ch4flow and error control

8/16/2019 Ch4Flow and Error Control

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Flow and ErrorControl

Chapter- 4

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Need of ow & error control

• For reliable and efcient datacommunication a great deal ocoordination is necessary between at

least two machines.• Constraints:

–Both sender and receiver have

limited speed –Both sender and receiver have

limited memory

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Requirements:

–! ast sender should not overwhelm a slow

receiver" which must perorm a certainamount o processing beore passing thedata on to the higher#level sotware.

– $ error occur during transmission" it is

necessary to devise mechanism to correct it %he most important unctions o &ata 'in(

layer to satisy the above re)uirements areerror control and ow control.

*ollectively" these unctions are (nown asdata lin( control

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Flow Control•

Flow *ontrol is a techni)ue so that transmitterand receiver with di,erent speed characteristicscan communicate with each other.

• Flow control ensures that a transmitting station"

such as a server with higher processingcapability" does not overwhelm a receivingstation" such as a des(top system" with lesserprocessing capability.

•

%his is where there is an orderly -ow otransmitted data between the source and thedestination.

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• is a set o procedures that tells the sender howmuch data it can transmit beore it must wait or

an ac(nowledgment rom the receiver.• %he -ow o data should not be allowed to

overwhelm the receiver.

• eceiver should also be able to inorm the

transmitter beore its limits 0this limit may beamount o memory used to store the incomingdata or the processing power at the receiver endare reached and the sender must send ewer

rames.• 2ence" Flow control reers to the set o

procedures used to restrict the amount o datathe transmitter can send beore waiting orac(nowledgment.

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Flow *ontrol• Flow control coordinates the amount o data that can be

sent beore receiving ac(nowledgement

• $t is one o the most important unctions o data lin( layer.

• Flow control is a set o procedures that tells the senderhow much data it can transmit beore it must wait or an

ac(nowledgement rom the receiver.• eceiver has a limited speed at which it can process

incoming data and a limited amount o memory in whichto store incoming data.

• eceiver must inorm the sender beore the limits are

reached and re)uest that the transmitter to send ewerrames or stop temporarily.

• 3ince the rate o processing is oten slower than the rateo transmission" receiver has a bloc( o memory 0bu,er

or storing incoming data until they are processed.

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Flow control techniques

1. Stop-and-wait

3top#and#wait is also (nown as e)uest/replysometimes.

e)uest/reply 03top#and#wait -ow control re)uireseach data pac(et to be ac(nowledged by theremote host beore the ne4t pac(et is sent.

. Slidin!-window.

3liding window algorithms" used by %*" permit

multiple data pac(ets to be in simultaneoustransit" ma(ing more efcient use o networ(bandwidth .

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Stop and "ait

• 3ource transmits rame• &estination receives rame and replies with

ac(nowledgement

• 3ource waits or !*8 beore sending ne4trame

• &estination can stop -ow by not send !*8

• #a$or draw%ac

– only one rame can be in transmission at a time"this leads to inefciency i propagation delay ismuch longer than the transmission delay.

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Stop and "ait

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'in (tili)ation in Stop-and-"ait

•

%ransmission time %he time it ta(es or a stationto transmit a rame 0normali;ed to a value o 1.

• ropagation delay %he time it ta(es or a bit totravel rom sender to receiver 0e4pressed as a. –

a < 1 %he rame is sufciently long such that the =rstbits o the rame arrive at the destination beore thesource has completed transmission o the rame.

– a > 1 3ender completes transmission o the entirerame beore the leading bits o the rame arrive at

the receiver.• %he lin( utili;ation ? @ 1/01Aa"

– a @ ropagation time / transmission time

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lin utili)ation*cont++..,

•

lin( utili;ation is strongly dependent on theratio o the propagation time to thetransmission time.

• hen the propagation time is small" as in

case o '!C environment" the lin( utili;ation isgood .

• But" in case o long propagation delays" as incase o satellite communication" the utili;ation

can be very poor .• %o improve the lin( utili;ation" we can use the

sliding#window protocol instead o using stop#and#wait protocol.

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Slidin! "indow*sender,•

%o (eep trac( o the rames" sender station sendsse)uentially numbered rames.

