北邮课件计算机网络ch3-datalink

Chapter3TheDataLinkLayerDatalinkLayerDataLinkLayerDesignIssuesServicesProvidedtotheNetworkLayerFramingErrorControl&FlowControlErrorDetectionandCorrectionErrorCorrectingCode(ECC)ErrorDetectingCodeElementaryDataLinkProtocolsSlidingWindowProtocolsOne-BitSlidingWindowProtocolProtocolUsingGoBackNUsingSelectiveRepeatPerformanceofSlidingWindowProtocolsExampleDataLinkProtocolsHDLCSLIPPPP3.1DataLinkLayerDesignIssuesDataLinkLayerDesignIssuesGoalsStudythealgorithmsforachievingreliable,efficientcommunicationbetweentwoadjacentmachinesAdjacent:twomachinesareconnectedbyacommunicationchannelthatactslikeawire,bitsaredeliveredinexactlythesameorderinwhichtheyaresentDataLinkLayerDesignIssuesServicesProvidedtotheNetworkLayerFramingErrorControlFlowControlServicesProvidedtotheNetworkLayerFunctionsoftheDataLinkLayerProvideserviceinterfacetothenetworklayerDealingwithtransmissionerrorsRegulatingdataflowSlowreceiversnotswampedbyfastsendersFunctionsoftheDataLinkLayer(2)Relationshipbetweenpacketsandframes.ServicesProvidedtoNetworkLayer(a)Virtualcommunication.(b)Actualcommunication.ServicesProvidedtoNetworkLayerUnacknowledgedconnectionlessservice.

MostLANsuseunacknowledgedconnectionlessservice

Acknowledgedconnectionlessservice.

Thisserviceisusefuloverunreliablechannels,suchaswirelesssystemsAcknowledgedconnection-orientedserviceTimerSequencenumberConnectionestablishmentPlacementofthedatalinkprotocolFramingFramingAsequenceofbitsissentfromnodeAtonodeBoverapoint-to-pointlink,NodeBmustrecognizeexactlywhatsetofbitsconstitutesaframeCharacterCountAcharacterstream.(a)Withouterrors.(b)Withoneerror.Charactercount(字符记数):Usesafieldintheheadertospecifythenumberofcharactersintheframe(DEC:DDCMP)ByteStuffing

(a)Aframedelimitedbyflagbytes.(b)Fourexamplesofbytesequencesbeforeandafterstuffing.Flagbyteswithbytestuffing(字符添充):Havingeachframestartandendwithspecialbytes(BISYNC,PPP)BitStuffing

(a)Theoriginaldata.(b)Thedataastheyappearontheline.(c)Thedataastheyarestoredinreceiver’smemoryafterdestuffing.Startingandendingflags,withbitstuffing(比特添充):Eachframebeginsandendswithaspecialbitpattern,01111110.Wheneverthesender'sdatalinklayerencountersfiveconsecutive1sinthedata,itautomaticallystuffsa0bitintotheoutgoingbitstream(Example:HDLC)ExamplewithPossibleErrors(BitStuffing)

PhysicalLayerCodingViolationsPhysicallayercodingviolations:物理层编码违例法(IEEE:802.3)

例如：编码方案：时钟周期内电压高变低表示1，低变高表示0，时钟周期内电压恒高或恒低，不表示数据，可用做定界符Manchester差分ManchesterHomework1,2,3,4,5ErrorControl&FlowControlErrorControlMakesureallframesaredeliveredtothedestinationandintheproperorderProvidethesenderwithsomefeedbackaboutwhatishappeningattheotherendofthelineHardwaretroublesmaycauseaframetovanishcompletelyframeorACKislost,thetimerwillgooff,...Managingtimersandsequencenumberssoastoensurethateachframeispassedtothedestinationexactlyonce,nomoreandnolessFlowControlTwoapproachesarecommonlyused

