redundancy Check (VRC)
Append a single bit at the end of data block such that the number of ones is even Even Parity (odd parity is similar)0110011 011001100110001 01100011
VRC is also known as Parity Check Performance
bull Detects all odd-number errors in a data block
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
17
Error Detection Methods
Longitudinal Redundancy Check (LRC) Organize data into a table and create a
parity for each column11100111 11011101 00111001 10101001
11100111110111010011100110101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-8
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
55 link virtualization MPLS
56 data center networking
57 a day in the life of a web request
Figure 9-1
Single-bit error
Figure 9-2
Multiple-bit errorFigure 9-3
Burst errorFigure 9-4
13
Error Detection Data transmission can contain errors
Single-bit Burst errors of length n
(n distance between the first and last errors in data block)
How to detect errors If only data is transmitted errors cannot be
detected Send more information with data that satisfies a special relationship Add redundancy
RedundancyFigure 9-5
Figure 9-6
16
Error Detection Methods Vertical Redundancy Check (VRC)
Append a single bit at the end of data block such that the number of ones is even Even Parity (odd parity is similar)0110011 011001100110001 01100011
VRC is also known as Parity Check Performance
bull Detects all odd-number errors in a data block
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
17
Error Detection Methods
Longitudinal Redundancy Check (LRC) Organize data into a table and create a
parity for each column11100111 11011101 00111001 10101001
11100111110111010011100110101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Figure 9-1
Single-bit error
Figure 9-2
Multiple-bit errorFigure 9-3
Burst errorFigure 9-4
13
Error Detection Data transmission can contain errors
Single-bit Burst errors of length n
(n distance between the first and last errors in data block)
How to detect errors If only data is transmitted errors cannot be
detected Send more information with data that satisfies a special relationship Add redundancy
RedundancyFigure 9-5
Figure 9-6
16
Error Detection Methods Vertical Redundancy Check (VRC)
Append a single bit at the end of data block such that the number of ones is even Even Parity (odd parity is similar)0110011 011001100110001 01100011
VRC is also known as Parity Check Performance
bull Detects all odd-number errors in a data block
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
17
Error Detection Methods
Longitudinal Redundancy Check (LRC) Organize data into a table and create a
parity for each column11100111 11011101 00111001 10101001
11100111110111010011100110101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Single-bit error
Figure 9-2
Multiple-bit errorFigure 9-3
Burst errorFigure 9-4
13
Error Detection Data transmission can contain errors
Single-bit Burst errors of length n
(n distance between the first and last errors in data block)
How to detect errors If only data is transmitted errors cannot be
detected Send more information with data that satisfies a special relationship Add redundancy
RedundancyFigure 9-5
Figure 9-6
16
Error Detection Methods Vertical Redundancy Check (VRC)
Append a single bit at the end of data block such that the number of ones is even Even Parity (odd parity is similar)0110011 011001100110001 01100011
VRC is also known as Parity Check Performance
bull Detects all odd-number errors in a data block
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
17
Error Detection Methods
Longitudinal Redundancy Check (LRC) Organize data into a table and create a
parity for each column11100111 11011101 00111001 10101001
11100111110111010011100110101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Multiple-bit errorFigure 9-3
Burst errorFigure 9-4
13
Error Detection Data transmission can contain errors
Single-bit Burst errors of length n
(n distance between the first and last errors in data block)
How to detect errors If only data is transmitted errors cannot be
detected Send more information with data that satisfies a special relationship Add redundancy
RedundancyFigure 9-5
Figure 9-6
16
Error Detection Methods Vertical Redundancy Check (VRC)
Append a single bit at the end of data block such that the number of ones is even Even Parity (odd parity is similar)0110011 011001100110001 01100011
VRC is also known as Parity Check Performance
bull Detects all odd-number errors in a data block
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
17
Error Detection Methods
Longitudinal Redundancy Check (LRC) Organize data into a table and create a
parity for each column11100111 11011101 00111001 10101001
11100111110111010011100110101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Burst errorFigure 9-4
13
Error Detection Data transmission can contain errors
Single-bit Burst errors of length n
(n distance between the first and last errors in data block)
How to detect errors If only data is transmitted errors cannot be
detected Send more information with data that satisfies a special relationship Add redundancy
RedundancyFigure 9-5
Figure 9-6
16
Error Detection Methods Vertical Redundancy Check (VRC)
