計算機(jī)網(wǎng)絡(luò)英文課件：Chapter3 The Data Link Layer

Chapter3

TheDataLinkLayerCollegeofComputerScienceChongqingUniversityContents3.0DefinitionandConcept3.1DataLinkLayerDesignIssues3.2ErrorDetectionandCorrection3.3ElementaryDataLinkProtocols3.4SlidingWindowProtocolsandARQS3.5ExampleDataLinkProtocols3.6Summary3.0DefinitionandConceptThedatalinklayerprovidesthefunctionalandproceduralmeanstotransferdata

betweennetworkentitiesandmightprovidethemeanstodetectandpossiblycorrecterrorsthatmayoccurinthephysicallayer,especiallytotransfersdatabetweenadjacentnetworknodesinawideareanetworkorbetweennodesonthesamelocalareanetworksegment.Originally,thislayerwasintendedforpoint-to-pointandpoint-to-multipointmedia,characteristicofwideareamediainthetelephonesystem(1)DataLinkLayerDefinition:(NoISODefinition)Purpose:(2)Link:PhysicalLink3.0DefinitionandConceptNode:hosts,switches,bridges,routersLink:communicationchannelsthatconnectadjacentnodesalongcommunicationpatharelinkswiredlinkswirelesslinksLANs“l(fā)ink”data-linklayerhasresponsibilityof

transferringdatagramfromonenodetoadjacentnodeoveralink(3)DataLink:LogicalLink(4)Frame:Header+Payload(DataFromUpperLayer)HostAHostBNode1Node2Node3UpperLayerProtocolsDataLinkLayerProtocolDataLinkLayerProtocolDataVirtualChannelDataVirtualChannelDataActualChannelDataActualChannelDataLink/LogicalLink

=(Physical)Link

+DataLinkLayerProtocol3.0DefinitionandConcept3.0DefinitionandConceptPlacementofthedatalinkpaotocol3.1DataLinkLayerDesignIssuesNo.Issues/FunctionsMemo1ServiceProvidedtoTheNetworkLayers2Framing3ErrorControlHandletheErrorsProducedByPhysicalLayer4FlowControlRateMatchbetweensenderandReceiver5TransparentTransmissionHowtohandlethecontrollinginformationinpayloads6LinkmanagementConnection-OrientedCommunication

7AddressingBroadcastChannel,ShareChannel8OtherIssuesShareChannel:MediaAccessStrategy/Mode3.1.1ServicesprovidedtoTheNetworkLayerUnacknowledgedconnectionlessserviceAcknowledgedconnectionlessserviceAcknowledgedconnection-orientedserviceConcept(1)Acknowledgedvs.Unacknowledged(2)Connection-orientedvs.ConnectionlessFeatures:

(1)NoConnectionBetweenSenderandReceiver

(2)

Receiverdoesn’tsendbackthereplymessageswhenreceivedtheframe

(3)

TheupperlayerdealswiththetransmissionerrorsUsages:(1)Appropriatewhentheerrorrateisverylow(2)Appropriateforreal-timetraffic,suchasreal-timevoice(3)mostLANS

UnacknowledgedconnectionlessserviceFeatures:(1)Unacknowledgedconnectionlessservice+AcknowledgedService

(2)Receiversendsbackthereplymessageswhenreceivedtheframe(3)

Senderadoptsthetimeoutmechanismtodealwiththereplymessages

sendersendstheframeagainifitdoesn’treceivethereplymessagesintimeUsages:

(1)

unreliablechannelswithnothighreliability(2)WirelesssystemsAcknowledgedconnectionlessserviceFeatures:(1)1ststep:establishaconnectionbetweensenderandreceiver(2)2ndstep:keepconnect,transmittheframesbetweensenderandreceiverReceiversendsbackthereplymessageswhenreceivedframesenderadoptsthetimeoutmechanismtohandlethereplymessages(3)3rdstep:

releaseconnectionServicePrimitives:

(1)

