Giuseppe Bianchi

EthernetEthernet

Carrier Sense Multiple AccessCarrier Sense Multiple Accesswith Collision Detectionwith Collision Detection

Giuseppe Bianchi

Ethernet AncestorsEthernet Ancestors

Late 1960: ALOHA networkNorman Abramson, University of HawaiiApplication: radio network among islands

Distributed, uncoordinated network!First random access mechanism

(Pure aloha / Slotted aloha)

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Birth of EthernetBirth of EthernetMay 22, 1973: Ethernet memo

Bob Metcalfe (Xerox Palo Alto Research Center)Carrier Sense Multiple Access with Collision

Detection and expo backoff3 mbps speed

US Patent 4.063.220“Multipoint Data CommunicationSystem with Collision Detection”end 1977

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On the birth of EthernetOn the birth of Ethernet [quoting from Ethernet – The definitive guide”]: In late 1972, Metcalfe and his Xerox PARC colleagues

developed the first experimental Ethernet system to interconnect the Xerox Alto, a personal workstation with a graphical user interface. The experimental Ethernet was used to link Altos to one another, and to servers and laser printers. The signal clock for the experimental Ethernet interface was derived from the Alto's system clock, which resulted in a data transmission rate on the experimental Ethernet of 2.94 Mbps. Metcalfe's first experimental network was called the Alto Aloha Network. In 1973 Metcalfe changed the name to "Ethernet," to make it clear that the system could support any computer--not just Altos--and to point out that his new network mechanisms had evolved well beyond the Aloha system. He chose to base the name on the word "ether" as a way of describing an essential feature of the system: the physical medium (i.e., a cable) carries bits to all stations, much the same way that the old "luminiferous ether" was once thought to propagate electromagnetic waves through space. Thus, Ethernet was born.

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Ethernet Standardization: Ethernet Standardization: from DIX to 802.3from DIX to 802.3

1980: DIX Ethernet StandardDIX = Digital-Intel-Xerox vendor consortium Interoperable products from the three founding companies

1982: Xerox relinquish “Ethernet” trademark

1985: IEEE 802.3Ethernet becomes an IEEE 802 standard

Minor modifications vs DIX standardPath towards worldwide interoperability

Speed: 10 mbpsMedium:

Thick coaxial, 500 mt max cable (10BASE5)Thin coaxial, 185mt max cable (10BASE2)Network extension via repeaters (up to a max limit)

Ethernet standard: the world’s FIRST open, multi-vendor standard!Quoting Metcalfe: “the invention of Ethernet as an open, non-proprietary,

industry-standard local network was perhaps even more significantthan the invention of Ethernet technology itself”

Thick-RG213

Thin-RG58

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A note on Ethernet terminologyA note on Ethernet terminology

speed Signal method medium

Speed10, 100, 1000, 10G

Signal methodBase, Broad

Broad = RF modulated on coax» only one case: 10BROAD36, now obsolete

MediumOld notation: 2,5 = 200/500 mt (thin/thick coax)More recent notation: T, Tx, T4, T2, FX, X, CX, SX, LX

Depends on which specific twisted pair category & fibre category;Different labels (e.g. T, TX, T4, T2) account for different encoding

details

EXAMPLE: 100Base-T, 1000Base-LX, …

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FDCBA E

FDCB EA

wrong!wrong!

Multiple Access shared transmission medium thick / thin coaxial cable

Historical Ethernet topology: Historical Ethernet topology: busbus

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Ethernet transceiver Ethernet transceiver (10base-5)(10base-5)

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Ethernet transceiver (10Base-2)Ethernet transceiver (10Base-2)

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Breakthrough idea (1990)Breakthrough idea (1990)

FDCB EA

FDCB EA

Collapsed BackboneFrom BusTo Star topology

Multi-PortRepeater(HUB)

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Twisted Pair revolutionTwisted Pair revolution

1990: 802.3i 10BASE-T twisted pair Invented by SynOptics Communications

Reuse structured cabling system standardsOvercomes management and installation

problems from coaxial cablingEthernet market takes-off!!

Alternatives UTP (Unshielded) FTP (Foiled)

1 shield for all the cable STP (Shielded):

One shield per pair

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Ethernet RJ45 connectorEthernet RJ45 connector

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Signal transmission & reception Signal transmission & reception on twisted pairon twisted pair

Pair 3 used for transmission Pair 2 used for reception Pairs 1 & 4 unused

In 10Base-T & 100Base-T(x) Ethernet Used in 1000Base-T

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Straight & crossoverStraight & crossover RJ45 meant to connect PC to Hub What About PC to PC or Hub to Hub connection?