Feedback-basedflowcontrol,thereceiversendsbackinformationtothesender,givingitpermissionortellingthesenderhowthereceiverisdoingRate-basedflowcontrol,limitstherateatwhichsendersmaytransmitdata3.2ErrorDetectionandCorrectionErrorCorrectingCode(ECC)Single-bitError&BurstErrorSingle-biterror,onlyonebithaschangedAbursterrormeansthat2ormorebitshavechanged.HammingDistanceHammingdistance:Thenumberofbitpositionsinwhichtwocodewordsdiffer(10001001and10110001,Hammingdistance=3)Todetectterrors,needadistancet+1codeTocorrectterrors,needadistance2t+1codeECC:error-correctingcodesFEC:forwarderrorcorrectionExampleofanError-CorrectingCodeConsideracodewithonlyfourvalidcodewords:,1111100000,1111111111Thiscodehasadistance5,whichmeansthatitcancorrect2errors.Ifthecodewordarrives,thereceiverknowsthattheoriginalmusthavebeenIfatripleerrorchangesinto,theerrorwillnotbecorrectedproperlyTheNumberofCheckBitsFindoutthenumberofcheckbitsneededtocorrectsingleerrorCodingm:messagebitsr:checkbitsn:codewordlength(n=m+r)2mlegalmessagesEachlegalmessagehasnillegalcodewordsatadistance1fromit,need(n+1)2mcodewords(n+1)2m

≤2n

(m+r+1)≤2r Example:m=7,r≥4Thebitsofthecodewordarenumbered,startwith1atleftend,bit2toitsimmediateright,andsoon.CheckbitsThebitsthatarepowersof2(1,2,4,8,16...)arecheckbitsEachcheckbitforcestheparityofsomecollectionofbits,includingitself,tobeeven(orodd).DatabitsTherest(3,5,6,...)arefilledupwiththemdatabitsHammingCodeMessage:‘H’(7-bitASCII100,1000)DataBits:3,5,6,7,9,10,114Checkbits,Bit1,2,4,8(evenparity)BitNumber:1

234567891011Codeword:00110010000发送前编码：

3=1+2，5=1+4，6=2+4，7=1+2+49=1+8，10=2+8，11=1+2+8接收者纠错：

b1⊕b3⊕b5⊕b6⊕b7⊕b9⊕b10⊕b11=0(k=1)

b2⊕b3⊕b5⊕b6⊕b7⊕b9⊕b10⊕b11=0(k=2)

b4⊕b3⊕b5⊕b6⊕b7⊕b9⊕b10⊕b11=0(k=4)

b8⊕b3⊕b5⊕b6⊕b7⊕b9⊕b10⊕b11=0(k=8)ExampleofHammingCodeHammingcode:CorrectbursterrorsUseofaHammingcodetocorrectbursterrors.其他前向纠错码（FEC）广泛用于无线链路，提高信道信噪比BCH码Bose，Ray-Chaudhuri，Hocquenhem里德-所罗门码(里所码)Reed-solomoncodes,RScodes戈帕码Goppacodes卷积码与块编码不同，卷积码可以连续地生成冗余位擦除码(ErasureCorrection)原始数据分成k块，编码后的编码数据n片从n片中的任意k片都可以恢复原先的k块数据k<n≤2k,常见n=2k例如：k=3,n=6a00x0+a01x1+a02x2=y0a10x0+a11x1+a12x2=y1a20x0+a21x1+a22x2=y2a30x0+a31x1+a32x2=y3a40x0+a41x1+a42x2=y4a50x0+a51x1+a52x2=y5(x0,x1,x2)为原始数据，(y0,y1,y2,y3,y4,y5)为编码后数据只需得到y0-y5的任三个(包括它们的行号)就可还原x0-x2精心设计系数矩阵A(非齐次线性方程问题，矩阵的秩)Homework9,11ErrorDetectingCodeErrorCorrecting&DetectingOnchannelsthatarereliable,suchasfiber,itischeapertouseanerrordetectingcodeandjustretransmittheoccasionalblockfoundtobefaultyOnchannelssuchaswirelesslinksthatmakemanyerrors,itisbettertoaddenoughredundancytoeachblockforthereceivertobeabletofigureoutwhattheoriginalblockwasExample(single-biterror)BitErrorRateBER=10-6,1Block=1000bits,Data=1MbitsTheoverheadErrordetection+retransmission:2001bitsHammingcode:10,000bitsErrorDetectingCodes