• 3ince the se)uence number to be used occupiesa =eld in the rame" it should be o limited si;e.

•

$ the header o the rame allows ( bits" these)uence numbers range rom : to ( 1.

• 3ender maintains a list o se)uence numbersthat it is allowed to send 0sender window.

•

%he si;e o the senderGs window is at most ( 1.

• %he sender is provided with a bu,er e)ual tothe window si;e.

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Slidin!"indow*Receier,

•

%he receiver ac(nowledges a rame by sendingan !*8 rame that includes the se)uencenumber o the ne4t rame e4pected.

• %his also e4plicitly announces that it is prepared

to receive the ne4t C rames" beginning with thenumber speci=ed.

• %his scheme can be used to ac(nowledgemultiple rames. $t could receive rames " +"

but withhold !*8 until rame has arrived.• By returning an !*8 with se)uence number 5" it

ac(nowledges rames " +" in one go.

• %he receiver needs a bu,er o si;e 1.

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Sender Slidin! "indow

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Receier Slidin! "indow

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Slidin! "indow Eample

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Sender

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Receier

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lin utili)ation in Slidin! "indow

• ? @ 1" or C > a A 1

C /01Aa" or C < a A 1

here C @ the window si;e"and a @ ropagation time /

transmission time

EH. 8@5 then C@I5#1@+1a@1: then ?@1 J

hen a@1:: then ?@+1/01A::

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Error /etection andCorrection

• 01pes of Errors

•

/etection• Correction


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2asic concepts

• Cetwor(s must be able to transerdata rom one device to another withcomplete accuracy.

&ata can be corrupted duringtransmission.

For reliable communication" errors

must be detected and corrected. Error detection and correction are

implemented either at the data lin(

layer or the transport layer o the D3$5/19/16 +


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01pes of Errors

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Sin!le-%it error

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Sin!le %it errors are the leastliel1 type o errors in serial data

transmission because the noise musthave a very short duration which isvery rare. 2owever this (ind o errorscan happen in parallel transmission.

Example:$ data is sent at 1Kbps then each bit

lasts only 1/1":::"::: sec. or 1 Ls.

For a single#bit error to occur" thenoise must have a duration o only 1Ls" which is very rare.

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2urst error

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%he term %urst error means thattwo or more bits in the data unit

have changed rom 1 to : or rom: to 1.

2urst errors does not necessarily mean that the errors occur inconsecutie %its" the length o

the burst is measured rom the=rst corrupted bit to the lastcorrupted bit. 3ome bits inbetween may not have been

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2urst error is most liel1 tohappen in serial transmission since

the duration o noise is normally longerthan the duration o a bit.

%he number o bits a,ected depends

on the data rate and duration o noise.Example:$ data is sent at rate @ 18bps then a noise

o 1/1:: sec can a,ect 1: bits.01/1::M1:::

$ same data is sent at rate @ 1Kbps then anoise o 1/1:: sec can a,ect 1:"::: bits.01/1::M1:6

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Error detection

Error detection means to decidewhether the received data is corrector not without having a copy o the

original message.

Error detection uses the concept of

redundanc1" which means addinge4tra bits or detecting errors at thedestination.

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Redundanc1

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Error detectiontechniques:

opular techni)ues are1.3imple arity chec(

.%wo#dimensional aritychec(+.*hec(sum

.*yclic redundancy chec(

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ur t1pes of redundanc1 checs are use

in data communications

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*3, arit1 Checin!:

– 3imple or %wo &imensional

– $n parity chec(" a parity bit is added toevery data unit so that the total number

o 1s is even or odd.

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*3, arit1 Checin!:

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*3, arit1 Checin!:

• 3uppose the sender wants to send theword world . $n !3*$$ the =ve charactersare coded as

• 3335333 3353333 33355353353355 3355355

• %he ollowing shows the actual bits sent

• 111:1115 11:11115 111::1:5 11:11::5 11::1::3

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i%l 4 %it d t d d


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ossi%le 4-%it data words andcorrespondin! code words

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*3, arit1 Checin!:• %wo#dimensional parity

• $n two#dimensional parity chec(" a bloc( o bits isdivided into rows and a redundant row o bits isadded to the whole bloc(.

