V.Parthipan assistant Professor ,Saveetha University ,chennai India

• Developed during the mid-1970Developed during the mid-1970 抯 抯 at Xerox Palo Alto Research at Xerox Palo Alto Research Center (Bob Metcalfe) Center (Bob Metcalfe) • Later refinement by DEC, Intel and Xerox (DIX standard)Later refinement by DEC, Intel and Xerox (DIX standard)• Became IEEE 802.3 in 1985Became IEEE 802.3 in 1985• Different frame format between Ethernet and IEEE 802.3Different frame format between Ethernet and IEEE 802.3

Data link layer

Physical layer

0.5 ?Coax0.5 ?Coax

taptap

BNC connectorBNC connector

transceivertransceiver

AUI cableAUI cable

NICNIC

station interface

data encapsulation

link management

encoding and decoding

transmission and receipt

hubhub

10Base2 - Thin Ethernet10Base2 - Thin Ethernet

10Base5 - Thick Ethernet10Base5 - Thick Ethernet

10BaseT-Twisted pair10BaseT-Twisted pair

routerrouter

serverserver

hubhub

bridgebridge

• Various standard defined for IEEE802.3 with a concise notationVarious standard defined for IEEE802.3 with a concise notation• 10Base5 -- thickwire coaxial10Base5 -- thickwire coaxial• 10Base2 -- thinwire coaxial or cheapernet10Base2 -- thinwire coaxial or cheapernet• 10BaseT -- twisted pair10BaseT -- twisted pair• 10BaseF -- fiber optics10BaseF -- fiber optics• 10Broad36 -- broadband10Broad36 -- broadband

• Fast EthernetFast Ethernet• 100BaseTX, 100BaseT4, 100BaseF and 100 VG-AnyLAN100BaseTX, 100BaseT4, 100BaseF and 100 VG-AnyLAN

data rate data rate in Mbpsin Mbps

signalingsignalingbaseband or baseband or broadband broadband

maximum segment maximum segment length in hundredslength in hundredsof metersof meters

10 Base 5

• tap : tap : cable does not to be cut cable does not to be cut• transceiver : transceiver : send/receive, collisonsend/receive, collison detection, electronics isolationdetection, electronics isolation• AUI : AUI : Attachment Unit InterfaceAttachment Unit Interface• Use for backbone networks Use for backbone networks

0.5?Coax0.5?Coax

vampire tapvampire tap

BNC connectorBNC connector

transceivertransceiver

AUI cableAUI cable

NICNIC

maximum segment length=500mmaximum segment length=500mmaximum number of stations per segment=100maximum number of stations per segment=100

minimum distance minimum distance between two stations = 2.5 mbetween two stations = 2.5 m

maximum network distance between maximum network distance between two stations = 2.8kmtwo stations = 2.8km

0.25 ?Coax0.25 ?Coax

BNC T-connectorBNC T-connector

NICNIC

• BNC connector BNC connector • No drop cableNo drop cable• use for office LANuse for office LAN

maximum segment length=185mmaximum segment length=185mmaximum number of stations per segment=30maximum number of stations per segment=30

minimum distance minimum distance between two stations = 0.5 mbetween two stations = 0.5 m

maximum network distance between maximum network distance between two stations = 925 m two stations = 925 m

NICNIC

• A hub functions as a repeaterA hub functions as a repeater• use for office LANuse for office LAN

hubhub

maximum segment maximum segment length = 100mlength = 100m

• 10BaseF specification enable long distance connections with 10BaseF specification enable long distance connections with the use of optical fiber. Three standards are:the use of optical fiber. Three standards are:

• 10BaseFP - 10BaseFP - A passive-star topology, up to 1 km linkA passive-star topology, up to 1 km link

• 10BaseFL - 10BaseFL - An asynchronous point-to-point linkAn asynchronous point-to-point link (a new FOIRL) , up to 2 km link(a new FOIRL) , up to 2 km link

• 10BaseFB - 10BaseFB - A synchonous point-to-point link , up to 2 km link A synchonous point-to-point link , up to 2 km link with 15 cascade repeaterswith 15 cascade repeaters

Station listens to the medium before transmittingStation listens to the medium before transmitting (listen before (listen before talking ;LBT)talking ;LBT)

Non persistent CSMANon persistent CSMA if medium is idle, transmitif medium is idle, transmit if medium busy, waits a random periods an then resense mediumif medium busy, waits a random periods an then resense medium