Odd&EvenParityOdd&Evenparity单个奇偶校验位性能可检出奇数个位翻转错误不能检出偶数个位翻转错误OddParityEvenParityEvenParityCheckOdd&EvenParityCoding(rcheckbits)Eachblockisregardedasak×rmatrixAparitybiteverycolumn,affixedtothematrixasthelastrowThematrixisthentransmittedonerowatatimePerformanceCandetectasingleburstoflength≤rCatchallerrorsconsistingofanoddnumberofinvertedbitsAburstoflength>rIfblockisgarbledbyalongburst，theprobabilityofbeingacceptedis2-r

1001000101010011101000010101011011001100111011011000100010011110PolynomialCodeAlsoknownasCRC(CyclicRedundancyCheck)FCS(FrameCheckSequence)M(x):Treatingbitstringsasrepresentationsofpolynomialswithcoefficientsof0and1Ak-bitframeisregardedasthecoefficientlistforapolynomialwithkterms,rangingfromxk-1tox0:110001=>x5+x4+x0Polynomialarithmeticisdonemodulo2,additionandsubtractionareidenticaltoXORG(x):GeneratorpolynomialSender:ComputingtheChecksumThealgorithmforcomputingthechecksum:Appendrzerobitstothelow-orderendoftheframeDividethebitstringcorrespondingtoG(x)T(x):SubtracttheremainderfromthebitstringExampleofCRCCalculationCalculationofthepolynomialcodechecksum.Receiver:VerifytheChecksumIfatransmissionerroroccurs,sothatinsteadofthebitstringforT(x)arriving,T(x)+E(x)arrives.Thereceivercomputes[T(x)+E(x)]/G(x).T(x)/G(x)is0,sotheresultisE(x)/G(x)ThoseerrorsthathappentocorrespondtopolynomialscontainingG(x)asafactorwillslipby;allothererrorswillbecaught.ThePowerofCRCrcheckbitswilldetectallbursterrorsoflength≤rTwoisolatedsingle-biterrorsaredetectedE(x)=xi+xj,=xj(xi-j+1).wherei>jChooseG(x):G(x)doesnotdividexk+1foranykuptomaximumframelengthForexample,x15+x14+1willnotdividexk+1foranyvalueofkbelow32,768(4096bytes)Catchallerrorsconsistingofanoddnumberofinvertedbits:bymakingx+1afactorofG(x)IfE(x)hasanoddnumberoftermsandisdivisiblebyx+1.E(x)=(x+1)Q(x),E(1)=(1+1)Q(1)=0.E(x)hasanoddnumberofterms,E(1)=1Otherbursterrors(length>r)Theprobabilityofabadframebeenacceptedis2-rGeneratorPolynomialCertainpolynomialshaveeinternationalstandardsCRC-16x16+x15+x2+1CRC-CCITT(HDLC)x16+x12+x5+1CRC-32(IEEE802)x32+x26+x23+x22+x16+x12+x11+x10+x8+x7+x5+x4+x2+x1+1CRC-64TheCalculationofCRC5(Hardware)TheCalculationofCRC5(Hardware)HardwareAsimpleshiftregistercircuitcanbeconstructedtocomputeandverifythechecksums生成多项式G(x)=x5+x4+x2+1(110101)初始时移位寄存器C0~C4清0信息位10比特发送之后，断开A接通B,发送5比特校验和，并顺便清0寄存器C0~C4为下次发送做好准备C4C3C2C1C0I(Input)Initial000001111Step1101011110Step2111111101Step3111100010Step4010011000Step5100101010Step6100010001Step7000101011Step8100011110Step9101110101Step1001110TheCalculationofCRC5(Hardware)手工计算CRC4：速算表生成多项式x4+x+1手工计算CRC4：利用速算表TheCalculationofCRC(Software)