DL-CONNECT.request,DL-CONNECT.indication,DL-CONNECT.response,DL-CONNECT.confirm(2)DL-DATA.request,DL-DATA.indication(3)DL-DISCONNECT.request,DL-DISCONNECT.indication,Usages:AreliablebitstreamAcknowledgedconnection-orientedserviceMethodsofbreakingthebitstreamupintoaframe:

(1)

TimeGaps

(2)

Charactercount

(3)

Flagbyteswithbytestuffing

(4)

Startingandendingflagswithbitstuffing

(5)

Physicallayercodingviolation

(6)

Hybridmethod:Ethernet,802.113.1.2FramingCharacterCountExample:DDCMP(DigitaldataCommunicationMessageProtocol(a)Aframedelimitedbyflagbytes.(b)Fourexamplesofbytesequencesbeforeandafterstuffing.FlagByteswithByteStuffingExample:BSC(BinarySynchronousCommunication,Bisync,IBM)PPP(Point-toPointProtocol,IETF,SLIP)Bitstuffing

(a)Theoriginaldata

(b)Thedataastheyappearontheline

(c)Thedataastheyarestoredinreceiver’smemoryafterdestuffingStartingandEndingFlagswithBitstuffingExample:SDLC(SynchronousDataLinkControlprotocol,IBM)HDLC(High-levelDatalinkControl,ISO)FlashPhysicallayercodingviolation(1)ManchesterCode(2)4B/5BCodeViolationCodeStartingFlag/EndingFlagViolationCode00001:StartingFlag10000:EndingFlag3.1.3ErrorControlError-Detectingdamaged(Error-DetectingCodes)outoforder(sequencenumbers)Duplicated(Sequencenumbers)Lost(timeratsender,timeoutmechanism)Error-Correcting

Retransmission:AutomaticRepeatreQuest(ARQ),OtherError-CorrectionCodes(FECForwardErrorCorrection)TwoapproachesFeedback-basedflowcontrol(usedatDLL)Rate-basedflowcontrol(notsuitableforDLL)3.1.4FlowControl

Asetofprocedurestocoordinatetheamountofdatatosend/receive

(1)theprocessingspeedatreceiverislimited(2)thebufferofreceiverhasalimitedsize(3)thereceivermustbeabletoinformthesendertohaltorslowdownthesending(4)thesendermustfollowtheindicationfromthereceivertocontrolitssendingFigureSingle-biterrorBursterrors3.2ErrorDetectionandCorrectionBiterrorsmode

Isolatederrors/Bursterrorsmultiple-biterrorsblockcoding(2)Structureofthereliablecommunicationsystem(3)Error-detectingcodesParitycheckChecksumsConstantRatioCodePositiveandnegativecodingCRC(CyclicRedundancyCheck)(4)Error-correctingcodesHammingcodesBinaryconvolutionalcodesReed-SolomoncodesLow-densityParityCheckCodesError-detecting/correctingCodes(5)ParityChecksOddnumbererrorscanbedetectediferrorsoccurinsomeline(row/column)Oddnumbererrorscanbecorrectediferrorsoccurinsomeline(row/column),except4,8,12..errorplaceslocatatvertexescanbeusedtodetectthebursterrorsEvenparityCheck:Cn-1⊕Cn-2⊕……⊕C0＝0OddparityCheck：Cn-1⊕Cn-2⊕……⊕C0＝1OddnumbererrorscanbedetectedEvennumbererrorscan’tbedetectedTwoDimensionParityChecksSimpleParityCheck(6)ChecksumThemessageisdividedinto16-bitwordsThevalueofthechecksumwordissetto0Allwordsincludingthechecksumareaddedusingone’scomplementaddition(4)ThesumiscomplementedandbecomesthechecksumThechecksumissentwiththedata異或反碼Totreatbitstringsasrepresentationsofpolynomialswithcoefficientsof0and1only.Ak-bitframeisregardedasthecoefficientlistforapolynomialwithkterms,rangingfromx(k-1)to1(x0).Forexample,110001x5+x4+1.Polynomialarithmeticisdonemodulo2,accordingtotherulesofalgebraicfieldtheory.Therearenocarriesforadditionorborrowsforsubtraction.BothadditionandsubtractionareidenticaltoexclusiveOR.(7)PolynomialcodeorCRC(CyclicRedundancyCheck)身份證LetrbethedegreeofG(x).DividethebitstringcorrespondingtoG(x)intothebitstringcorrespondingtoM(x)xr,usingmodulo2division,i.e.r(x)=M(x)xr