– erormance can be improved by using two#dimensional parity chec(" which organi;es thebloc( o bits in the orm o a table.

– arity chec( bits are calculated or each row"which is e)uivalent to a simple parity chec( bit.

– arity chec( bits are also calculated or all

columns then both are sent along with the data.

– !t the receiving end these are compared withthe parity bits calculated on the received data

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0wo-dimension arit1 Checin!

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6ertical Redundanc1 Chec


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6ertical Redundanc1 Chec 6RC

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erformance

$t can detect single bit error

$t can detect burst errors only i thetotal number o errors is odd.

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'on!itudinal Redundanc1 Chec 'RC

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erformance

'* increases the li(elihood odetecting burst errors.

$ two bits in one data units aredamaged and two bits in e4actlythe same positions in another data

unit are also damaged" the '*chec(er will not detect an error.

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6RC and 'RC

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*, Checsum – 0he sender follows these steps:

• %he unit is divided into ( sections" each o n bits.

• !ll sections are added using oneGs complement to getthe sum.

• %he sum is complemented and becomes the

chec(sum.• %he chec(sum is sent with the data.

– 0he receier follows these steps:• %he unit is divided into ( sections" each o n bits.

• !ll sections are added using oneGs complement to getthe sum.

• %he sum is complemented.

• $ the result is ;ero" the data are accepted otherwise"reNected.

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Eample art 3

• 3uppose the ollowing bloc( o 16 bits is to be sentusing a chec(sum o 7 bits.

• 1:1:1::1 ::111::1

• %he numbers are added using oneGs complement

• 1:1:1::1

• ::111::1 ############3um 111:::1:

• *hec(sum 55533353

• %he pattern sent is 1:1:1::1 ::111::155533353

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Eample part • Cow suppose the receiver receives the pattern sent in

E4ample and there is no error.

• 1:1:1::1 ::111::1 :::111:1

• hen the receiver adds the three sections" it will get all 1s"which" ater complementing" is all :s and shows that there

is no error.

• 1:1:1::1

• ::111::1

• :::111:1

• 3um11111111

• *omplement 55555555 means that the pattern is D8.

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C1clic Redundanc1 Checs *CRC,

• %his is the most powerul and easy to implement

techni)ue.• ?nli(e chec(sum scheme" which is based on addition"

** is based on binary division.

• $n **" a se)uence o redundant bits" called cyclicredundancy chec( bits" are appended to the end o dataunit so that the resulting data unit becomes e4actlydivisible by a second" predetermined binary number.

• !t the destination" the incoming data unit is divided bythe same number.

• $ at this step there is no remainder" the data unit isassumed to be correct and is thereore accepted.

• ! remainder indicates that the data unit has beendamaged in transit and thereore must be reNected

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C1clic Redundanc1


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C1clic Redundanc1Chec • Oiven a (#bit rame or message"

the transmitter generates an n#bitse)uence" (nown as a frame checksequence (FCS) , so that the resultingrame" consisting o 0(An bits" ise4actly divisible by somepredetermined number.

•

%he receiver then divides theincoming rame by the samenumber and" i there is noremainder" assumes that there was

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C1clic Redundanc1 Chec*CRC,

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2asic scheme for C1clic Redundanc1 Checin!

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• $n mathematical operation dividing a sample #bitnumber by 1:11" using the modulo# arithmetic.

• Kodulo# arithmetic is a binary addition process

without any carry over" which is Nust theE4clusive#D operation.

• *onsider the case where (@11:1. 2ence we haveto divide 11:1::: 0i.e. ( appended by + ;eros by

1:11" which produces the remainder r@::1" sothat the bit rame 0(Ar @11:1::1 is actuallybeing transmitted through the communicationchannel.

•

!t the receiving end" i the received number" i.e."11:1::1 is divided by the same generatorpolynomial 1:11 to get the remainder as :::" itcan be assumed that the data is ree o errors.

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2inar1 /iision

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Division in CRC encoder

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Division in the CRC decoder for two cases

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Error Control

• Error control includes both error detectionand error correction.

• $t allows the receiver to inorm the sender i arame is lost or damaged during transmission

and coordinates the retransmission o thoserames by the sender.