staticunsignedshortcrctab[256]={0x0000,0x1189,0x2312,0x329b,0x4624,0x57ad,0x6536,0x74bf,0x8c48,0x9dc1,0xaf5a,0xbed3,0xca6c,0xdbe5,……0xd49d,0xc514,0xb1ab,0xa022,0x92b9,0x8330,0x7bc7,0x6a4e,0x58d5,0x495c,0x3de3,0x2c6a,0x1ef1,0x0f78};unsignedshortcrc16(unsigned

shortcrc,unsignedchar*p,intlen){

while(len--)crc=(crc>>8)^crctab[(crc^*p++)&0xff];

returncrc;}其他哈希函数及应用哈希函数CRC算法为一个数据段计算出一个长度固定的哈希值。哈希算法的哈希值位数越多，校验能力越强，64位以上的哈希函数常用于安全领域，实现电子签名的功能，防止数据被恶意修改在计算性能上的特点：便于32位计算机高效率的软件实现MessageDigest5(MD5)：128bitsSecureHashAlgorithm1(SHA-1)：160bits哈希函数的其他应用(用来比较两段数据是否相同)哈希值不同，数据必不同；哈希值相同，数据不同的概率很低（弱哈希函数和强哈希函数）用CRC32比较两台计算机上的数据是否相同，例如：Cisco的OSPF链路状态数据库Bittorent,eMulemd5sum增量拷贝rsync（同时利用弱哈希函数和强哈希函数）Homework14,15,163.3ElementaryDataLinkProtocolsElementaryDataLinkProtocolsAnUnrestrictedSimplexProtocolASimplexStop-and-WaitProtocolASimplexProtocolforaNoisyChannelProtocolDefinitionsElementaryDataLinkProtocolsSomeoftheassumptionsPhysicallayer,datalinklayer,andnetworklayerareindependentprocessesMachineAwantstosendalongstreamofdatatomachineB,usingareliable,connection-orientedserviceMachinesdonotcrashLibraryprocedureswait_for_eventsfrom_network_layer/to_network_layerto_physical_layer/from_physical_layertimeroperationsenable_network_layer/disable_network_layersequencenumberoperationProtocolDefinitionsSomedefinitionsneededintheprotocolstofollow.Thesearelocatedinthefileprotocol.h.Protocol1:UtopiaUnrestricted

Simplex

ProtocolProtocol2:Stop-and-WaitProtocolforaError-freeChannelSimplexStop-and-WaitProtocolProtocol3:ASimplexProtocolforaNoisyChannelProblemsSendDATAandthenwaitforACK……Case1:ProtocolfailedonreceiverThenetworklayeronAgivespacket1toitsdatalinklayer.ThepacketiscorrectlyreceivedatBandpassedtothenetworklayer.BsendsanACKtoATheACKgetslostcompletelyThedatalinklayeronAeventuallytimesout.NothavingreceivedACK,resendstheframeTheduplicateframealsoarrivesB,...Case2:ProtocolfailedonSenderAdelayedACKSequenceNumberDistinguishanewframefromaretransmissionPutasequencenumberintheheaderofeachframeWhatistheminimumnumberofbitsneededforthesequencenumber?（停等协议）链路层：1比特，编号空间大小为2传输层：1比特编号空间太小，需要较大的编号空间。如：TFTP（16比特编号）seq_nr=0从网络层获取一个分组放入buffer发送buffer中的数据(seq_nr)，新启动定时器label1:wait_for_event()switch(event){case收到了坏帧(校验和错):

重发缓冲在buffer里的数据(seq_nr)，重新启动定时器case定时器超时：重发缓冲在buffer里的数据(seq_nr)，新启动定时器case收到校验和正确的帧：

if（ack序号正确）{

关闭旧定时器

inc(seq_nr)从网络层获取下一个分组放入buffer

发送buffer中的数据(seq_nr)，新启动定时器

}else

重发缓冲在buffer里的数据(seq_nr)，重新启动定时器}gotolabel1Senderseq_nr=0从网络层获取一个分组放入bufferlabel1:发送buffer中的数据(seq_nr)，新启动定时器wait_for_event()switch(event){case收到了坏帧(校验和错)://重发缓冲在buffer里的数据(seq_nr)，重新启动定时器case定时器超时：