/G(x)

SubtracttheremainderfromthebitstringcorrespondingtoM(x)xrusingmodulo2subtraction.Theresultisthechecksummedframetobetransmitted.CallitspolynomialT(x)=.M(x)xr+r(x)Clearly,T(x)isdivisible(modulo2)byG(x).PolynomialcodeorCRC(CyclicRedundancyCheck)WhenthereceivergetsthechecksummedframerepresentedbyT’(X),ittriesdividingitbyG(x).Ifthereisaremainder,therehasbeenatransmissionerrorPolynomialcodeorCRCerrorsdetectionability-CRC16=17bursterrors:99.9969%≤16bursterrors:100%(8)HammingCodeLetmbethelengthoftransmittedmessagesLetrbethenumberofcheckbitsortheredundancybitsThen,thelengthofcodeism+r2r

m+r+1TodesignacodewithmmessagebitsandrcheckbitsthatwillallowallsingleerrorstobecorrectedThen,thereism=4,r=3m=11,r=4r1=bits1,3,5,7,9,11r2=bits2,3,6,7,10,11r4=bits4,5,6,7r8=bits8,9,10,11HammingCodeHammingCode3.3

Stop-and-WaitDataLinkProtocolAnUnrestrictedSimplexProtocol

(UtopianSimplexProtocol)ASimplexStop-and-WaitProtocolforError-freeChannel

(Error-FreeChannel+RateMatching)ASimplexProtocolforaNoisyChannel

(NoisyChannel+RateMatching)AStop-and-WaitProtocolforActualapplicationPerformanceAnalysisoftheSimplexStop-and-WaitARQProtocol3.3.1AnUnrestrictedSimplexProtocolDataaretransmittedinonedirectiononlyBoththetransmittingandreceivingnetworklayersarealwaysreadyProcessingtimecanbeignoredInfinitebufferspaceisavailableThecommunicationchannelbetweenthedatalinklayersneverdamagesorlosesframesABDATADATAtimeDATANoFlowControlNoErrorControl3.3.2ASimplexStop-and-WaitProtocolDataaretransmittedinonedirectiononlyThecommunicationchannelbetweenthedatalinklayersneverdamagesorlosesframesButreceiver’sprocessingrateisslowerthanthesender’s,andhasfinitebufferspaceABDATAACKDATAACKtimeHowtopreventthesenderfromfloodingthereceiver?ThesenderinsertadelaytoslowitselfdownThereceiverprovidefeedbacktothesender(stop-and-wait)NoErrorControl+FlowControl3.3.3ASimplexProtocolforaNoisyChannelFrameisdamagedintransitFrameislostAcknowledgementframeislostABDATA0ACKDATA1ACKtimeABDATA0NAKDATA0ACKABDATA0timeoutACKABDATA0ACKDATA0ACKXdamagedlosttimeoutlostARQprotocolAreallproblemsresolved?(No.thesequence.)ErrorControl+FlowControlR=0FrameErrorNormalErrorFrameisdamagedintransitFrameislostAcknowledgementframeislostDuplicatedDataFrameErrorControlFlowControlDualCommunication3.3.4AStop-and-WaitProtocolforActualApplicationFrameLostFrameLost:retransmissionACKLost:retransmissionNecessary,why?R=0DelayedACKACKmustbenumbered,why?BidirectionaltransmissionPiggybacking(R=0)R=0FlashSenderReceiverReceiversenderSenderReceiverReceiversenderPiggybacking:DualCommunicationProblem:HowlongshouldthedatalinklayerwaitforapacketontowhichtopiggybacktheacknowledgementSimplextransmissionfull-duplextransmission3.3.45Performanceanalysistf=Lf/C tout=tp+tpr+ta+tp+tpr(Usually,

