Networking lecture 4 data link_layer_by_Mamun sir
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Transcript of Networking lecture 4 data link_layer_by_Mamun sir
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Data Link Layer
By Md. Abdullah al mamun
Sr. Lecturer
Computer Networks
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Data Link Layer
• Provides a well-defined service interface to the network layer.
• Determines how the bits of the physical layer are grouped into frames (framing).
• Deals with transmission errors (CRC and ARQ).
• Regulates the flow of frames.
• Performs general link layer management.
Data Link Layer 2
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Data Link Layer: IntroductionTerminology:• Hosts and routers/ bridges/
switches are nodes
• Communication channels that connect adjacent nodes along communication path are links, e.g.:– Wired links
– Wireless links
– LANs
• 2-PDU is a frame, encapsulates datagram
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“link”
Data-link layer has the responsibility of transferring datagram from one node to adjacent node over a link
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Data Link Layer Services (1)• Datagram transferred by different link protocols over
different links: – Ethernet, FDDI,802.11, etc– Different protocols different services
• Framing, link access:– Encapsulate datagram into frame, adding header, trailer– Channel access if shared medium– Physical addresses used in frame headers to identify source,
destination
• Reliable delivery between adjacent nodes– Rarely used on low bit error link (e.g., fiber)– Essential in links with high error rates
• e.g., wireless
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Link Layer Services (2)
• Flow Control:– Pacing between adjacent sending and receiving nodes
• Error Detection:– Errors are caused by signal attenuation, noise. – Receiver detects presence of errors
• Either signals sender for retransmission or drops frame
• Error Correction:– Receiver identifies and corrects bit error(s) without resorting to
retransmission– Recovery depends on the encoding scheme
• Half-duplex and full-duplex– Half duplex nodes at both ends of link can transmit, but not at
same time
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Adaptors Communicating
• Link layer implemented in network “adapter” – Ethernet card, PCMCI card,
802.11 card, …
• Sending side:– Encapsulates datagram in a
frame– Adds error checking bits, flow
control, etc.
• Receiving side– Looks for errors, flow control,
etc
– Extracts datagram, passes to receiving node
• Adapter is semi-autonomous link & physical layers
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sendingnode
frame
receivingnode
datagram
frame
adapter adapter
link layer protocol
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Error Detection
• EDC: Error Detection and Correction bits (redundancy)• D: Data protected by error checking,
– May include header fields
• Note: Error detection not 100% reliable!– Depends on the encoding– Larger EDC better error detection/correction capabilities
• Plus more overhead!7
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Parity Checking
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Single Bit Parity:Detect single bit errors
Two Dimensional Bit Parity:Detect and correct single bit errors
0 0
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CRC: Cyclic Redundancy Check
• View data bits, D, as a binary number
• Choose r+1 bit pattern (generator), G
• Goal: Choose r CRC bits, R, such that– <D,R> exactly divisible by G (modulo 2)
– Receiver knows G, divides <D,R> by G.• If non-zero remainder error detected
• CRC can detect all burst errors less than r+1 bits
• Very practical, e.g., ATM, HDCL
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CRC ExampleWant:
D.2r XOR R = nG
D.2r = nG XOR R Divide D.2r by G & take
the remainder R
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Data Link Layer 11
3 2 11 2 3 2 11 2
21
Medium
1
2
Physical layer entity
Data link layer entity3 Network layer entity
PhysicalLayer
Data linkLayer
PhysicalLayer
Data linkLayer
A B
A B
Packets Packets
Frames
(a)
(b)
Figure 5.2
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Data Link Layer 12
1 2 3 4 5
Data Data Data
ACK/NAK
Data
1 2 3 4 5Data Data Data Data
ACK/NAK
ACK/NAK
ACK/NAK
ACK/NA
K Figure 5.7
End to End
Hop by Hop
Leon-Garcia & Widjaja: Communication Networks
Copyright ©2000 The McGraw Hill Companies
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Data Link Layer 13
Packetsequence
Error-free packet
sequence
Informationframes
Control frames
Transmitter Receiver
CRC
Informationpacket
Header
Station A Station B
Information Frame
Control frame
CRC Header
Figure 5.8Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies
Basic Elements of ARQ
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Data Link Layer 14
ack seq kindinfo
buffer
physical layer
network layer
data link layer
frame
packet
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In parts (a) and (b) transmitting station A acts the same way, but part (b) receiving station B accepts frame 1 twice.
Data Link Layer
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(a) Frame 1 lost
A
B
frame 0
frame1
ACK
frame1
ACK
timeTime-out
frame2
(b) ACK lost
A
B
frame 0
frame1
ACK
frame1
ACK
timeTime-out
frame2
ACK
Figure 5.9
Ambiguities with Stop-and-Wait[unnumbered frames]
Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies
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Transmitting station A misinterprets duplicate ACKs
Data Link Layer 16
A
B
frame 0 frame
0ACKframe
1ACK
timetime-out
frame2
Figure 5.10Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies
PAR [OLD] problemAmbiguities when ACKs are not numbered
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Data Link Layer 17
Transmitter Receiver
SlastRnext
0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
(0,0) (0,1)
(1,0) (1,1)
Timer
Global State:(Slast, Rnext)
Error-free frame 0arrives at receiver
ACK forframe 0
arrives attransmitter
ACK forframe 1
arrives attransmitter Error-free frame 1
arrives at receiver
Station A Station BRnext
Slast
Figure 5.11Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies
State Machine for Stop-and-Wait
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Sliding Window Protocols[Tanenbaum]
• Must be able to transmit data in bothdirections.