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Ethernet FrameEthernet Frame

Preamble: For bit-level and frame-level synchronization

Destination address: Who’s this frame for (who is the intended receiver) Hardware address (48 bits address of the network interface card)

Source address: Who’s this frame from (who is the transmitter) Hardware address (48 bits address of the network interface card)

Type: Which type of information is carried by the frame (which upper layer protocol)

Frame Check Sequence Verifies if the frame has been received properly (not corrupted)

preamble Ethernet header DATA FCS8 bytes 14 bytes 46-1500 bytes 4 bytes

Source address TypeDestination address

6 bytes 6 bytes 2 bytes

64-1518 bytes

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Transmission of bitsTransmission of bitsIdea:

send data + clock in the same signal!10 mbps Manchester encoding

1 1 1 0 00

When a frame is being transmitted, a signal level transition occurs every 0,2 s Allows to detect when transmissions are NOT occurring on the channel (IDLE channel)

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Preamble (10 mbps)Preamble (10 mbps)8 bytes

7 bytes preamble 7 x (10101010) square wave @ 5 MHz for clock recovery

Last byte: SFD (Start Frame Delimiter) 1 x (10101011) signals the start of the frame

1 0 1 0 1 0 1 1SFD

bit sequence

Manchester Encoding

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48 bit addresses48 bit addresses Typically referred to as

Interface addressHardware addressMAC address

First bit:0 = physical address of an interface

Unicast address1 = group address

Second bit:0 = globally administered address

Assigned by the manifacturer1 = locally administered address

Can be configured through driver

First 24 bits: OUI(Organization Unique Identifier)(unique for each vendor)

Typically written in hexe.g.: F0-11-00-4F-A2-1C

Each byte transmitted from LSB to MSB

0000.1111.1000.1000.0000.0000.1111.0010.0100.0101.1000.0011

mcast addresses: start with 1 (first octet LSB!)

Why destination first? Station who does not match dest may ignore rest of frame!

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ExamplesExamples

Individual unicast: xy-xx-xx-xx-xx-xx y multiple of 4802.3 & 802.4: transmitted from LSB to MSB802.5 & FDDI: transmitted from MSB to LSB

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Multiple Access: Ethernet vs Multiple Access: Ethernet vs AlohaAloha

Aloha:Transmit without

listeningRegardless of

channel activity

Transmit all the frame

Ethernet:Listen before

transmittingTransmit only if

channel idle

Listen while transmitting, and stop transmission if collision is detected

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Ethernet vs Aloha: listen before Ethernet vs Aloha: listen before txtx

A

BALOHA

A

BETHERNET

Listen before transmit

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Ethernet vs Aloha: Ethernet vs Aloha: collision handlingcollision handling

A

BALOHA

A

BETHERNET

Detect collision; enforce collision with JAM sequence; end transmission

Collision lasts until the end of the transmitted frames; waste of channel capacity

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Carrier Sense Multiple Carrier Sense Multiple AccessAccess

1. Listen before talking

1. NIC ready totransmit a frame

2. Listen for at least an Inter Frame Spacing(channel must beidle meanwhile)

3. Transmit frame

Ethernet Notation = Inter Packet Gap (IPG)802.3 Notation = Inter Frame Spacing (IFS)Minimum: 96 bits (@ 10 Mbps = 9.6 s)

IFS

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Carrier Sense Multiple Carrier Sense Multiple AccessAccess

2. If channel detected busy: defer

2. Listen

4. Defer

1. Frameready 3. Busy

Detect5. Listen for ≥ IFS

(similar defer situation if channel immediately busy)

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Collision DetectionCollision Detection

3. Listen while talking

If collision detected:Continue to transmit other 32 bits of signal

(Collision Enforcement Jam Signal)Collision detection: media-dependent

On coax:Monitor average DC voltageWhen more than 1 station transmits, voltage gets greater than

given threshold

On point-to-point fiber or twisted pair links:Collision detected by the simultaneous occurrence of activity on

both transmit and receive paths

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Why collisions occur?Why collisions occur?distance d (m)

prop delay d/200 s

Speed of EM signal in cable: ~200.000 km/s 200m/s

IFS

time

IFSStart tx

Detect collisionJam sequence

Start tx

Detect coll.Jam seq.

Collision occurs if stations access the channelin instants of time which differ for less than their

mutual propagation delay

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When a frame is “too When a frame is “too short”…short”…

prop delay d/200 sIFS

time

IFSStart tx

Detect collision

Start tx

Can’tdetect collision!!

Thinks Tx was OK

TransmittingStation

Destination station

Interferer

Collision!

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Network diameterNetwork diameter

Essential condition: a station MUST be able to

detect a collision Shortest possible

frame:6+6+2+46+4 = 64 bytes =

= 512 bits (excl preamble)

Condition on network diameter: a collision MUST be detected

on shortest possible frameBound on maximum RTT

Stations placed at opposite network edges

RTT=2 x prop must be lower than 512 bitstx time

64 bytes512 bitsframe

@ 10 Mbps: 512/10 [s] > 2d [m] / 200 [m/s] d < 5120 [m]@ 100 Mbps: 512/100 [s] > 2d [m] / 200 [m/s] d < 512 [m]NETWORK DIAMETER REDUCES AS SPEED INCREASES!!