• Error control in the data lin( layer is based onautomatic repeat re)uest 0!P. henever

an error is detected" speci=ed rames areretransmitted.

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0wo main methods of errorcorrection

•

Forward error correction is the process in whichthe receiver tries to guess the message byusing redundant bits.

• %his is possible" i the number o errors is small.

•*orrection by retransmission is a techni)ue inwhich the receiver detects the occurrence o anerror and as(s the sender to resend themessage.

•

esending is repeated until a message arrivesthat the receiver believes is error#ree 0usually"not all errors can be detected.

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Error Correction can %e handled intwo wa1s.

• Dne is when an error is discoveredQthe receiver can have the senderretransmit the entire data unit. %his is

(nown as %acward errorcorrection.

• $n the other" receiver can use an error#correcting code" which automaticallycorrects certain errors. %his is (nownas forward error correction.

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Error Control 0echniques

• hen an error is detected in a message" thereceiver sends a re)uest to the transmitterto retransmit the ill#ated message orpac(et.

• %he most popular retransmission scheme is(nown as !utomatic#epeat#e)uest 0!P.

• 3uch schemes" where receiver as(stransmitter to re#transmit i it detects anerror" are (nown as reverse error correctiontechni)ues.

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Error control techniques

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Error Control #echanisms

• 3top#and#ait

• Oo#Bac(#C !P

•

3elective#epeat !P

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- -"ait

3ender (eeps a copy o the last


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"aitp py

rame until it receives anac(nowledgement.

For identi=cation" both data ramesand ac(nowledgements 0!*8 ramesare numbered alternatively : and 1.

3ender has a control variable 03 that

holds the number o the recently sentrame. 0: or 1

eceiver has a control variable R

that holds the number o the ne4trame e4pected 0: or 1.

3ender starts a timer when it sends a

rame. $ an !*8 is not receivedwithin a allocated time period" the

sender assumes that the rame waslost or damaged and resends it

eceiver send only positive !*8 ithe rame is intact.

!*8 number always de=nes thenumber o the ne4t e4pected rame

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3top#and#ait !P" lost


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3top and ait !P" lostrame

• hen a receiverreceives adamaged rame" itdiscards it and

(eeps its value o.

• !ter the timer atthe sender

e4pires" anothercopy o rame 1 issent.

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78C9 frame


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8C9 frame

• $ the sender

receives a damaged!*8" it discards it.

• hen the timer othe sender e4pires"the sender

retransmits rame 1.• eceiver has

already receivedrame 1 ande4pecting to receive

rame : 0@:. %hereore it discardsthe second copy orame 1.

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78C9 frame


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8C9 frame• %he !*8 can be

delayed at the

receiver or due tosome problem

• $t is received ater thetimer or rame : hase4pired.

• 3ender retransmitted

a copy o rame :.2owever" @1 meansreceiver e4pects tosee rame 1. eceiverdiscards the duplicaterame :.

• 3ender receives !*8s" it discards thesecond !*8.

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i!!1%acin!


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i!!1%acin!• ! method to

combine a data

rame with !*8.• 3tation ! and B

both have data tosend.

• $nstead o sending

separately" station! sends a datarame that includesan !*8.

• 3tation B does the

same thing.• iggybac(ing saves

bandwidth.

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&isadvantage o 3top#and#


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&isadvantage o 3top#and#ait• $n stop#and#wait" at any point in time" there

is only one rame that is sent and waiting tobe ac(nowledged.

•

%his is not a good use o transmissionmedium.

• %o improve efciency" multiple ramesshould be in transition while waiting or !*8.

• %wo protocol use the above concept" – o-2ac-N 8R;

– Selectie Repeat 8R;

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Oo#Bac(#C !P

• e can send up to rames beoreworrying about !*8s.

• e (eep a copy o these rames until

the !*8s arrive.• %his procedure re)uires additional

eatures to be added to 3top#and#

ait !P.

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3e)uence Cumbers

• Frames rom a sender are numbered se)uentially.

• e need to set a limit since we need to include these)uence number o each rame in the header.

• $ the header o the rame allows m bits or

se)uence number" the se)uence numbers rangerom : to m 1. or m @ +" se)uence numbersare :"1" " +" " 5" 6" .

• e can repeat the se)uence number.