Append a single bit at the end of data block such that the number of ones is even Even Parity (odd parity is similar)0110011 011001100110001 01100011
VRC is also known as Parity Check Performance
bull Detects all odd-number errors in a data block
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
17
Error Detection Methods
Longitudinal Redundancy Check (LRC) Organize data into a table and create a
parity for each column11100111 11011101 00111001 10101001
11100111110111010011100110101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
13
Error Detection Data transmission can contain errors
Single-bit Burst errors of length n
(n distance between the first and last errors in data block)
How to detect errors If only data is transmitted errors cannot be
detected Send more information with data that satisfies a special relationship Add redundancy
RedundancyFigure 9-5
Figure 9-6
16
Error Detection Methods Vertical Redundancy Check (VRC)
Append a single bit at the end of data block such that the number of ones is even Even Parity (odd parity is similar)0110011 011001100110001 01100011
VRC is also known as Parity Check Performance
bull Detects all odd-number errors in a data block
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
17
Error Detection Methods
Longitudinal Redundancy Check (LRC) Organize data into a table and create a
parity for each column11100111 11011101 00111001 10101001
11100111110111010011100110101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
RedundancyFigure 9-5
Figure 9-6
16
Error Detection Methods Vertical Redundancy Check (VRC)
Append a single bit at the end of data block such that the number of ones is even Even Parity (odd parity is similar)0110011 011001100110001 01100011
VRC is also known as Parity Check Performance
bull Detects all odd-number errors in a data block
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
17
Error Detection Methods
Longitudinal Redundancy Check (LRC) Organize data into a table and create a
parity for each column11100111 11011101 00111001 10101001
11100111110111010011100110101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Figure 9-6
16
Error Detection Methods Vertical Redundancy Check (VRC)
Append a single bit at the end of data block such that the number of ones is even Even Parity (odd parity is similar)0110011 011001100110001 01100011
VRC is also known as Parity Check Performance
bull Detects all odd-number errors in a data block
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
17
Error Detection Methods
Longitudinal Redundancy Check (LRC) Organize data into a table and create a
parity for each column11100111 11011101 00111001 10101001
11100111110111010011100110101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
16
Error Detection Methods Vertical Redundancy Check (VRC)
Append a single bit at the end of data block such that the number of ones is even Even Parity (odd parity is similar)0110011 011001100110001 01100011
VRC is also known as Parity Check Performance
bull Detects all odd-number errors in a data block
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
17
Error Detection Methods
Longitudinal Redundancy Check (LRC) Organize data into a table and create a
parity for each column11100111 11011101 00111001 10101001
11100111110111010011100110101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
17
Error Detection Methods
Longitudinal Redundancy Check (LRC) Organize data into a table and create a
parity for each column11100111 11011101 00111001 10101001
11100111110111010011100110101001
10101010
11100111 11011101 00111001 10101001 10101010
Original Data LRC
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Winter 2005
ECE
ECE 766Computer Interfacing and Protocols
18
Error Detection Methods
Performancebull Detects all burst errors up to length n
(number of columns)bull Misses burst errors of length n+1 if there are n-1
uninverted bits between the first and last bitbull If the block is badly garbled the probability of
acceptance is n21
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
CRCFigure 9-10
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Binary Division
Figure 9-11
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Polynomial
Figure 9-12
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Figure 9-13
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Polynomial and Divisor
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Standard PolynomialsFigure 9-14
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Example-CRC
Data Link Layer 5-24
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
ChecksumFigure 9-15
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
bull Used by upper layer protocolsbull Similar to LRC uses onersquos complement arithmetic
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Figure 9-16
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Data Unit and Checksum
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Error CorrectionFigure 9-17
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Hamming CodeFigure 9-18
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Figure 9-19
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Figure 9-19-continued
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Hamming Code
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Figure 9-20