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Network diameter in Network diameter in practicepractice

Max RTT lower than 51,2 s 802.3 standard max RTT = 46,38 s

Max link length Thick coax up to 500 mt Fiber link betw. repeaters up to 2 km Transceiver cable up to 50 mt

Topological limits 5-4-3 rule: between any two nodes on

the network, there can only be a maximum of five segmentsconnected through four repeatersonly three of the five segments may

contain user connections. Repeaters add delay!

Must account for repeater delaysMin_TX_time >

2 x d/200 + N x Repeater_delay

MAX network diameter for 10Base-5: 2800 mt

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The need for random backoffThe need for random backoff

Retransmission after collision must occur at different instant of times Otherwise collision will repeat forever

Deterministic rule? Hard, for a fully distributed mechanism

Idea: generate a random time after a collisionDifferent stations will extract (sooner or later) different

backoff times

A

B

IFS

IFS

IFS

IFS

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Rationale for backoff “slots”Rationale for backoff “slots”

Max RTT delay: < 51,2 s

Detect coll & Jam

Retransmit immediately(neglect IFS for simplicity)

Detect coll & Jam

Schedule RetransmAfter random time T…… busy channel: defer!

T greater or equal than 51,2 us NO COLLISION with another STA that has immediately rtx-edT greater or equal than N x 51,2 us NO COLLISION with another STA that rtx-ed at (N-1) x 51,2

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Backoff slot-timeBackoff slot-time Set to 512 bits

As minimum frame size As maximum RTT

guarantees that 1) A station transmitting at the beginning of the previous

backoff slot will be ALWAYS detected;2) A station transmitting for more than 512 bytes will acquire

for sure the channelo No “late collision” possibleo If they occur misconfiguration or hardware failures

Ethernet Backoff rule: extract a DISCRETE value B, and schedule transmission at timeB x 51,2 us

First 64 bytes

Collision possible

Remaining frame bytes (up to 1518 minus 64)

Collision impossible(Late Collision)

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Ethernet (truncated) Ethernet (truncated) Exponential backoff rulesExponential backoff rules

Once a frame collides:End transmission with Jam signalLet N be the retry index

N=0 for a new frame; N=1 for first rtx; N=2 for second rtx, etcAbort transmission after N=16 unsuccessful retriesLet K=min(10,N)Extract a random integer number R in the range:

0 ≤ R < 2K

Example: if K=6, extract R in (0, 31)Max range: when K=10 R in (0, 1023)

Schedule retransmission at time:Backoff = R x 51,2 us (10 mbps case)

» Where 51,2 us = tx time of 512 bits @ 10 Mbps» 100 Mbps Ethernet: slot-time = 5,12 us

Retransmit at given time only if channel idle (otherwise defer until channel becomes idle for an IFS)

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Ethernet (R)EvolutionEthernet (R)Evolution

From multiple access to From multiple access to point-to-point collision-free networkspoint-to-point collision-free networks

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Ethernet RepeatersEthernet Repeaters Physical layer device 3R functions

Re-ShapingRestores the proper signal waveform

Re-TimingRestores the proper impulse duration (clock recovery)

Re-TransmittingRetransmits signal on “other” portRegardless of who’s the signal intended for

R ABCD

A sends frame to BC & D receive signal, too

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Repeaters & collisionsRepeaters & collisionsRetransmits collisions, too

Actually, regenerates (extending them to 96 bits) 010101… jam sequences

And generates collisions when signal is received simultaneously on both ends

R ABCD

A sends frame to BC sends frame to D Repeater generates collision (Jam signal) on both links

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Multi-port repeater (hub)Multi-port repeater (hub)

Signal regenerated & retransmitted on all repeater ports except the port from which the signal has arrived

R

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Multiport repeaters Multiport repeaters star star topologytopology

Coax & transceivers no more necessary!

1990: emergence of 10Base-T technologyTransmission

medium = twisted pair point-to-point link

Hub distributes signal across all stations

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Hub-based star topologies:Hub-based star topologies:Why collision detection?Why collision detection?

How collision is detected?How collision is detected?

HUBTX

RX

RX

TX

TX

TX RX

A

B

C

D

A&B detect collision by listening on RX

RX

A CB D

Signal-level collision occurs on C&D RX links

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Ethernet bridgeEthernet bridge Data-link layer device Signal retransmission takes into account destination

address

B ABCD

A sends frame to B; Bridge recognizes that station B is on the same side the signal has been received and DOES NOT retransmit

B ABCD

A sends frame to D; Bridge recognizes that station D is on the other side: retransmits

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Multiport bridge Multiport bridge Switch Switch

HUBbroadcasts signal on all linksLogically behaves as a busOnly one tx at a time

SWITCH Repeats signal on specifically

addressed linkBridging functionMany tx-rx pairs at a time

More bw!

