Point-to-PointLinks Outline Encoding Framing ErrorDetection - - PDF document

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Spring2002 CS461 1

Outline

Encoding Framing ErrorDetection SlidingWindowAlgorithm

Point-to-PointLinks

Spring2002 CS461 2

Encoding

• Signalspropagateoveraphysicalmedium

– modulateelectromagneticwaves – e.g.,varyvoltage

• Encodebinarydataontosignals

– e.g.,0aslowsignaland1ashighsignal – knownasNon-Returntozero(NRZ)

Bits NRZ 1 1 1 1 1 1 1

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Problem:Consecutive1sor0s

• Lowsignal(0)maybeinterpretedasnosignal
• Highsignal(1)leadstobaselinewander
• Unabletorecoverclock

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AlternativeEncodings

• Non-returntoZeroInverted(NRZI)

– makeatransitionfromcurrentsignaltoencodeaone; stayatcurrentsignaltoencodeazero – solvestheproblemofconsecutiveones

• Manchester

– transmitXORoftheNRZencodeddataandtheclock – only50%efficient(bitrate=1/2baudrate)

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Encodings (cont)

• 4B/5B

– every4bitsofdataencodedina5-bitcode – 5-bitcodesselectedtohavenomorethanoneleading0 andnomorethantwotrailing0s – thus,nevergetmorethanthreeconsecutive0s – resulting5-bitcodesaretransmittedusingNRZI – achieves80%efficiency

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Encodings(cont)

Bits NRZ Clock Manchester NRZI 1 1 1 1 1 1 1

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Framing

• Breaksequenceofbitsintoaframe
• Typicallyimplementedbynetworkadaptor

Frames Bits Adaptor Adaptor NodeB NodeA

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Approaches

• Sentinel-based

– delineateframewithspecialpattern:01111110 – e.g.,HDLC,SDLC,PPP – problem:specialpatternappearsinthepayload – solution:bitstuffing

• sender:insert0afterfiveconsecutive1s
• receiver:delete0thatfollowsfiveconsecutive1s

Header Body 8 16 16 8 CRC Beginning sequence Ending sequence Spring2002 CS461 9

Approaches(cont)

• Couter-based

– includepayloadlengthinheader – e.g.,DDCMP – problem:countfieldcorrupted – solution:catchwhenCRCfails

SYN Header Body 8 8 42 14 16 8 SYN Class CRC Count

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Approaches(cont)

• Clock-based

– eachframeis125uslong – e.g.,SONET:SynchronousOpticalNetwork – STS-n (STS-1=51.84Mbps)

Overhead Payload 90columns 9rows

STS-1 Hdr STS-1 Hdr STS-1 Hdr STS-3c Hdr Spring2002 CS461 11

CyclicRedundancyCheck

• Addkbitsofredundantdatatoann-bitmessage

– wantk <<n – e.g.,k =32andn =12,000(1500bytes)

• Representn-bitmessageasn-1degreepolynomial

– e.g.,MSG=10011010asM(x)=x7 +x4+x3 +x1

• Letk bethedegreeofsomedivisorpolynomial

– e.g.,C(x)=x3 +x2 +1

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CRC(cont)

• TransmitpolynomialP(x)thatisevenlydivisible

byC(x)

– shiftleftk bits,i.e.,M(x)xk – subtractremainderofM(x)xk /C(x)fromM(x)xk

• ReceiverpolynomialP(x)+E(x)

– E(x)=0impliesnoerrors

• Divide(P(x)+E(x))byC(x);remainderzeroif:

– E(x)waszero(noerror),or – E(x)isexactlydivisiblebyC(x)

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SelectingC(x)

• Allsingle-biterrors,aslongasthexk andx0 termshave

non-zerocoefficients.

• Alldouble-biterrors,aslongasC(x)containsafactorwith

atleastthreeterms

• Anyoddnumberoferrors,aslongasC(x)containsthe

factor(x +1)

• Any‘burst’error(i.e.,sequenceofconsecutiveerrorbits)

forwhichthelengthoftheburstislessthank bits.

• Mostbursterrorsoflargerthank bitscanalsobedetected
• SeeTable2.6onpage102forcommonC(x)

Spring2002 CS461 14

InternetChecksumAlgorithm

• Viewmessageasasequenceof16-bitintegers;sumusing

16-bitones-complementarithmetic;takeones-complement

• ftheresult.

u_short cksum(u_short*buf, int count) { registeru_longsum=0; while(count--) { sum+=*buf++; if(sum&0xFFFF0000) { /*carryoccurred,sowraparound*/ sum&=0xFFFF; sum++; } } return~(sum&0xFFFF); }

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Acknowledgements&Timeouts

Sender Receiver Frame ACK Timeout Time Sender Receiver Frame ACK Timeout Frame ACK Timeout Sender Receiver Frame ACK Timeout Frame ACK Timeout Sender Receiver Frame Timeout Frame ACK Timeout (a) (c) (b) (d)

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Stop-and-Wait

• Problem:keepingthepipefull
• Example

– 1.5Mbpslinkx 45msRTT=67.5Kb(8KB) – 1KBframesimplies1/8thlinkutilization

Sender Receiver

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SlidingWindow

• Allowmultipleoutstanding(un-ACKed)frames
• Upperboundonun-ACKedframes,calledwindow

Sender Receiver Time

… …

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SW:Sender

• Assignsequencenumbertoeachframe(SeqNum)
• Maintainthreestatevariables:

– sendwindowsize(SWS) – lastacknowledgmentreceived(LAR) – lastframesent(LFS)

• Maintaininvariant:LFS - LAR <=SWS
• AdvanceLAR whenACKarrives
• BufferuptoSWS frames

≤SWS

LAR LFS

… …

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SW:Receiver

• Maintainthreestatevariables

– receivewindowsize(RWS) – largestframeacceptable(LFA) – lastframereceived(NFE)

• Maintaininvariant:LFA - LFR <=RWS
• FrameSeqNum arrives:

– ifLFR <SeqNum <=LFA accept – ifSeqNum <=LFR orSeqNum >LFA discarded

• SendcumulativeACKs

≤ RWS

NFE LFA

… …

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SequenceNumberSpace

• SeqNum fieldisfinite;sequencenumberswraparound
• Sequencenumberspacemustbelargerthennumberof
• utstandingframes
• SWS <=MaxSeqNum-1 isnotsufficient

– suppose3-bitSeqNum field(0..7) – SWS=RWS=7 – sendertransmitframes0..6 – arrivesuccessfully,butACKslost – senderretransmits0..6 – receiverexpecting7,0..5,butreceivessecondincarnationof0..5

• SWS <(MaxSeqNum+1)/2 iscorrectrule
• Intuitively,SeqNum “slides”betweentwohalvesof

sequencenumberspace

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ConcurrentLogicalChannels

• Multiplex8logicalchannelsoverasinglelink
• Runstop-and-waitoneachlogicalchannel
• Maintainthreestatebitsperchannel

– channelbusy – currentsequencenumberout – nextsequencenumberin

• Header:3-bitchannelnum,1-bitsequencenum

– 4-bitstotal – sameasslidingwindowprotocol

• Separatesreliabilityfrom order