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Example of Hamming Code
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Single-bit error
Figure 9-21
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Figure 9-22
WCBMcGraw-Hill The McGraw-Hill Companies Inc 1998
Error Detection
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-34
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-35
Multiple access protocols It avoids two or more nodes from
transmitting at the same time over a broadcast channel
If different nodes send data at the same time collisions occurs and the receiver will not be able to understand the signal
Classes of MAP Partitioning the channel eg FDM amp TDM Taking turns eg Token based polling based amp
reservation based Contention based protocol eg ALOHA and
Ethernet
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-36
Multiple access protocols Different multiple access protocolsRandom access protocols-
channel not divided allow collisions ldquorecoverrdquo from collisions
Controlled access protocols- Nodes take turns but nodes with more to
send can take longer turnsChannelization protocols-
divide channel into smaller ldquopiecesrdquo (time slots frequency code)
allocate piece to node for exclusive use
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-37
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Reservation Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-38
Multiple access linkstwo types of ldquolinksrdquo point-to-point
PPP for dial-up access point-to-point link between Ethernet switch host
broadcast (shared wire or medium) old-fashioned Ethernet upstream HFC 80211 wireless LAN
shared wire (eg cabled Ethernet) (eg 80211 WiFi)
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-39
An ideal multiple access protocolgiven broadcast channel of rate R bpsdesiderata
1 when one node wants to transmit it can send at rate R
2 when M nodes want to transmit each can send at average rate RM
3 fully decentralizedbull no special node to coordinate transmissionsbull no synchronization of clocks slots
4 simple
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-40
Channel partitioning MAC protocols TDMA
TDMA time division multiple access access to channel in rounds each station gets fixed length slot
(length = pkt trans time) in each round unused slots go idle example 6-station LAN 134 have pkt
slots 256 idle
1 3 4 1 3 4
6-slotframe
6-slotframe
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-41
FDMA frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go
idle example 6-station LAN 134 have pkt
frequency bands 256 idle
freq
uenc
y ba
nds
time
FDM cable
Channel partitioning MAC protocols FDMA
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-42
Random access protocols when node has packet to send
transmit at full channel data rate R no a priori coordination among nodes
two or more transmitting nodes ldquocollisionrdquo
random access MAC protocol specifies how to detect collisions how to recover from collisions (eg via
delayed retransmissions) examples of random access MAC
protocols Pure (Unslotted )ALOHA slotted ALOHA CSMA CSMACD CSMACA
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-43
ALOHA It signifies a simple communications
schemes in which each transmitter within a network sends data whenever there is a frame to send
Next frame is sent only if the previous frame successfully reaches the destination
If frame fails to reach at the destination it is sent again
Types of ALOHA Pure ALOHA Slotted ALOHA
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-44
Pure (unslotted) ALOHA unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-45
Pure (unslotted) ALOHA
unslotted Aloha simpler no synchronization when frame first arrives
transmit immediately collision probability increases
frame sent at t0 collides with other frames sent in [t0-1t0+1]
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-46
Pure ALOHA efficiencyP(success by given node) = P(node transmits)
P(no other node transmits in [t0-1t0]
P(no other node transmits in [t0-1t0]
= p (1-p)N-1 (1-p)N-1
= p (1-p)2(N-1)
hellip choosing optimum p and then letting n
= 1(2e) = 18
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-47
Slotted ALOHA
assumptions all frames same size time divided into
equal size slots (time to transmit 1 frame)
nodes start to transmit only slot beginning
nodes are synchronized
if 2 or more nodes transmit in slot all nodes detect collision
operation when node obtains fresh
frame transmits in next slot if no collision node
can send new frame in next slot
if collision node retransmits frame in each subsequent slot with prob p until success
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-48
Pros single active node
can continuously transmit at full rate of channel
highly decentralized only slots in nodes need to be in sync
simple
Cons collisions wasting
slots idle slots nodes may be able to
detect collision in less than time to transmit packet
clock synchronization
Slotted ALOHA1 1 1 1
2
3
2 2
3 3
node 1
node 2
node 3
C C CS S SE E E
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-49
suppose N nodes with many frames to send each transmits in slot with probability p
prob that given node has success in a slot = p(1-p)N-1