FDCBA E

HUBHUB

FDCBA E

SWITCHSWITCH

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Switches & collisionsSwitches & collisions

A sends frame to C; D sends frame to E No collision

Simultaneous transmission occurs

FDCBA E

SWITCHSWITCH

FDCBA E

SWITCHSWITCH

A sends frame to C; E sends frame to C

Switch understand that two frames are destined to a same link

“Collision” solved by buffering one frame & transmitting the other

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Broadcast domain vs collision Broadcast domain vs collision domaindomain

SwitchHUB

HUB

Without Switching

LANCollisionDomain

Broadcast Domain

With switching

Switch

CollisionDomain

CollisionDomain

CollisionDomain

CollisionDomain

Broadcast Domain

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Full-duplexingFull-duplexing Switched star network: only possible collision is on a same

link When both station and switch simultaneously start transmission

But rx and tx are carried out on different pairs collision is fictitious detected by the network interface card (activity sensed on Rx pair while NIC is transmitting) But it is not a real collision between the signals!

it would be with a central hub; it is not with central switch

1997: full duplex standard (802.3x) Simultaneously transmit and receive (2x speed increase) In a switched star network collision is no more possible!! CSMA/CD no more necessary Network diameter no more a concern

Arbitrary increase of the link speed is now possible!

1998-1999: Gigabit Ethernet 2002 (july): 10 GigaEthernet Ethernet in the metropolitan area (e.g. Fastweb, Telecom IT) Future: 40 Giga-Ethernet, 160 Giga-Ethernet

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X A

B

C

D

BufferingBuffering

A B A

A

A

A

B

D

DD

C

C

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X A

B

C

D

bufferingbuffering

A B A

A

A

A

B

D

DD

C CC

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X A

B

C

D

bufferingbuffering

A

A

A

A B

A B

D

DD

C C

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X A

B

C

D

bufferingbuffering

AA

A

A B

A B

D

DD

C C

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X A

B

C

D

bufferingbuffering

A

AAA B

A B

D

DD

C C

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BuffersBuffersSolve output port contention

1 packet transmittedOther stored in waiting line

May have finite capacityMax number of stored bytes or packetsIntroduce loss!!

Packet arriving when buffer full gets lost Introduce latency!!

Latency is NOT deterministic, but statistical

QUEUEING THEORY: we need a mathematical tool to quantitatively evaluate loss/delay introduced by networking devices

…. and much more

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Bridge/Switch operationBridge/Switch operation

PADPreamble +

SFDData

LEN ortype

FCSDEST SRC

lookup Store & Forward: read frame (memorize into onboard buffer)Check CRC

Discard frame if » CRC fails» too short (<64 bytes, “runt”) » too long (>1518 bytes, “giant”)

Look up destination into forwarding (switching) table

Forward packet to outgoing port Cut-through

Just read first few bytes (until destination address)

Don’t do any checkLook up forwarding table and select destination forward frame while receiving it

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Store & forward vs cut-through Store & forward vs cut-through latencylatency

1518 bytes frame

Assume full 8 bytes preamble receivedS&F @ 10 mbps ≥ 1526*8/10 s = 1222 sC-T @ 10 mbps ≥ 14*8/10 s = 11.2 s

S&F @ 100 mbps ≥ 122 sC-T @ 100 mbps ≥ 1.1 s

Not a real problem at high rate

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S&F vs CT: Fragment-free modeS&F vs CT: Fragment-free mode

Compromise between cut-through and store-and-forwardReads first 64 bytes

includes the frame (+LLC) headerThen starts send packet

before the entire data field is read and the FCS is checked. Advantages:

Verify reliability of header information (addresses, frame type, LLC header information)

Detects & discards runts & collisions

PADPreamble +

SFDData

LEN ortype

FCSDEST SRC

Cut-through Store & ForwardFragment-free

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Further issues with C-TFurther issues with C-TCut-through possible only if source

and destination ports have same bit rateSymmetric switching.

Different rates buffering necessary S&F only Asymmetric switching

Asymmetric switching typical in client/server environmentsMore bandwidth dedicated to the server port to

prevent a bottleneck

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Forwarding databaseForwarding database

Mapping between MAC addresses and ports Ports: module/port-#

Static entries: Configured by sysadmin Permanent database

Dinamic entries: “Learned” Expire after ageing process

reaches upper valueE.g. 300 secondsconfigurable

Dest MAC Address Ports Age----------------- ----- ---00-00-08-11-aa-01 1/1 100-b0-8d-13-1a-f1 1/7 4a8-11-06-00-0b-b4 2/3 008-01-00-00-a7-64 2/4 100-ff-08-10-44-01 2/6 5

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Address Learning /1Address Learning /1

STA 100-11-22-33-44-01

STA 208-55-66-77-88-02

STA 308-aa-bb-cc-dd-03

00-11-22-33-44-01 P108-55-66-77-88-02 P108-aa-bb-cc-dd-03 P208-01-02-f1-f2-04 P3

P1

P2

STA 408-01-02-f1-f2-04

Frame arrives at port XHence it has come

from LAN attached to port X

SRC address used to update forwarding DBSRC MAC Port

P3

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Address Learning /2Address Learning /2

STA 100-11-22-33-44-01

STA 208-55-66-77-88-02

STA 308-aa-bb-cc-dd-03

00-11-22-33-44-01 P1 508-55-66-77-88-02 P1 708-aa-bb-cc-dd-03 P2 008-01-02-f1-f2-04 P3 608-00-0f-cc-cc-a2 P1 0