//重发缓冲在buffer里的数据(seq_nr)，新启动定时器case收到校验和正确的帧：

if（ack序号正确）{

关闭旧定时器

inc(seq_nr)从网络层获取下一个分组放入buffer//发送buffer中的数据(seq_nr)，新启动定时器

}else//重发缓冲在buffer里的数据(seq_nr)，重新启动定时器}gotolabel1SenderSenderframe_expected=0while(true){wait_for_event()switch(event){case坏帧：

do_nothingcase收到校验和正确的数据帧:if(序号==frame_expected){向网络层上交分组

回送ACK(序号为frame_expected)inc(frame_expected)}else{回送ACK(序号为dec(frame_expected))}}}Receiverframe_expected=0while(true){wait_for_event()switch(event){case坏帧：

do_nothingcase收到校验和正确的数据帧:if(序号==frame_expected){向网络层上交分组inc(frame_expected)

}回送ACK(序号为dec(frame_expected))}}ReceiverReceiverApositiveacknowledgementwithretransmissionprotocol.(PAR)TimerTimeoutIfthetimeoutintervalissettooshort,thesenderwilltransmitunnecessaryframes.Whiletheseextraframeswillnotaffectthecorrectnessoftheprotocol,theywillhurtperformance3.4SlidingWindowProtocolsPiggybackingForfull-duplexdatatransmissionUsetwoseparatecommunicationchannelsanduseeachoneforsimplexdatatrafficUsethesamecircuitfordatainbothdirectionsPiggybackingWhenadataframearrives,insteadofimmediatelysendingaseparatecontrolframe,thereceiverrestrainsitselfandwaitsuntilthenetworklayerpassesitthenextpacketACKisattachedtotheoutgoingdataframe(usingtheackfieldintheframeheader)Sendingwindow&ReceivingwindowSendingwindowUsewindowtocontrolnumberofoutstandingframesAtanyinstantoftime,thesendermaintainsasetofsequencenumberscorrespondingtooutstandingframes.TheseframesaresaidtofallwithinthesendingwindowMaximumofSendingWindowSizeReceivingwindowReceiveralsomaintainsareceivingwindowcorrespondingtothesetofframesitispermittedtoacceptSlidingWindowProtocolsAOne-BitSlidingWindowProtocolAProtocolUsingGoBackNAProtocolUsingSelectiveRepeatOne-BitSlidingWindowProtocolSlidingWindowofSize1Aslidingwindowofsize1,witha3-bitsequencenumber.(a)Initially.(b)Afterthefirstframehasbeensent.(c)Afterthefirstframehasbeenreceived.(d)Afterthefirstacknowledgementhasbeenreceived.Protocol4:One-BitSlidingWindow发送窗口最大值1One-BitSlidingWindowProtocol(1)/*两端都执行相同的程序protocol4()*//*双向均有无穷尽数据发向对方*/One-BitSlidingWindowProtocol(2)TwoScenariosforProtocol4Twoscenariosforprotocol4.Thenotationis(seq,ack,packet)Anasteriskindicateswhereanetworklayeracceptsapacket(a)Normalcase.(b)Abnormalcase.ProblemConsidera50-kbpssatellitechannelwitha520-msecround-trippropagationdelay,send1000-bitframest=0:senderstartssendingt=20msec:theframehasbeencompletelysentt=270msec:framefullyarrivedatthereceivert=520msec:ACKarrivedbackatthesender,underthebestofcircumstances(nowaitinginthereceiverandashortacknowledgementframe).ConclusionSenderwasblockedduring500/520or96%ofthetime.only4%oftheavailablebandwidthwasusedLineUtilization