prob that any node has a success = Np(1-p)N-1
max efficiency find p that maximizes Np(1-p)N-1
for many nodes take limit of Np(1-p)N-1
as N goes to infinity gives
max efficiency = 1e = 37
efficiency long-run fraction of successful slots (many nodes all with many frames to send)
at best channelused for useful transmissions 37of time
Slotted ALOHA efficiency
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-50
CSMA (carrier sense multiple access)
CSMA listen before transmitif channel sensed idle transmit entire
frame if channel sensed busy defer
transmission
human analogy donrsquot interrupt others
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-51
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-52
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull In this if a station wants to transmit a
frame and it finds that the channel is busy then it will wait for fixed interval of time
bull After some time it again checks the status of the channel
bull If channel is free it will transmitbull 1-persistent CSMAbull P-persistent CSMA
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-53
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMA
bull The station which wants to transmit continuously monitors the channel until it is idle and then transmits immediately
bull If two are waiting then they will transmit simultaneously and collision take place then require retransmission
bull P-persistent CSMA
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-54
CSMA (carrier sense multiple access)
bull Non-persistent CSMAbull 1-persistent CSMAbull P-persistent CSMA
bull All the waiting stations are not allowed to transmit simultaneously as soon as the channel become idle
bull Possibility of collision and retransmission is reduced
bull A station is assumed to be transmitted with a probability p
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-55
CSMACD (collision detection)
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-56
CSMACD (collision detection)CSMACD carrier sensing deferral as in
CSMA collisions detected within short time colliding transmissions aborted reducing
channel wastage collision detection
easy in wired LANs measure signal strengths compare transmitted received signals
difficult in wireless LANs received signal strength overwhelmed by local transmission strength
human analogy the polite conversationalist
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-57
CSMACD (procedure)
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-58
CSMACD efficiency Tprop = max prop delay between 2 nodes in LAN ttrans = time to transmit max-size frame
so ttrans gt= 2Tprop
iLRgt=2DSLgt=2DRS so of less than frame length then collision detection mechanism not possible
efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity
better performance than ALOHA
transprop ttefficiency
51
1
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Station ready to transmit senses the the line by using one of persistent technique
As soon as it find the line to be idle the station waits for a time equal to an IFG(Interframe Gap)
It then wait for some random time and sends the frame
After sending the frame It sets a timer and waits for the ack from the receiver
If the ack is received before expiry of the timer then the transmitter is successful
But if the transmitting station does not receive the expected ack before the timer expiry then the increments the back off parameter waits for the back off time and senses the line again
Data Link Layer 5-59
CSMACA (Collision Avoidance)
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Data Link Layer 5-60
CSMACA (Collision Avoidance)
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-61
Multiple access protocols Different multiple access protocols Random access protocols
ALOHA CSMA CSMACD
Controlled access protocols Polling Token passing
Channelization protocols FDMA TDMA
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-62
bull It requires one of the nodes to be designated as a master node
bull The master node polls each of the nodes in a round-robin fashion
bull the master node first sends a message to node 1 saying that it (node 1) can transmit up to some maximum number of frames
bull After node 1 transmits some frames the master node tells node 2 it (node 2) can transmit up to the maximum number of frames
Controlled access protocols-Polling
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-63
bull The master node can determine when a node has finished sending its frames by observing the lack of a signal on the channel
The procedure continues in this manner with the master node polling each of the nodes in a cyclic manner
It eliminates the collisions and allows polling to achieve a much higher efficiency
Controlled access protocols-Polling
master
slaves
poll
data
data
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-64
Disadvantage - bullPolling delaymdashthe amount of time required to notify a node that it can transmit bullif the master node fails the entire channel becomes inoperative
Controlled access protocols-Polling
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
5-65
In this protocol there is no master node
A small special-purpose frame known as a token is exchanged among the nodes in some fixed order