P1

P2

STA 408-01-02-f1-f2-04

Incoming frame whose SRCaddr not in forwarding DB:Create new entryAgeing-time=0

Incoming frame whose SRCaddr already in forwarding DB:Refresh ageing-timeAgeint-time=0

P3

08-00-0f-cc-cc-a2

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Address Learning /3Address Learning /3

STA 100-11-22-33-44-01

STA 208-55-66-77-88-02

STA 308-aa-bb-cc-dd-03

00-11-22-33-44-01 P1 508-55-66-77-88-02 P1 708-aa-bb-cc-dd-03 P2 408-01-02-f1-f2-04 P3 608-00-0f-cc-cc-a2 P1 208-00-0f-cc-cc-a2 P2 0

P1

P2

STA 408-01-02-f1-f2-04

Incoming frame whose SRCaddr already in forwarding DB but associated to different port:Update associated portRefresh ageing time

P3

08-00-0f-cc-cc-a2

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Frame forwardingFrame forwarding Very first operation performed by the bridge/switch

upon frame receptionBefore learning

PADPreamble +

SFDData

LEN ortype

FCSDEST SRC

1. Frame OK? CRC check Only for Store & Forward

2. Incoming port enabled (in forwarding state)? Switch port may be disabled

e.g. to isolate malfunctioning stations/LANs3. If DEST is NOT in forwarding DB

broadcast frame (flooding) forward frame to all ports EXCEPT incoming one

4. If DEST is in forwarding DB Check whether DEST port = incoming port

If YES, discard packet (dest on same LAN of src) If NO, forwards packet to output port

» Unless output port blocked

Port X

Port Y

Flooding occurs also for broadcast frames (obvious) and for multicast frames (unless more sophisticated policies are set)

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Example / 1Example / 1start-upstart-up

P1

P2

P3

Initial state: forwarding DB = empty

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Example / 2Example / 2STA 1 STA 1 STA 2 STA 2

00-11-22-33-44-01 P1 0

P1

P2

P3

STA 1 transmits frame to STA 2Flooding occurs (STA2 not registered in

DB)Bridge learns STA1=P1

STA 100-11-22-33-44-01

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Example / 3Example / 3STA 2 STA 2 STA 1 STA 1

00-11-22-33-44-01 P1 200-aa-bb-cc-dd-02 P3 0

P1

P2

P3

STA 2 may respond depends on involved protocol/app rules (e.g. TCP handshake)

transmits frame to STA 1 Destination selected Bridge learns STA2=P3

STA 100-11-22-33-44-01

STA 200-aa-bb-cc-dd-02

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Example / 4Example / 4STA 3 STA 3 STA 1 STA 1

00-11-22-33-44-01 P1 1200-aa-bb-cc-dd-02 P3 1008-80-f0-00-ff-03 P1 0

P1

P2

P3

STA 3 on LAN 1 transmits to STA 1 Frame arrives to STA1 on LAN 1

But arrives also to Bridge Bridge discards frame (STA1 on same port of incoming

frame) This operation is referred to as FILTERING FUNCTION

Bridge learns STA3=P1

STA 100-11-22-33-44-01

STA 200-aa-bb-cc-dd-02

STA 308-80-f0-00-ff-03

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Example / 5Example / 5STA 1 moves; STA 1 STA 1 moves; STA 1 STA 3 STA 3

00-11-22-33-44-01 P1 1300-aa-bb-cc-dd-02 P3 1108-80-f0-00-ff-03 P1 100-11-22-33-44-01 P2 0

P1

P2

P3

STA 1 moves on LAN 2 Then transmits to STA 3 Frame arrives to Bridge on P2, and forwarded to P1

According to forwarding DB information Bridge learns that STA 1 moved

Deletes previous entry with P1 Adds new entry with P2

STA 100-11-22-33-44-01

STA 200-aa-bb-cc-dd-02

STA 308-80-f0-00-ff-03

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Example / 6Example / 6STA 2 moves; STA 1 STA 2 moves; STA 1 STA 2 STA 2

00-aa-bb-cc-dd-02 P3 1308-80-f0-00-ff-03 P1 300-11-22-33-44-01 P2 2P1

P2

P3

STA 2 moves on LAN 1 STA 1 transmit frame to STA 2 Frame forwarded on old port P3!!

Bridge will learn only when STA2 will transmit first frameOR when ageing time will expire

and STA2 P3 entry will be removed from forwarding DB

STA 100-11-22-33-44-01

STA 200-aa-bb-cc-dd-02

STA 308-80-f0-00-ff-03

???

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Why a station should move?Why a station should move?Fault-tolerant architectures!Fault-tolerant architectures!