ProtocolUsingGoBackNPipelining&ErrorRecoveryPipelininganderrorrecovery.Effectonanerrorwhen(a)Receiver’swindowsizeis1.(b)Receiver’swindowsizeislarge.AboutACKBA012ACK0ACK2ACK1BA012ACK2GoBackNBasedonslidingwindowUsewindowtocontrolnumberofoutstandingframesIfReceiverdetectserror,discardthatframeandallfutureframesuntilerrorframereceivedcorrectly(silent)Transmittermust

gobackandretransmitthatframeandallsubsequentframesDamagedFrameIfDataFrameisDamagedReceiverdetectserrorinframeiReceiverdiscardthatframei

andallsubsequentWhentimeout,transmitterretransmitsframeiandallsubsequentIfACKFrameisDamagedReceivergetsframei

andsendacknowledgementACK(i)whichislostAcknowledgementsarecumulative,sonextACK(i+n)mayarrivebeforetransmittertimesoutonframeiIftransmittertimesout,retransmitsframeiandallsubsequentMaximumofSendingWindowSizeProblemThesendersendsframes0through7.ACKforframe7comesbacktothesender.Thesendersendsanothereightframes,againwithsequencenumbers0through7.Didalleightframesbelongingtothesecondbatcharrivesuccessfully,ordidalleightgetlost?

原则发送方发完窗口内所有帧，接收方正确接收到所有发送来的帧，接收窗口向前推移。此时(未回送ACK)，必须保证发送窗口与接收窗口在序号上不重叠(考虑所有ACK丢失的情况:发送窗口不滑动，接收窗口最大滑动)。Protocol5:Go-Back-NProtocol发送窗口最大值7函数between

判断序号b是否落在窗口(a,b)内判断：a到c不回绕c>a(b位于a~c之间)判断：a到c回绕a>c(b位于a~7,或者0~c-1)-Protocol5改进和效率设置ACK帧和ACK定时器发送数据量并非一个源源不断的分组流无法及时搭载ACK时，发送短的ACK帧(不含数据信息)用于回送ACK的定时器操作start_ack_timer和stop_ack_timer,数据重传所用定时器操作(与上述两个操作有区别)start_timer与stop_timerACK定时器时限的设计NAK帧:激发重传不需等到超时协议参数与线路利用率超时定时器时限的设计（考虑线路往返时间延迟，ACK定时器时限，对方物理层发送排队时延）滑动窗口即MAX_SEQ大小TimerQueueinSoftwareSimulationofmultipletimersinsoftware.UsingSelectiveRepeatSelectiveRepeatAlsocalledselectiveRejectOnlyrejectedframesareretransmittedSubsequentframesareacceptedbythereceiverandbufferedMinimizesretransmissionReceivermustmaintainlargeenoughbufferMorecomplexWindowSizeofSelectiveRepeat

(a)Initialsituationwithawindowsizeseven.(b)Aftersevenframessentandreceived,butnotacknowledged.(c)Initialsituationwithawindowsizeoffour.(d)Afterfourframessentandreceived,butnotacknowledgedSequenceNumber,BuffersThemaximumwindowsizeshouldbeatmosthalftherangeofthesequencenumbersHowmanybuffersmustthereceiverhave?Thenumberofbuffersneededisequaltothewindowsize,nottotherangeofsequencenumbersSendWindow&ReceiveWindows