When a node receives a token it holds onto the token only if it has some frames to transmit otherwise it immediately forwards the token to the next node
Controlled access protocols-Token Passing T
data
(nothingto send)
T
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-66
bull If a node does have frames to transmit when it receives the token it sends up to a maximum number of frames and then forwards the token to the next node
bull It is decentralized and highly efficientDisadvantage- The failure of one node can crash the
entire channel if a node accidentally neglects to
release the token then some recovery procedure must be invoked to get the token back in circulation
Controlled access protocols-Token Passing
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-67
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches VLANS
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-68
MAC addresses and ARP
32-bit IP address network-layer address for interface used for layer 3 (network layer) forwarding
MAC (or LAN or physical or Ethernet) address function used lsquolocallyrdquo to get frame from one
interface to another physically-connected interface (same network in IP-addressing sense)
48 bit MAC address (for most LANs) burned in NIC ROM also sometimes software settable
eg 1A-2F-BB-76-09-ADhexadecimal (base 16) notation(each ldquonumberrdquo represents 4 bits)
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-69
LAN addresses and ARPeach adapter on LAN has unique LAN address
adapter
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN(wired orwireless)
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-70
LAN addresses (more)
MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness)
analogy MAC address like Social Security Number IP address like postal address
MAC flat address portability can move LAN card from one LAN to another
IP hierarchical address not portable address depends on IP subnet to which node
is attached
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-71
ARP address resolution protocol
ARP table each IP node (host router) on LAN has table
IPMAC address mappings for some LAN nodes
lt IP address MAC address TTLgt
TTL (Time To Live) time after which address mapping will be forgotten (typically 20 min)
Question how to determineinterfacersquos MAC address knowing its IP address
1A-2F-BB-76-09-AD
58-23-D7-FA-20-B0
0C-C4-11-6F-E3-98
71-65-F7-2B-08-53
LAN
137196723
137196778
137196714
137196788
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-72
ARP protocol same LAN A wants to send
datagram to B Brsquos MAC address not in
A rsquos ARP table A broadcasts ARP query
packet containing Bs IP address dest MAC address = FF-
FF-FF-FF-FF-FF all nodes on LAN receive
ARP query B receives ARP packet
replies to A with its (Bs) MAC address frame sent to Arsquos MAC
address (unicast)
A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state information
that times out (goes away) unless refreshed
ARP is ldquoplug-and-playrdquo nodes create their
ARP tables without intervention from net administrator
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-73
walkthrough send datagram from A to B via R focus on addressing ndash at IP (datagram) and MAC layer (frame) assume A knows Brsquos IP address assume A knows IP address of first hop router R assume A knows Rrsquos MAC address
Addressing routing to another LAN
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-74
Addressing routing to another LAN
IPEthPhy
IP src 111111111111 IP dest 222222222222
A creates IP datagram with IP source A destination B A creates link-layer frame with Rs MAC address as dest
frame contains A-to-B IP datagram
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-75
Addressing routing to another LAN
IPEthPhy
frame sent from A to R
IPEthPhy
frame received at R datagram removed passed up to IP
MAC src 74-29-9C-E8-FF-55 MAC dest E6-E9-00-17-BB-4B
IP src 111111111111 IP dest 222222222222
IP src 111111111111 IP dest 222222222222
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-76
Addressing routing to another LAN
IP src 111111111111 IP dest 222222222222
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-77
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
IPEthPhy
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
R
1A-23-F9-CD-06-9B222222222220
111111111110E6-E9-00-17-BB-4BCC-49-DE-D0-AB-7D
111111111112
11111111111174-29-9C-E8-FF-55
A
22222222222249-BD-D2-C7-56-2A
22222222222188-B2-2F-54-1A-0F
B
Link Layer 5-78
Addressing routing to another LAN
R forwards datagram with IP source A destination B R creates link-layer frame with Bs MAC address as dest
frame contains A-to-B IP datagram
IP src 111111111111 IP dest 222222222222
MAC src 1A-23-F9-CD-06-9B MAC dest 49-BD-D2-C7-56-2A
IPEthPhy
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-79
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-80
Ethernet 8023bull IEEE 8023 (CSMACD - Ethernet) standard ndash
originally 2Mbpsbull IEEE 8023u standard for 100Mbps Ethernetbull IEEE 8023z standard for 1000Mbps Ethernet Most popular packet-switched LAN technology Bandwidths 10Mbps 100Mbps 1Gbps Max bus length 2500m
500m segments with 4 repeaters Bus and Star topologies are used to connect host For transmission 8023 LAN it make use of 1-