tpr<<tp,ta<<tp)

tout=2tptT=tf+tout=tf+2tpDATADATAACKtptptftouttTtprtatprtimeABAssumingerror-freetransmissionofACKandNAK

frames,andassumingindependenceforeachframetransmissionThentheprobabilityofrequiringexactlykattemptstosuccessfullyreceiveagivenframeispk=pk(1-p),k=1,2,3,……(wherepistheframeerrorprobability)TheaveragenumberoftransmissionsNtthatarerequiredbeforeaframeisacceptedbythereceiveristhengivenbyTheaverageoveralltransmissiontimeTvforanacceptedframe,isthengivenbyTheaveragenumberofretransmissionsNrforanacceptedframeisNt-1,whichisNr=Nt-1=Propagationtimedelay:Pathlength/PropagationRateTransmissiontimedelay:TheFrameSize/TransmissionRatetosendthe1000bitframebya50kbpssatellitechannelwitha500msecround-trippropagationtimedelayTd:TotaltimedelayofoneframetransmittedsuccessfullybytheStop-and-WaitprotocolTd=1000/50K*1000+500=520Att=0msec:sendingthefirstframe.Att=20msec:theframebeingcompletelysent.Att=270msec:theframearrivedatthereceiver.Att=520msec:theacknowledgementarrivedbackatthesenderTransmissionEfficiencyRate=20/520

Conclusion:Thelonground-triptimecanhaveimportantimplicationsfortheefficiencyofthebandwidthutilizationTheStop-and-WaitProtocolisnotsuitableforsatellitechannelARQ:AutomaticRepeatreQuestPAR:PositiveAcknowledgementwithRetransmissionAone-bitSlidingProtocol:Stop-and-WaitProtocolContinualARQProtocol:Go-Back-NSelectiveARQ(2)SlidingWindowProtocolbeusedtodescribeandimplementtheARQsProtocolifWT=1,WR=1thenSlidingWindowProtocol=one-bitStop-and-WaitProtocolifWT>1,WR=1thenSlidingWindowProtocol

=ContinualARQifWT>1,WR>1thenSlidingWindowProtocol=SelectiveARQ3.4

SlidingWindowProtocolandARQs3.4.1SlidingWindowProtocolsBasicsofslidingwindowprotocol:TheSendingWindowAtanyinstantoftime,thesendermaintainsasetofsequencenumberscorrespondingtoframesitispermittedtosend.TheseframesaresaidtofallwithinthesendingwindowTheReceivingWindowSimilarly,thereceiveralsomaintainsareceivingwindowcorrespondingtothesetofframesitispermittedtoacceptAllowmultipleframestobeintransitReceiverhasbufferWlongEachframeisnumberedACKincludesnumberofnextframeexpectedSequencenumberboundedbysizeoffield(k)Framesarenumberedmodulo2kFigureSendingWindowThesequencenumberswithinthesender'swindowrepresentframessentbutasyetnotacknowledged.Usedforretransmission.AnewpacketUpdatestheupperedgeofthesendingwindow.AnewacknowledgementUpdatestheloweredgeofthesendingwindow.Ifthesendingwindowevergrowstoitsmaximumsize,thesendingdatalinklayermustdisablethenetworklayeruntilanotherbufferbecomesfree.Thereceivingdatalinklayer'swindowcorrespondstotheframesitmayaccept.Anyframefallingoutsidethewindowisdiscardedwithoutcomment.Whenaframewhosesequencenumberisequaltotheloweredgeofthewindowisreceived,itispassedtothenetworklayer,anacknowledgementisgeneratedandthewindowisrotatedbyone.Thereceivingwindowalwaysremainsatitsinitialsize.ReceivingWindowFlashThesizeofsendingwindowis3thesizeofreceivingwindowis12-bitsequencenumberExample1:SlidingWindowWT＝3；WR＝1WT＝5；WR＝1;3-bitsequencenumberSendingwindowReceivingwindow017234565framessent01723456ACK1received01723456ACK4received01723456toreceiveframe001723456toreceiveframe101723456Toreceive4Example2:SlidingWindow3.4.2ContinualARQProtocol:Go-Back-NSender’sruleofactionkeepstransmittingframescontinuouslyIfanyframesisnotacknowledgedToutaftertheframeistransmitted(orreceivesitsNAK),assumethattheframeislostandretransmitallframesfromthelostframeReceiver’sruleofaction