1-persistent CSMA1-persistent CSMA if medium is idle, transmitif medium is idle, transmit if medium busy, continue to listen until the channel is sensed idle; then if medium busy, continue to listen until the channel is sensed idle; then transmit immediatelytransmit immediately

More than two station send frame at the same time More than two station send frame at the same time Station transmits its data while checking the cahannel for Station transmits its data while checking the cahannel for collisioncollision If average DC voltage level exeeds the CD threshold, collision is If average DC voltage level exeeds the CD threshold, collision is detected detected

How to detect How to detect Station send frame and sense the mediumStation send frame and sense the medium Collision detected if Collision detected if

station sense the exeeded signal strength (coax)station sense the exeeded signal strength (coax) there is signal on more than one inut (UTP)there is signal on more than one inut (UTP)

How to doHow to doTransmitting nodes send a jamming signal after a collision is Transmitting nodes send a jamming signal after a collision is detected.detected.

jamjam

IEEE 802.3 IEEE 802.3

PA : Preamble - 10101010s for synchronizationPA : Preamble - 10101010s for synchronizationSFD : Start of Frame delimiter -- 10101011 to start frameSFD : Start of Frame delimiter -- 10101011 to start frameDA: Destination Address -- MAC addressDA: Destination Address -- MAC addressSA: Source Address -- MAC addressSA: Source Address -- MAC addressLEN: Length -- Number of data bytesLEN: Length -- Number of data bytesType: identify the higher -level protocol Type: identify the higher -level protocol LLC PDU+pad -- minimum 46 bytes, maximum 1500 LLC PDU+pad -- minimum 46 bytes, maximum 1500 FCS : Frame Check Sequence -- CRC-32FCS : Frame Check Sequence -- CRC-32

7 1 6 6 2 46-1500 4 7 1 6 6 2 46-1500 4

FCSPA SA LENSFD DA LLC PDU Pad

8 6 6 2 46-1500 4 8 6 6 2 46-1500 4

FCSSA TypePA DA Data Pad EthernetEthernet

calculation of the FCScalculation of the FCS

64-1518 bytes64-1518 bytes

I/G =0 Individual addressI/G =0 Individual addressI/G =1 Group addressI/G =1 Group addressU/L=0 Global administered addressU/L=0 Global administered addressU/L=1 Local administered addressU/L=1 Local administered address

Unicast : Unicast : define a single destination define a single destination Broadcast : Broadcast : FFFFFFFF each station on the network receive and accept frames FFFFFFFF each station on the network receive and accept frames multicast : multicast : a group address defines multiple recepienta group address defines multiple recepient

I/G U/L 46-bit address

1 1 46

I/G 15-bit address

1 15

Two addressformat:Two addressformat: 48 bits 48 bits 16 bits 16 bits

Obligatory 9.6ms interval between the emitted frame and Obligatory 9.6ms interval between the emitted frame and the new onethe new one To enable other stations wishing to transmit to take over at To enable other stations wishing to transmit to take over at this timethis time

FCSSA TypePA DA Data Pad PA

Interframe gapInterframe gap> 9.6 > 9.6 ss

Assemble frame

carrier sense signal ON ?

Wait interframe gap timeStart trnasmission

collision detected?

N

N

Y

Y send jam sequenceIncrement attempt.

attempt limited?

Discard frame

N

YCompute backoff

and wait backoff time transmission done?

tranmit OK

N

Y

done receiving?

Pass frame to next layer

Y

Y

N

N

Discard frame

Y

N

start receiving

Matched DA

FCS and frame size OK?

collision detected?

Y

send frame

Discard frame

Y

random wait between 0 and delay-1

double delay

limited delay<1024

N

max delay is limited at1023*51.2 s=52.4 ms

>16 attempts

delay= 2

transmission done?

tranmit OK

N

Y

N

1st wait 0 or 1 slot time2nd wait 0,1,2 or 3 slot time3rd wait 0,1,2,..7 slot timekth wait 0.. 2k slot time

after 10th collisions the interval is frozen at 1023 slots after 16th collisions , frame is discarded and report failure back to the upper layer

(1) packet starts at time 0

(2) packet almost at B at t-

(3) B send packet; collision occurs at t

(4) jam signal gets back to A at 2t

A frame must take more than 2t to send to prevent the situation that the sender incorrect conclude that the frame was successfully sent. This slot times is = 51.2 s corresponds to 512 bit = 64 bytes The minimum frame length is 64 bytes (excluding preamble) This answers why data field must have 46 bytes minimum

A

A

A

A

B

B

B

B

A and B locates at the far end of the cableA and B locates at the far end of the cable