P1

P2

Link 1 Link 2

As link 1 fails, server switches on link 2 server MOVES from original port P2 to new port P1 !!(need to reduce ageing time – but trade-off required: too short ageing time, too much burden on switch)(effective solution: i) periodically send “advertisement” frames ii) send frame after switching to link 2)

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Wireless LANs (Wi-Fi)Wireless LANs (Wi-Fi)

Just a very basic overviewJust a very basic overview

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WLAN historyWLAN history Original goal:

Deploy “wireless Ethernet” First generation proprietary solutions (end ’80, begin ’90):

WaveLAN (AT&T) HomeRF (Proxim)

Abandoned by major chip makers (e.g. Intel: dismissed HomeRF in april 2001)

IEEE 802.11 Committee formed in 1990 Charter: specification of MAC and PHY for WLAN First standard: june 1997

1 and 2 Mbps operation Reference standard: september 1999

Multiple Physical Layers Two operative Industrial, Scientific & Medical (ISM) shared unlicensed band

» 2.4 GHz: Legacy; 802.11b/g» 5 GHz: 802.11a

1999: Wireless Ethernet Compatibility Alliance (WECA) certification Later on named Wi-Fi Boosted 802.11 deployment!!

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WLAN data ratesWLAN data rates Legacy 802.11

Work started in 1990; standardized in 1997 1 mbps & 2 mbps

The 1999 revolution: PHY layer impressive achievements 802.11a: PHY for 5 GHz

published in 1999 Products available since early 2002

802.11b: higher rate PHY for 2.4 GHz Published in 1999 Products available since 1999 Interoperability tested (wifi)

2003: extend 802.11b 802.11g: OFDM for 2.4 GHz

Published in june 2003 Products available, though no extensive

interoperability testing yet Backward compatibility with 802.11b Wi-Fi

Ongoing standardization effort: 802.11n

Launched in september 2003 Minimum goal: 108 Mbps (but higher numbers

considered)

Standard Transfer Method

Freq. Band

Data Rates Mbps

802.11 legacy FHSS, DSSS, IR

2.4 GHz, IR

1, 2

802.11b DSSS, HR-DSSS

2.4 GHz 1, 2, 5.5, 11

"802.11b+" non-standard

DSSS, HR-DSSS, (PBCC)

2.4 GHz 1, 2, 5.5, 11,

22, 33, 44

802.11a OFDM 5.2, 5.5 GHz

6, 9, 12, 18, 24, 36, 48, 54

802.11g DSSS, HR-DSSS, OFDM

2.4 GHz 1, 2, 5.5, 11; 6, 9, 12, 18, 24, 36, 48, 54

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Why multiple rates?Why multiple rates?“Adaptive” (?) “Adaptive” (?)

coding/modulationcoding/modulation

Example: 802.11a case

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Wireless Medium Wireless Medium UnreliabilityUnreliability

11 Mbps 802.11b outdoor measurements - Roma 2 Campus - roof nodes

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Must rely on explicit ACKsMust rely on explicit ACKs

Successful DATA transmission:ONLY IF an ACK is received

ACK transmission provided by MAC layerImmediate retransmission

» Don’t get confused with higher layer rtx

DATA-ACK exchange:Also called two-way handshakeOr Basic Access Mechanism

SENDER RECEIVER

DATA

ACK

Giuseppe Bianchi

Must forget Collision Detection!Must forget Collision Detection! One single RF circuitry

Either TX or RX… Half-duplex

Even if two simultaneous TX+RX: large difference (100+ dB!) in TX/RX signal power Impossible to receive while transmitting

On a same channel, of course

Collision detection at sender: meaningless in wireless! Ethernet = collision detection at sender Wireless = large difference in the interference

power between sender & receiver! Collision OCCURS AT THE RECEIVER

STA

tx

rx

CA B

A detects a very low interference (C is far) no “collision”

B detects a disructive interference (C is near) collision occurs

Giuseppe Bianchi

Carrier Sense Multiple Carrier Sense Multiple AccessAccess Station may transmit ONLY IF senses channel IDLE for a

DIFS time DIFS = Distributed Inter Frame Space

Key idea: ACK replied after a SIFS < DIFS SIFS = Short Inter Frame Space

Other stations will NOT be able to access the channel during the handshake Provides an atomic DATA-ACK transaction

DIFSDATA

SIFS ACK

TX

RX

Packet arrival

OTHERSTA

DIFS

Packet arrival

Must measure a whole DIFS

OK!

Giuseppe Bianchi

Grasping wi-fi (802.11b) Grasping wi-fi (802.11b) numbersnumbers

DIFS = 50 sRationale: 1 SIFS + 2 slot-times

Slot time = 20 s, more later

PHY MAC header 24 (30) Payload FCS

SIFS = 10 sRationale: RX_TX turnaround time

The shortest possible!

DATA frame: TX time = f(rate) Impressive PHY overhead!

192 s per every single frame Total data frame time (1500

bytes) @1 Mbps: 192+12288= 12480 s

» PHY+MAC overhead = 3.3% @11 Mbps: 192+ 1117.1 = 1309.1 s

» PHY+MAC overhead = 16.% Overhead increases for small frames!

ACK frame: TX at basic rate Typically 1 mbps but 2 mbps possible… ACK frame duration (1mbps): 304 s

Preamble SFD PLCP hdr

128 16 48

1 mbps DBPSK

192 s

(28+payload) [bytes] x 8 / TX_rate [mbps] = s

PHY ACK 14

192 s

DATA

ACK

112 s

Giuseppe Bianchi

And when an ACK is And when an ACK is “hidden”?“hidden”?