• Choices for utilization of the reverse channel:– mix DATA frames with ACK frames.
– Piggyback the ACK• Receiver waits for DATA traffic in the opposite direction.
• Use the ACK field in the frame header to send sequence number of frame being ACKed.
– better use of the channel capacity.
Data Link Layer 18
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Sliding Window Protocols
• ACKs introduce a new issue – how long does receiver wait before sending ONLY an ACK frame.We need an ACKTimer!!
sender timeout period needs to set longer.
• The protocol must deal with the premature timeout problem and be “robust” under pathological conditions.
Data Link Layer 19
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Sliding Window ProtocolsEach outbound frame must contain a sequence number. With n
bits for the sequence number field, maxseq = 2**n - 1 and the numbers range from 0 to maxseq.
Sliding window :: sender has a window of frames and maintains a list of consecutive sequence numbers for frames that it is permitted to send without waiting for ACKs.
receiver has a window that is a list of frame sequence numbers it is permitted to accept.
Note – sending and receiving windows do NOT have to be the same size.
Windows can be fixed size or dynamically growing and shrinking.
Data Link Layer 20
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Sliding Window Protocols• Host is oblivious, message order at transport level
is maintained.
sender’s window :: frames sent but not yet ACKed.
– new packets from the Host cause the upper edge inside sender window to be incremented.
– ACKed frames from the receiver cause the lower edge inside window to be incremented.
• All frames in the sender’s window must be saved for possible retransmission and we need one timer per frame in the window.
Data Link Layer 21
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Sliding Window Protocols
• If the maximum sender window size is B, the sender needs B buffers.
• If the sender window gets full (i.e., reaches its maximum window size, the protocol must shut off the Host (the network layer) until buffers become available.
• receiver windowFrames received with sequence numbers outside
the receiver window are not accepted.
Data Link Layer 22
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Sliding Window Protocols
receiver window– Frames received with sequence numbers
outside the receiver window are not accepted.
– The receiver window size is normally static.
The set of acceptable sequence numbers is rotated as “acceptable” frames arrive.
a receiver window size = 1 the protocol only accepts frames in order.
There is referred to as Go Back N.
Data Link Layer 23
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Data Link Layer 24
Standard Ways to ACK
1. ACK sequence number indicates the last frame successfully received.
2. ACK sequence number indicates the next frame the receiver expects to
receive.Both of these can be strictly individual
ACKs or represent cumulative ACKing.Cumulative ACKing is the most common
technique.
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Data Link Layer 25
A
B
fr0
timefr1
fr2
fr3
fr4
fr5
fr6
fr3
ACK1 error
Out-of-sequence frames
Go-Back-4: 4 frames are outstanding; so go back 4
fr5
fr6
fr4
fr7
fr8
fr9
ACK2
ACK3
ACK4
ACK5
ACK6
ACK7
ACK8
ACK9
Figure 5.13Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies
Go Back N
ACKing next frame expected
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Data Link Layer 26
A
B
fr0
timefr1
fr2
fr3
fr4
fr5
fr1
fr2
ACK1
error
Out-of-sequence
frames
Go-Back-7:
fr4
fr5
fr3
fr6
fr7
fr0
NAK1
ACK3
ACK4
ACK5
ACK6
ACK7
ACK2
Transmitter goes back to frame 1
Figure 5.17
Go Back Nwith NAK error recovery
Copyright ©2000 The McGraw Hill Companies
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Data Link Layer 27
A
B
fr0
timefr1
fr2
fr3
fr4
fr5
fr6
fr2
ACK1 error
fr8
fr9
fr7
fr10
fr11
fr12
ACK2
NAK2
ACK7
ACK8
ACK9
ACK10
ACK11
ACK12
ACK2
ACK2
ACK2
Figure 5.21
Selective Repeatwith NAK error recovery
Leon-Garcia & Widjaja: Communication NetworksCopyright ©2000 The McGraw Hill Companies
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Multiple Access Links and Protocols
• Link types:– Point-to-point, e.g.:
• PPP for dial-up access
• Point-to-point link between Ethernet switch and host
– Broadcast (shared wire or medium), e.g.:• traditional Ethernet
• 802.11 wireless LAN
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Multiple Access Protocols
• Single shared broadcast channel
• Simultaneous transmissions interference only one node can send successfully at a time
• Multiple access protocol:
A distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit.
• Note: Communication about channel sharing must use channel itself!
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Protocol Requirements
• Assume broadcast channel of R bps
• Requirements:– When one node wants to transmit, it can send at rate R.