persistant CSMACD protocol When collision they wait for random time to
select random time Ethernet use Binary Exponential Backoff Algo
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-81
Ethernet CSMACD algorithm1 NIC receives datagram
from network layer creates frame
2 If NIC senses channel idle starts frame transmission If NIC senses channel busy waits until channel idle then transmits
3 If NIC transmits entire frame without detecting another transmission NIC is done with frame
4 If NIC detects another transmission while transmitting aborts and sends jam signal
5 After aborting NIC enters binary (exponential) backoff after mth collision NIC
chooses K at random from 012 hellip 2m-1 NIC waits K512 bit times returns to Step 2
longer backoff interval with more collisions
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-82
Ethernet frame structure
sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame
preamble 7 bytes with pattern 10101010 followed
by one byte with pattern 10101011 used to synchronize receiver sender
clock rates
destaddress
sourceaddress
data (payload) CRCpreamble
type
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
CS 640 83
Ethernet Frames
Preamble is a sequence of 7 bytes each set to ldquo10101010rdquo Used to synchronize receiver before actual data is sent
Addresses unique 48-bit unicast address assigned to each adapter
bull example 80e4b12bull Each manufacturer gets their own address range
broadcast all 1s multicast first bit is 1
Type field is a demultiplexing key used to determine which higher level protocol the frame should be delivered to
Body can contain up to 1500 bytes of data
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-84
Ethernet frame structure (more) addresses 6 byte source destination MAC
addresses if adapter receives frame with matching destination
address or with broadcast address (eg ARP packet) it passes data in frame to network layer protocol
otherwise adapter discards frame type indicates higher layer protocol key
(mostly IP but others possible) CRC cyclic redundancy check at receiver
error detected frame is dropped
destaddress
sourceaddress
data (payload) CRCpreamble
type
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-85
Ethernet physical topology bus
all nodes in same collision domain (can collide with each other)
star prevails today active switch in center each ldquospokerdquo runs a (separate) Ethernet protocol
(nodes do not collide with each other)
switch
bus coaxial cablestar
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
CS 640 86
Ethernet Technologies 10Base2
10 10Mbps 2 under 185 (~200) meters cable length
Thin coaxial cable in a bus topology
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
CS 640 87
10BaseT and 100BaseT 10100 Mbps rate T stands for Twisted Pair Hub(s) connected by twisted pair facilitate ldquostar
topologyrdquo Distance of any node to hub must be lt 100M
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
CS 640 88
Physical Layer Configurations for 8023
Physical layer configurations are specified in three parts Data rate (10 100 1000)
10 100 1000Mbps Signaling method (base broad)
Basebandbull Digital signaling
Broadbandbull Analog signaling
Cabling (2 5 T F S L) 5 - Thick coax (original Ethernet cabling) F ndash Optical fiber S ndash Short wave laser over multimode fiber L ndash Long wave laser over single mode fiber
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
CS 640 89
Fast and Gigabit Ethernet
Fast Ethernet (100Mbps) has technology very similar to 10Mbps Ethernet Uses different physical layer encoding (4B5B) Many NICrsquos are 10100 capable
bull Can be used at either speed Gigabit Ethernet (1000Mbps)
Compatible with lower speeds Uses standard framing and CSMACD algorithm Distances are severely limited Typically used for backbones and inter-router
connectivity Becoming cost competitive How much of this bandwidth is realizable
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
CS 640 90
Experiences with Ethernet
Ethernets work best under light loads Utilization over 30 is considered heavy
bull Network capacity is wasted by collisions Most networks are limited to about 200 hosts
Specification allows for up to 1024 Most networks are much shorter
5 to 10 microsecond RTT
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-91
Link layer LANs outline
51 introduction services
52 error detection correction
53 multiple access protocols
54 LANs addressing ARP Ethernet switches
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Switch
Device that channels incoming data from any of multiple input ports to the specific output port Example-traditional telephone network
In Internet a switch determines from the IP address in each packet which output port to use for the next part of its trip to the intended destination
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-93
Ethernet switch link-layer device takes an active role
store forward Ethernet frames examine incoming framersquos MAC address
selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment uses CSMACD to access segment
transparent hosts are unaware of presence of
switches plug-and-play self-learning
switches do not need to be configured
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-94