SENDER RECEIVERSTA

1) Sender TX Receiver RX

STA defers

BUSY DETECT (DATA)

SENDER RECEIVERSTA

2)Receiver ACKs(after SIFS)STA cannot hear…

SIFSACK

STA STA TX!DIFS

SENDER RECEIVERSTA

3)STA tranmitsAnd destroys ACK!

Giuseppe Bianchi

The Duration FieldThe Duration Field

FrameControl

Duration / ID

Address 1 Address 2 Address 3SequenceControl

Address 4 DataFramecheck

sequence

2 2 2 40-23126666

0# microseconds1514131211109876543210

When bit 15 = 1 NOT used as duration (used by power-saving frames to specify station ID)

DIFSDATA

SIFS ACK

OTHERSTA

Physical carrier sensing

NAV (data)

Allows “Virtual Carrier Sensing”Other than physically sensing the channel, each station keeps a Network

Allocation Vector (NAV)Continuously updates the NAV according to information read in the

duration field of other frames

Virtual carrier sensing

Giuseppe Bianchi

And when a terminal is And when a terminal is “hidden”?“hidden”?

RECEIVER SENDERSTA

… this can be “solved” by increasing the sensitiveness of the Carrier Sense…Quite stupid, though (LOTS of side effects – out of the goals of this lecture)

SENDERSTA

… this can’s be “solved” by any means!

RECEIVER

The Hidden Terminal Problem SENDER and STA cannot hear each

other SENDER transmits to RECEIVER STA wants to send a frame

Not necessarily to RECEIVER…

STA senses the channel IDLE Carrier Sense failure

Collision occurs at RECEIVER Destroys a possibly very

long TX!!

Giuseppe Bianchi

DIFSDATA

SIFS ACK

TX

RX

Packet arrival

RTS

SIFS CTS SIFS

The RTS/CTS solutionThe RTS/CTS solution

TX

RX

hidden

others

RTS

NAV (RTS)

RTS/CTS: carry the amount of time the channelwill be BUSY. Other stations may update a Network Allocation Vector, and defer TX

even if they sense the channel idle (Virtual Carrier Sensing)

CTS CTS

NAV (CTS)

(Update NAV)

data

Giuseppe Bianchi

Why backoff?Why backoff?

DIFSDATA

SIFS ACKSTA1

STA2

STA3

DIFS

Collision!

RULE: when the channel is initially sensed BUSY, station defers transmission;THEN,when channel sensed IDLE again for a DIFS, defer transmission of a further random time (Collision Avoidance)

Giuseppe Bianchi

Slotted BackoffSlotted Backoff

STA2

STA3

DIFS

Extract random number in range (0, W-1)Decrement every slot-time

w=7

w=5

Note: slot times are not physically delimited on the channel!Rather, they are logically identified by every STA

Slot-time values: 20s for DSSS (wi-fi)Accounts for: 1) RX_TX turnaround time

2) busy detect time3) propagation delay

Giuseppe Bianchi

Backoff freezingBackoff freezing

When STA is in backoff stage:It freezes the backoff counter as long as the channel is

sensed BUSYIt restarts decrementing the backoff as the channel is sensed

IDLE for a DIFS period

DIFS DATA

SIFS ACK

STATION 1

DIFS

SIFS ACK 6 5

DIFS

Frozen slot-time 4BUSY medium

STATION 2

DIFS3 2 1

Giuseppe Bianchi

Why backoff between Why backoff between consecutive tx?consecutive tx?

A listening station would never find a slot-time after the DIFS (necessary to decrement the backoff counter)

Thus, it would remain stuck to the current backoff counter value forever!!

DIFS DATA

SIFS ACK

S 1

DIFS6 5

DIFS

Frozen slot-time 4

BUSY medium

S 2

DIFS3

DATA

SIFS ACK

DIFS

BUSY medium DIFS

Giuseppe Bianchi

Backoff rulesBackoff rules

First backoff value:Extract a uniform random number in range (0,CWmin)

If unsuccessful TX:Extract a uniform random number in range (0,2×(CWmin+1)-1)

If unsuccessful TX:Extract a uniform random number in range (0,22×(CWmin+1)-1)

Etc up to 2m×(CWmin+1)-1Exponential Backoff!For 802.11b:

CWmin = 31CWmax = 1023 (m=5)

Giuseppe Bianchi

Further backoff rulesFurther backoff rules Truncated exponential backoff

After a number of attempts, transmission fails and frame is droppedBackoff process for new frame restarts from CWminProtects against cannel capture

unlikely when stations are in visibility, but may occur in the case of hidden stations

Two retry limits suggested:Short retry limit (4), apply to frames below a given thresholdLong retry limit (7), apply to frames above given threshold (loose) rationale: short frames are most likely generated bu realk time

stationsOf course not true in general; e.g. what about 40 bytes TCP ACKs?