TransmitsanacknowledgeframetothesenderwheneveraframeisdeliveredwithoutanyerrorIfduplicatedframearrives,assumethatthepreviousacknowledgementfortheframeislost(3)SendermustsetatimerforeachoutstandingframeEDDACK1ACK2NAK2AsenderBreceiver012342340123ACK3ACK4Example1:AContinualARQProtocol(lostordamaged)exampleinbookusually,ackishouldbeisacki+1(page254)ReceiverreceivesthedatainorderExample2:AContinualARQProtocol0123456701234567012345670123456701234567012345670123456701234567012345670123456701234567012345670,1,2ACK33,4,5,6NAK44,5,6Wt=7Wr=1ACK7slidingthewindowwaitingwaitingGobackto4,sendingframe4,5,6againReceiverreceivesthedatainorderExample3:AContinualARQProtocolMaximumsizeofthesendingwindow:wt=2m-1Receiverreceivesthedatainorder3.4.3SelectiveARQProtocolSender’sruleofactionkeepstransmittingframescontinuouslyIfanyframesisnotacknowledgedToutaftertheframeistransmitted(orreceivesitsNAK),assumethattheframeislostandretransmitonlythelostframeReceiver’sruleofaction

DoesthesameactionasintheStop-and-WaitARQ,butneverdiscardallframesdeliveredwithouterrorsandhasthecapabilitytoreordertheminthecorrectorder(3)Sendermustsetatimerforeachoutstandingframe(4)Wt=WrFrame2istransmittedonlyFramesacceptedandacknowledgedtimeusually,ackishouldbeacki+1(page254)Frame2,3,4,5,6willpassedtolayernetworkinorderBufferedframesMaximumsizeofthesendingwindow:wt≤m/2=2n-1Example1:SelectiveARQProtocolWT<=2m-1WR<=2m-1ACK1ACK2ACK1ACK2ACK3Example2:SelectiveARQProtocol3.5

ExampleDataLinkProtocolDataLinkProtocolsAsynchronousProtocolsSynchronousProtocolsXModemYModemZModemKermitBit-oriented(HDLC)Character-orientedByte-oriented(BSC)Usedprimarilyinmodems.Designedtotransferfilefortelephone-linecommunicationbetweenPCs.Half-duplexstop-and-waitARQProtocolForLANs,MANs,andWANs3.5.1BSC(BinarySynchronousProtocol)