• 3e)uence numbers are:" 1" " +" " 5" 6" " :" 1" " +" " 5" 6" " :" 1" S

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"indow


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"indow• !t the sending site"

to hold the

outstanding ramesuntil they areac(nowledged" weuse the concept o a

window.• %he si;e o the

window is at mostm #1 where m is the

number o bits orthe se)uencenumber.

• 3i;e o the windowcan be variable" e.g.

%*.5/19/16

"indow


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"indow• 3i;e o the window

at the receivingsite is always 1 inthis protocol.

• eceiver is alwaysloo(ing or a

speci=c rame toarrive in a speci=corder.

• !ny rame arriving

out o order isdiscarded andneeds to be resent.

• eceiver window

slides as shown in5/19/16 5

Control 6aria%les


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• 3ender has + variables 3" 3F" and 3'

• 3 holds the se)uence number o recently sent rame

• 3F holds the se)uence number o the =rst rame

• 3' holds the se)uence number o the last rame

• eceiver only has the one variable" " that holds these)uence number o the rame it e4pects to receive.$ the se). no. is the same as the value o " therame is accepted" otherwise reNected.

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!c(nowledgement


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!c(nowledgement• eceiver sends positive !*8 i a rame arrived sae and in

order.

• $ the rames are damaged/out o order" receiver is silent anddiscard all subse)uent rames until it receives the one it ise4pecting.

• %he silence o the receiver causes the timer o the

unac(nowledged rame to e4pire.• %hen the sender resends all rames" beginning with the one

with the e4pired timer.

• For e4ample" suppose the sender has sent rame 6" but the

timer or rame + e4pires 0i.e. rame + has not been

ac(nowledged" then the sender goes bac( and sendsrames +" " 5" 6 again. %hus it is called Oo#Bac(#C#!P

• %he receiver does not have to ac(nowledge each ramereceived" it can send one cumulative !*8 or several rames.

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o-2ac-N 8R;7 normal operation


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• %he sender (eeps trac( o the outstandingrames and updates the variables and

windows as the !*8s arrive.

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o-2ac-N 8R;7dama!ed or lost frame

• Frame islost

•

hen thereceiverreceivesrame +" itdiscards

rame + as itis e4pectingrame 0according to

window.• !ter the

timer orrame

e4pires at the5/19/16 9

Frames in error in !o-


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!2ac-N 8R;

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'ost Frames in o-2ac-


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N 8R;

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o-2ac-N 8R; dama!ed


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o-2ac-N 8R;7 dama!ed


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8R;

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o-2ac-N 8R;7 sender window si)e


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• 3i;e o the sender window must be less than m.3i;e o the receiver is always 1. $ m @ " window

si;e @ m

1 @ +.• Fig compares a window si;e o + and .

!cceptsas the1st rame inthe ne4t

cycle#an5/19/16 7 receier windows


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• Oo#Bac(#C !P simpli=es the process at the receiver site.eceiver only (eeps trac( o only one variable" and there is noneed to bu,er out#o#order rames" they are simply discarded.

• 2owever" Oo#Bac(#C !P protocol is inefcient or noisy lin(. $tbandwidth inefcient and slows down the transmission.

• $n 3elective epeat !P" only the damaged rame is resent.Kore bandwidth efcient but more comple4 processing atreceiver.

• $t de=nes a negative !*8 0C!8 to report the se)uencenumber o a damaged rame beore the timer e4pires.

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Selectie Repeat 8R;7 lost frame• Frames :and 1 are


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and 1 areacceptedwhen

receivedbecausethey are inthe rangespeci=ed bythe receiverwindow.3ame orrame +.

• eceiversends a

C!8 toshow thatrame hasnot beenreceived and

then sender5/19/16 76

Selectie Repeat 8R;7 sender window si)e

3i th d d i i d t b t t


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• 3i;e o the sender and receiver windows must be at most one#hal o m. $ m @ " window si;e should be m / @ . Figcompares a window si;e o with a window si;e o +. indow

si;e is + and all !*8s are lost" sender sends duplicate o rame:" window o the receiver e4pect to receive rame : 0part o thewindow" so accepts rame :" as the 1st rame o the ne4t cycle an error.

ch4flow and error control

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