Ws+Wr<=2n Ws>=Wr

ACKTimerACKtimerAfteranin-sequencedataframearrives,anauxiliarytimerisstartedbystart_ack_timer.Ifnoreversetraffichaspresentedbeforethistimerexpires,aseparateACKframeissentNAKWheneverthereceiverhasreasontosuspectthatanerrorhasoccurred,itsendsNAKframebacktothesenderNAKframeisarequestforretransmissionTherearetwocaseswhenthereceivershouldbesuspiciousAdamagedframehasarrivedAframeotherthantheexpectedonearrivedAvoidmakingmultiplerequestsforretransmissionofthesameframeReceiverkeepstrackofwhetheraNAKhasalreadybeensentforagivenframeProtocol6:SelectiveRepeat发送窗口最大4，接收窗口最大4ContinuedContinuedContinued-3.5PerformanceofSlidingWindowProtocolsPerformanceStop-and-waitwithouterrorPerformanceStop-and-waitwithouterrorPerformanceStop-and-waitwitherrorPerformanceSlidewindowwithouterrorHomework172026,27,28,29,30,31,323.6ExampleDataLinkProtocolsExampleDataLinkProtocolsHDLC–High-LevelDataLinkControlTheDataLinkLayerintheInternetSLIP(SerialLineIP)PPP(Point-to-PointProtocol)HDLC(High-LevelDataLinkControl)HistoryIBMSDLCSynchronousDataLinkControlANSIADCCPAdvancedDataCommunicationControlProcedureISOHDLCHigh-levelDataLinkControlCCITT(ITU-T)LAPBLAP(LinkAccessProcedure)LAPB(LinkAccessProcedure–Balancedmode)LAPD(IDSND-Channel)HDLCStationTypesPrimarystationControlsoperationoflinkFramesissuedarecalledcommandsMaintainsseparatelogicallinktoeachsecondarystationSecondarystationUndercontrolofprimarystationFramesissuedcalledresponsesCombinedstationMayissuecommandsandresponsesHDLCLinkConfigurationsUnbalancedOneprimaryandoneormoresecondarystationsSupportsfull-duplexandhalf-duplexBalancedTwocombinedstationsSupportsfull-duplexandhalf-duplexHDLCFrameformatFrameformatforbit-orientedprotocols.HDLCFramesControlfieldof(a)Aninformationframe.(I)(b)Asupervisoryframe.(RR/RNR/REJ/SREJ)(c)Anunnumberedframe.(SABM,UA,DISC,FRMR)FirstoneortwobitsofcontrolfiledidentifyframetypeI-frameIfthefirstbitofthecontrolfieldis0,thismeanstheframeisanI-frame.3-bitN(S)1-bitP/F3-bitN(R)S-frameControlFieldIfthefirst2bitsofthecontrolfieldare10,thismeanstheframeisanS-frame.CodeRR,RNR,REJ,SREJ(3-bitN(R))将ACK帧分两类(RR/RNR),将NAK帧分两类(REJ/SREJ),why?U-frameCodingU-frame建立连接SABM/SABME/SNRM/SNRME（应答UA)断开连接DISC（应答UA)FRMR帧（FrameReject)indicatethataframewithacorrectchecksumbutimpossiblesemanticsarrivedUI帧(UnnumberedInformation)无编号数据帧，可以含有较长的数据域可用UI帧在链路层两端提供“无连接-不可靠”数据传输P/FBitUsedependsoncontext

UnbalancedconfigurationPoll:InvitetheterminaltosenddataFinal:Alltheframessentbytheterminal,exceptthefinalone,havetheP/Fbitsetto0.Thefinaloneis1BalancedconfigurationCommand(Pbit):P=1tosolicit(poll)responsefrompeerResponse(Fbit):F=1indicatesresponsetosolicitingcommand

ForcetheothermachinetosendaSupervisoryframeimmediatelyratherthanwaitingforreversetrafficontowhichtopiggybackthewindowinformationACKFrame:RR&RNRRR:ReceiverReadyACKframeusedtoindicatetheN(R)frameexpectedThisframeisusedwhenthereisnoreversetraffictouseforpiggybackingRNR:ReceiverNotReadyItacknowledgesallframesuptobutnotincludingN(R),justasRRdoes,butittellsthesendertostopsending(Flowcontrol)NAKFrame:REJ&SREJREJ:RejectIndicatethatatransmissionerrorhasbeendetectedTheN(R)

fieldindicatesthefirstframeinsequencenotreceivedcorrectly(i.e.,theframetoberetransmitted)Thesenderisrequiredtoretransmitalloutstandingframesstartingat

N(R)

SREJ:SelectiveRejectCallsforretransmissionofonlytheframespecifiedbyN(R)Ifareceiverwishestobufferout-of-sequenceframesforpotentialfutureuse,itcanforcetheretransmissionofanyspecificframeusingSREJHDLCOperationExchangeofinformation,supervisoryandunnumberedframesInformation-datatobetransmittedtouserFlowanderrorcontrolpiggybackedoninformationframesSupervisory-ARQwhenpiggybacknotusedUnnumbered-supplementarylinkcontrolThreephrasesInitializationDatatransferDisconnectExampleofHDLCOperationExampleofHDLCOperationSLIP(SerialLineIP)SLIP–SerialLineInternetProtocolRFC1055Framing:Ch