Datasentasblocksofcharacters(synchronous)Usedonlyforhalf-duplextransmissionUsedForASCII,EBCDICetcPointtopointandmultipoint(polling)UsesspecialcharactersforcontrolLineControlFlowControlErrorControlARQapproachforerrorchecking(ACK1,ACK0,NAK)CharacterASCIICodeACK0ACK1DLEENQEOTETBETXITBNAKNULRVISOHSTXSYNTTDWACKDLEand0DLEand1DLEENQEOTETBETXUSNAKNULLDLEand<SOHSTXSYNSTXandENQDLEand;FunctionGoodevenframereceivedorreadytoreceiveGoododdframereceivedDatatransparencymakerRequestforaresponseSenderterminatingEndoftransmissionblock;ACKrequiredEndoftextinamessageEndofintermediateblockinamultiblocktransmissionBadframereceivednothingtosendFillercharacterUrgentmessagefromreceiverHeaderinformationbeingsTextbeingsAlertsreceivertoincomingframeSenderispausingbutnotrelinquishingthelineGoodframereceivedbutnotreadytoreceivemoreControlCharactersforBSCPAD(01010101,85)SYN(00010110,22)DLE（00010000,16)SOH(00000001,1)ETX(00000011，3)ENQ(00000101,5)STX(00000010,2)EOT(00000100,4)ACK(00000110,6)ETB(00010111,23)NAK(00010101,21)ITB(00011111,31)BSCFramesDataframesSimpledataframeBSCFramewithHeaderMultiblockFrameControlFramesConnectionDisconnectionFlowcontrolErrorcontrolBSCFramesDataFrames(1)SimpleDataFrame(2)BSCFramewithHeaderSenderaddress,receiveraddressandframeID(3)Multi-blockFrameThereceiversendsasingleacknowledgmentfortheentireframeMulti-frametransmissionControlFramesSYNSYNENQBidpoint-topointconnectrequestSYNSYNP/SaddressP:PollPrimarypollssecondaryaskifthesecondaryisreadytosenddataoutS:selectPrimaryselectssecondaryaskthesecondaryifisreadytoreceivedataENQSYNSYNACKPositiveresponsetobid//selectReadytoreceivedataSYNSYNNAKNegativeresponsetobid//selectNotreadytoreceivedataSYNSYNEOTNegativeresponsetopollNotreadytosenddata(1)Connectionestablishment(2)ConnectionterminationSYNSYNEOTEndoftransmissionFinishedsendingdata(2)FlowanderrorcontrolSYNSYNACK0PositiveACKofevenframesFramenumber0receivedSYNSYNACK1PositiveACKofoddframesFramenumber1receivedSYNSYNNAKNegativeACKofframesErrorsintheFramereceivedSYNSYNWACKPositiveACKofpreviousframeNotreadytoreceivemoreSYNSYNRV1ReverseinterruptRequestforinterruption,ReceiverwantstosendouturgentdataSYNSYNTTDSenderispausingthetransmissionButdoesnotclosetheconnectionDataTransparencyConfusionbetweencontrolinformationanddataiscalledalackofdatatransparencyanycombinationofbitsasdatacanbesentoutandbereceivedbytestuffingDLE=datalinkescape3.5.2HDLCSDLC(SynchronousDataLinkControl)developedbyIBMin1975,usedinSNAADCCP(AdvancedDataCommunicationControlProcedure)developedbyANSIin1975basedonSDLCHDLC(High-LevelDataLinkControl)developedbyISOin1979basedonSDLCLAPs(LAPB,LAPD,LAPM,LAPX,etc)developedbyITU-Tsince1981,usedinX.25basedonHDLCPPP,framerelaydevelopedbyITU-TandANSIbasedonHDLCbit-orienteddatalinkprotocolsUsagecomputer-computercomputer-terminalterminal-terminalFeaturesAllbit-orientedprotocolsarerelatedtohigh-leveldatalinkcontrol(HDLC)Supportsbothhalf-duplex&full-duplexmodesinpoint-to-pointandmultipointlinksSupportsslidingwindowflowcontrolmechanismsItcanbecharacterizedbystationtypesConfigurationresponsemodesFrametypesHDLCBasicsPrimary:sendscommandreceiveresponselinkmanagement(initial,flowcontrol,errorrecovery)Secondary:receivecommandsendresponseassisttheprimarystationinperforminglinkmanagementCombined:sameasPrimary+secondaryStationTypeHDLCConfigurationPoint-to-point/multipointMaster/slaveconfigurationHDLCdoesnotsupportbalancedmultipoint.ThisnecessitatedtheinventionofmediaaccessprotocolsforLANsModeofcommunicationAmodeinHDLCistherelationshipbetweentwodevicesinvolvedinanexchange.ThemodedescribeswhocontrolsthelinkNRM(NormalResponseMode)ARM(AsynchronousResponseMode)ABM(AsynchronousBalancedMode)NRM(NormalResponseMode)referstothestandardprimary-secondaryrelationshipsecondarydevicemusthavepermissionfromtheprimarydevicebeforetransmittingARM(AsynchronousResponseMode)secondarymayinitiateatransmissionwithoutpermissionfromtheprimarywheneverthechannelisidledonotaltertheprimary-secondaryrelationshipinanyotherwayABM(AsynchronousBalancedMode)allstationsareequalandthereforeonlycombinedstationsconnectedinpoint-to-pointareusedEithercombinedstationmanyinitiatetransmissionwiththeothercombinedstationwithoutpermissionThreemodesofcommunicationinHDLCNRM(NormalResponseMode)referstothestandardprimary-secondaryrelationshipsecondarydevicemusthavepermissionfromtheprimarydevicebeforetransmittingARM(AsynchronousResponseMode)secondarymayinitiateatransmissionwithoutpermissionfromtheprimarywheneverthechannelisidledonotaltertheprimary-secondaryrelationshipinanyotherwaysecondarycaninitiateifidle,alltransmissionsaremadetoprimarystation.AllcommunicationsarethroughthePrimary(evenforsecondarytosecondary)Point-to-pointABM(AsynchronousBalancedMode)allstationsareequalandthereforeonlycombinedstationsconnectedinpoint-to-pointareusedEithercombinedstationmanyinitiatetransmissionwiththeothercombinedstationwithoutpermissionFramesinHDLCbit888variable168