Switch multiple simultaneous transmissions hosts have dedicated
direct connection to switch
switches buffer packets Ethernet protocol used on
each incoming link but no collisions full duplex each link is its own
collision domain switching A-to-Arsquo and B-
to-Brsquo can transmit simultaneously without collisions
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-95
Switch forwarding table
Q how does switch know A rsquo reachable via interface 4 Brsquo reachable via interface 5
switch with six interfaces(123456)
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6 A each switch has a switch table each entry (MAC address of host
interface to reach host time stamp)
looks like a routing tableQ how are entries created maintained in switch table
something like a routing protocol
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-96
Switch self-learning switch learns which
hosts can be reached through which interfaces when frame
received switch ldquolearnsrdquo location of sender incoming LAN segment
records senderlocation pair in switch table
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
Switch table (initially empty)
A 1 60
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-97
Switch frame filteringforwardingwhen frame received at switch
1 record incoming link MAC address of sending host2 index switch table using MAC destination address3 if entry found for destination
then if destination on segment from which frame arrived
then drop frame else forward frame on interface indicated by
entry else flood forward on all interfaces except
arriving interface
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
A
Arsquo
B
Brsquo C
Crsquo
1 2
345
6
Link Layer 5-98
Self-learning forwarding example
A Arsquo
Source ADest Arsquo
MAC addr interface TTL
switch table (initially empty)
A 1 60
A ArsquoA ArsquoA ArsquoA ArsquoA Arsquo
frame destination Arsquo locaton unknownflood
Arsquo A
destination A location known
Arsquo 4 60
selectively send
on just one link
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer 5-99
Interconnecting switches switches can be connected together
Q sending from A to G - how does S1 know to forward frame destined to F via S4 and S3A self learning (works exactly the same as in single-switch case)
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer5-100
Self-learning multi-switch exampleSuppose C sends frame to I I responds to C
Q show switch tables and packet forwarding in S1 S2 S3 S4
A
B
S1
C D
E
FS2
S4
S3
H
I
G
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer5-101
Institutional network
to externalnetwork
router
IP subnet
mail server
web server
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-
Link Layer5-102
Switches vs routersboth are store-and-forward routers network-layer devices (examine network-layer headers)switches link-layer devices (examine link-layer headers)
both have forwarding tablesrouters compute tables using routing algorithms IP addressesswitches learn forwarding table using flooding learning MAC addresses
applicationtransportnetwork
linkphysical
networklink
physical
linkphysical
switch
datagram
applicationtransportnetwork
linkphysical
frame
frame
frame
datagram
- UNIT -5 The Link layer and Local Area Networks
- Link layer LANs outline
- Link layer introduction
- Link layer context
- Link layer services
- Link layer services (more)
- Where is the link layer implemented
- Slide 8
- PowerPoint Presentation
- Slide 10
- Slide 11
- Slide 12
- Error Detection
- Slide 14
- Slide 15
- Error Detection Methods
- Slide 17
- Slide 18
- Slide 19
- Slide 20
- Slide 21
- Slide 22
- Slide 23
- Example-CRC
- Slide 25
- Slide 26
- Slide 27
- Slide 28
- Slide 29
- Slide 30
- Slide 31
- Slide 32
- Slide 33
- Slide 34
- Multiple access protocols
- Slide 36
- Slide 37
- Multiple access links
- An ideal multiple access protocol
- Channel partitioning MAC protocols TDMA
- Channel partitioning MAC protocols FDMA
- Random access protocols
- ALOHA
- Pure (unslotted) ALOHA
- Slide 45
- Pure ALOHA efficiency
- Slotted ALOHA
- Slide 48
- Slotted ALOHA efficiency
- CSMA (carrier sense multiple access)
- Slide 51
- Slide 52
- Slide 53
- Slide 54
- CSMACD (collision detection)
- Slide 56
- CSMACD (procedure)
- CSMACD efficiency
- CSMACA (Collision Avoidance)
- Slide 60
- Slide 61
- Controlled access protocols-Polling
- Slide 63
- Controlled access protocols-Polling
- Controlled access protocols-Token Passing
- Slide 66
- Slide 67
- MAC addresses and ARP
- LAN addresses and ARP
- LAN addresses (more)
- ARP address resolution protocol
- ARP protocol same LAN
- Addressing routing to another LAN
- Slide 74
- Slide 75
- Slide 76
- Slide 77
- Slide 78
- Slide 79
- Ethernet 8023
- Ethernet CSMACD algorithm
- Ethernet frame structure
- Ethernet Frames
- Ethernet frame structure (more)
- Ethernet physical topology
- Ethernet Technologies 10Base2
- 10BaseT and 100BaseT
- Physical Layer Configurations for 8023
- Fast and Gigabit Ethernet
- Experiences with Ethernet
- Slide 91
- Switch
- Ethernet switch
- Switch multiple simultaneous transmissions
- Switch forwarding table
- Switch self-learning
- Switch frame filteringforwarding
- Self-learning forwarding example
- Interconnecting switches
- Self-learning multi-switch example
- Institutional network
- Switches vs routers
-