Giuseppe Bianchi

DCF overheadDCF overhead

min_

[ ]

[ ] / 2station

Frame Tx

E payload

E T DIFS CWS

_Frame Tx MPDU ACKT T SIFS T

TxCTSPLCPCTS

TxRTSPLCPRTS

TxACKPLCPACK

TxMPDUPLCPMPDU

RTT

RTT

RTT

RLTT

_

_

_

_

/148

/208

/148

/)28(8

_Frame Tx RTS CTS MPDU ACKT T SIFS T SIFS T SIFS T

Giuseppe Bianchi

Example: maximum achievable Example: maximum achievable throughput for 802.11bthroughput for 802.11b

DATA SIFSACK

DIFS

backoffDATA

Cycle time

Data Rate = 11 mbps; ACK rate = 1 mbps

Payload = 1500 bytes

MbpsThr

BackoffE

DIFSSIFS

T

T

ACK

MPDU

07.631050304101303

81500

310202

31][

50;10

3041/148192

130311/)150028(8192

Data Rate = 11 mbps; ACK rate = 1 mbps

Payload = 576 bytes

MbpsThr

BackoffE

DIFSSIFS

T

T

ACK

MPDU

53.33105030410631

8576

310202

31][

50;10

3041/148192

63111/)57628(8192

REPEAT RESULTS FOR RTS/CTS Not viable (way too much overhead) at high rates!

Giuseppe Bianchi

DCF overhead (802.11b)DCF overhead (802.11b)

0 2000 4000 6000 8000

Transmssion Time (usec)

Basic

RTS/CTS

Basic

RTS/CTS

DIFS Ave Backoff RTS+SIFS CTS+SIFS Payload+SIFS ACK

Giuseppe Bianchi

DCF overhead (802.11b)DCF overhead (802.11b)

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

100

300

500

700

900

1100

1300

1500

1700

1900

2100

2300

Payload Size (Bytes)

Nor

mal

ized

Thr

ough

put

BAS- 2Mbps

RTS- 2Mbps

BAS- 11Mbps

RTS- 11Mbps

Giuseppe Bianchi

Performance AnomalyPerformance Anomaly

Question 1: Assume that throughput measured for a single 11 mbps greedy station is approx 6 mbps.

What is per-STA throughput when two 11 mbps greedy stations compete? Answer 1:

Approx 3 mbps (easy )

Question 2: Assume that throughput measured for a single 2 mbps greedy station is approx 1.7 mbps.

What is per-STA throughput when two 2 mbps greedy stations compete? Answer 2:

Approx 0.85 mbps (easy )

Question 3: What is per-STA throughput when one 11 mbps greedy station compete with one 2 mbps

greedy station? Answer 3:

...

Giuseppe Bianchi

Understanding Answers 1&2Understanding Answers 1&2(neclect collision – indeed rare – just slightly reduce computed (neclect collision – indeed rare – just slightly reduce computed

value)value)

STA 1 SIFS

ACKDIFS

backoff

Cycle time

STA 2 SIFS

ACKDIFS

Frozen backoff

Data Rate = 11 mbps; ACK rate = 1 mbps Payload = 1500 bytes

][]2[]1[

81500

][

][]2[]1[

backoffEDIFSACKSIFSTDIFSACKSIFSTtimecycleE

payloadEThrThr

MPDUMPDU

MbpsThr

BackoffE

DIFSSIFS

T

T

ACK

MPDU

3.3310)50304101303(2

81500

310202

31][

50;10

3041/148192

130311/)150028(8192

Data Rate = 2 mbps; ACK rate = 1 mbps Payload = 1500 bytes

MbpsThr

BackoffE

DIFSSIFS

T

T

ACK

MPDU

88.0310)50304106304(2

81500

310202

31][

50;10

3041/148192

63042/)150028(8192

Giuseppe Bianchi

Emerging “problem”: Emerging “problem”: long-term fairness!long-term fairness!

If you have understood the previous example, you easily realize that

802.11 provides FAIR access to stations

in terms of EQUAL NUMBER of transmission opportunities in the long term!

But this is INDEPENDENT OF transmission speed!STA1 STA2 STA1 STA1STA2 STA2

Giuseppe Bianchi

Computing answer 3Computing answer 3

STA (2mbps) SIFS

ACKDIFS

Cycle time

STA 11 SIFS

ACKDIFS

Frozen backoff

RESULT: SAME THROUGHPUT (in the long term)!!

!!!!!!39.1310)5030410(213036304

81500

][]2[]1[

81500

][

][]2[]1[

Mbps

backoffEDIFSACKSIFSTDIFSACKSIFST

timecycleE

payloadEThrThr

MPDUMPDU

DRAMATIC CONSEQUENCE: throughput is limited bySTA with slowest rate (lower that the maximum throughput achievable by the slow station)!!

Giuseppe Bianchi

Performance anomaly into Performance anomaly into actionaction

Why the network is soooo slow today? We’re so Close, we have a 54 mbps and“excellent” channel, and we getLess than 1 mbps …

Hahahahahah!!Poor channel, Rate-fallbacked @ 1mbps