Information

Flag

Flag

Address

ControlFCSIntervalfortransparenttransmissionIntervalforFCSFrameframeframeFrameFramescanbetransmittedcontinuallyFlag:0111110FCS:CRC-CCITT16,

G(x)=x16

+x12+x5+1Transparenttransmission:bitstuffing,extra0after5

consecutive

1sAddress:secondaryaddress(unbalanced),responsestationaddress(balanced)Bitstuffingbit888variable168

Information

Flag

Flag

Address

ControlFCSP/F:poll/finalbitPrimarySecondaryP=1nowpollingP=0nopollingSecondaryPrimaryF=1nowfinalF=0notyetfinalN(S):thesequencenumberoftheframeN(R):acknowledgmentnumber(whenpiggybacking,ACK)ifcorrectN(R)=nextexpectedframeseq.elseN(R)=numberofdamagedframe(needretransmission)S:supervisoryfunctionbits.M:unumberedframebitsI-FrameS-FrameU-FrameI-Frame:datatobetransmitted,supportingtheduplexmodeFrameUseofP/FbitinpollingandselectionS-frameisusedforacknowledgement,flowcontrol,anderrorcontrol00:Receiveready(RR)

notifiesthereceiptofasafeandsoundframe(s)N(R):ACKnumber10:Receivenotready(RNR)notifiesthereceiptofframe(s)butannouncesthereceivercannotreceivemoreframes(akindofcongestioncontrol)N(R):ACKnumber01:Reject(REJ)NAKframeusedinGo-Back-NARQN(R):NAKnumber11:Selectivereject(SREJ)NAKframeusedinSelective-repeatARQN(R):NAKnumberS-FrameRR(ReceiveReady)ACKUsedbyareceivingstationtoreturnapositiveacknowledgmentN(R)fieldhaving3bits(upto8frames)PollWhentransmittedbyprimarywithP/Fbitset,RRasksifithasanythingtosend

NegativeresponsetopollRRtellsprimarythatsecondaryhasnothingtosend.Ifthesecondarydoeshavedatatotransmit,itrespondstopollwithanI-frame,notanS-framePositiveresponsetoselectWhenasecondaryisabletoreceiveatransmissionfromtheprimaryS-FrameRNR(ReceiveNotReady)ACKRNRreturnedbyareceivertoasendingstationacknowledgesreceiptofallframesupto,butnoincluding,thefameindicatedintheN(R)fieldSelectWhenaprimarywishestotransmitdatatoaspecificsecondary,italertsthesecondarybysendinganRNRframewiththeP/F(usedasP)set.NegativeresponsetoselectWhenaselectedsecondaryisunabletoreceivedata,itreturnsanRNR.S-FrameREJ(Reject)thenegativeacknowledgmentreturnedbyareceiverinago-back-nARQerrorcorrectionsystemInanREJframe,theN(R)fieldcontainsthenumberofthedamagedframeSREJ(Selective-reject)thenegativeacknowledgmentreturnedby