– When M nodes want to transmit, each can send at average rate R/M
– Fully decentralized:• no special node to coordinate transmissions
• no synchronization of clocks, slots
– Simple cheap to implement
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Taxonomy of MAC Protocols
• Channel Partitioning– Divide channel into smaller “pieces”
• time slots• frequency• code
– Allocate piece to node for exclusive use
• Random Access– Channel not divided, allow collisions– Handle collisions
• Turn-taking– Tightly coordinate shared access to avoid collisions
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Channel Partitioning: TDMA• TDMA: Time Division Multiple Access
– Slice bandwidth into time slots
– Each station gets fixed length slot (length = packet transmission time) in each round
– Unused slots go idle
• Example: – 6-station LAN,
– 1,3,4 have pkt, slots 2,5,6 idle:
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Channel Partitioning: FDMA• 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, – 1,3,4 have pkt, frequency bands 2,5,6 idle
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freq
uenc
y ba
nds
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Channel Partitioning: CDMA
• Used mostly in wireless broadcast channels – cellular, satellite, etc
• CDMA: Code Division Multiple Access) – All users share same frequency, but
– each user has own “chipping” sequence (code) to encode data
– Allows multiple users to transmit simultaneously with minimal interference
• Encoding:– signal = (original data) X (chipping sequence)
• Decoding: – inner-product of encoded signal and chipping sequence
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CDMA Encoding/Decoding
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CDMA: Handling Interference
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Random Access Protocols
• When node has packet to send:– Transmits at full channel data rate R– with no a priori coordination among nodes
• Two or more transmitting nodes collision• Random access MAC protocol specifies:
– How to detect collisions– How to recover from collisions
• e.g., via delayed retransmissions
• Examples of random access MAC protocols:– Slotted ALOHA– ALOHA– CSMA, CSMA/CD, CSMA/CA
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Slotted ALOHA (1)
Assumptions• All frames same size• Time is divided into equal
size slots == time to transmit 1 frame
• Nodes start to transmit frames only at beginning of slots
• Nodes are synchronized• If 2 or more nodes transmit
in slot, all nodes detect collision
Operation
• When node obtains fresh frame, it transmits in next slot
• No collision node can send new frame in next slot
• Collision, node retransmits frame in each subsequent slot with probability p until success
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Slotted ALOHA (2)
Pros• Single active node can
continuously transmit at full rate of channel
• Decentralized: – only slots in nodes need to be
in sync
• Simple
Cons
• Collisions wasted slots
• Idle slots
• Nodes may be able to detect collision in less than time to transmit packet
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Slotted ALOHA Efficiency
Analysis:• Suppose N nodes with many
frames to send, each transmits in slot with probability p
• Probability that 1st node has success in a slot:
p(1-p)N-1
• Probability 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:
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Efficiency:long-run Efficiency:long-run fraction of successful slots given many nodes with many frames to send
At best: channel used At best: channel used for useful transmissions ~37% of time!
epNpN
/11lim **
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Pure (Unslotted) ALOHA• Same as slotted but no synchronization:
– When frame first arrives transmit immediately
Collision probability increases!
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Efficiency = 1/(2e)Efficiency = 1/(2e)
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CSMA: Carrier Sense Multiple Access
• CSMA:– Listen before transmit:
• If channel sensed idle: transmit entire frame
• If channel sensed busy, defer transmission
• Human analogy– Don’t interrupt others!
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CSMA Collisions
• Collisions can still occur:– propagation delay means
two nodes may not hear each other’s transmission
• On collision:– entire packet transmission
time wasted
• Collision probability– depends on propagation
delay
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CSMA/CD (Collision Detection)• CSMA/CD: Carrier sensing, deferral as in CSMA
– Collisions detected within short time– Colliding transmissions aborted
• Collision detection:– On wired LANs:
• measure signal strengths, • compare transmitted & received signals
– On wireless LANs: • receiver shut off while transmitting
• Human analogy: – the polite conversationalist
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CSMA/CD Collision Detection
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Turn-Taking MAC Protocols
• Channel partitioning:– High load efficiently and fair– Low load inefficient
• 1/N bandwidth allocated even if only 1 active node!
• Random access:– Low load efficient
• single node can fully utilize channel
– High load inefficient• collision overhead
• Turn-taking protocols:Look for best of both worlds!
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“Taking Turns” MAC protocolsPolling:• Designated “master”
node• Master node invites
“slave” nodes to transmit– Round robin– Max frames
• Concerns:– Polling overhead – Latency– Single point of failure
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Token passing: Control token passed from
one node to next sequentially. Concerns:
Token overhead Latency Single point of failure (token)
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Summary of MAC Protocols
• Sharing of shared media:– Channel (static) partitioning
• by time, frequency, code• TDMA, FDMA, CDMA
– Random (dynamic) partitioning • carrier sensing: collision detection• easy in wire technologies,harder in wireless• ALOHA, S-ALOHA, CSMA, CSMA/CD• CSMA/CD used in Ethernet
– Turn-Taking• polling from a central site• token passing
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