Chapter7Datalinkcontrol

Becauseofthepossibilityoftransmissionerrors,Itisnecessarytoimposealayerofcontrolineachcommunicatingdevicethatprovidesfunctionssuchasflowcontrol,errordetection,anderrorcontrol.Thislayerofcontrolisknownasadatalinkcontrolprotocol..7.1FlowcontrolFlowcontrolenablesareceivertoregulatetheflowofdatafromasendersothatthereceiver’sbuffersdonotoverflow.Itisatechniqueforassuringthatatransmittingentitydoesnotoverwhelmareceivingentitywithdata..(1)Stop-and-waitFlowControlAsourceentitytransmitsaframe.Afterthedestinationentityreceivestheframe,itindicatesitswillingnesstoacceptanotherframebysendingbackanacknowledgmenttotheframejustreceived.Thesourcemustwaituntilitreceivestheacknowledgmentbeforesendingthenextframe.Thedestinationcanthusstoptheflowofdatasimplybywithholdingacknowledgment..(2).Sliding-window-flowcontrolInsituationswherethebitlengthofthelinkisgreaterthantheframelength,seriousinefficienciesresult.Efficiencycanbegreatlyimprovedbyallowingmultipleframestobeintransitatthesametime.7.2ErrorDetectionErrordetectiontechniques:Foragivenframeofbits,additionalbitsthatconstituteanerror-detectingcodeareaddedbythetransmitter.Thiscodeiscalculatedasafunctionoftheothertransmittedbits.Adetectederroroccursifandonlyifthereisamismatch..1.ParityCheckToappendaparitybittotheendofablockofdataEvenparity(evennumberof1s)Oddparity(oddnumberof1s)2.CyclicRedundancyCheck(CRC)Givenakbitblockofbits,thetransmittergeneratesanbitsequence,knownasaframechecksequence(FCS).Theresultingframeconsistsofk+nbits.Thereceiverthendividestheincomingframebythepredeterminednumber,ifthereisnoremainder,noerroroccurs.(1).Modulo2ArithmeticPolynomials7.3ErrorcontrolErrorcontrolreferstomechanismstodetectandcorrecterrorsthatoccurinthetransmissionofframes.Dataaresentasasequenceofframes;framesarrivedinthesameorderinwhichtheyaresent;andeachtransmittedframesuffersanarbitraryandpotentiallyvariableamountofdelaybeforereception.Inadditionweadmitthepossibilityoftwotypesoferrors:Lostframe:Aframefailstoarriveattheotherside,Forexample,anoiseburstmaydamageaframetotheextentthatthereceiverisnotawarethataframehasbeentransmitted.Damagedframe:Arecognizableframedoesarrive,butsomeofthebitsareinerror(havebeenalteredduringtransmission)TechniquesusedforerrorcontrolErrordetection:Asdiscussedintheprecedingsection.Positiveacknowledgement:thedestinationreturnsapositiveacknowledgmenttosuccessfullyreceived,error-freeframes.Retransmissionaftertimeout:Thesourceretransmitsaframethathasnotbeenacknowledgedafterapredeterminedamountoftime.Negativeacknowledgmentandretransmission:Thedestinationreturnsanegativeacknowledgmenttoframesinwhichanerrorisdetected.Thesourceretransmitssuchframes.Collectively,thesemechanismsareallreferredtoasautomaticrepeatrequest(ARQ);theeffectofARQistoturnanunreliabledatalinkintoareliableone.ThreeversionofARQhavebeenstandardized:.Stop-and-WaitARQ.Go-back-NARQ.Selective–rejectARQ1.ARQ(automaticrepeatrequest)ACK(acknowledgment),RR(receiveready),REJ(reject)(1)stop-and-waitARQ(2)Go-back-NGo-Back-NARQTheformoferrorcontrolbasedonsliding-windowflowcontrolthatismostcommonlyusediscalledgo-backNARQ.Inthismethod,astationmaysendaseriesofframessequentiallynumberedmodulosomemaximumvalue.Thenumberofunacknowledgedframesoutstandingisdeterminedbywindowsize.Usingthesliding-windowsflowcontroltechnique.Whilenoerroroccur.Thedestinationwillacknowledge(RR=receiveready,orpiggybackedacknowledgment)incomingframesasusual.Ifthedestinationstationdetectsanerrorinframe,itsendsanegativeacknowledgment(REJ=reject)forthatframe.Thedestinationstationwilldiscardthatframeandallfutureincomingframesuntilltheframeinerroriscorrectlyreceived.Thus,thesourcestation,whenitreceivesaREJ,mustretransmittheframeinerrorplusallsucceedingframesthatweretransmittedintheinterim(過渡期）.(3)Selective–RejectARQSelective-RejectARQWithselective-rejectARQ,theonlyframesretransmittedarethosereceiveanegativeacknowledgment.InthiscasecalledSREJ,orthosethattimeout.7.4High-Leveldatalinkcontrol(HDLC)

BasicCharacteristics(1)ThreestationtypesPrimarystation:Responsibleforcontrollingtheoperationofthelink.Framesissuedbytheprimaryarecalledcommand.Secondarystation:Operatesunderthecontroloftheprimarystation.Framesissuedbyasecondaryarecalledresponses.Combinedstation:Combinesthefeaturesofprimaryandsecondary.Acombinedstationmayissuebothcommandsandresponses.(2)TwolinkconfigurationsUnbalancedconfiguration:Balancedconfiguration.(3)ThreetransfermodesNormalresponsemode(NRM)Asynchronousbalancedmode.(ABM)Asynchronousresponsemode(ABM)2.Framestructure(1)Flagfields

Flagfieldsdelimittheframeatbothendswiththeuniquepattern01111110.Bitstuffing:thetransmitterwillinsertanextra0bitaftereachoccurrenceoffive1sintheframe.Datatransparency:Withtheuseofbitstuffing,arbitrarybitpatternscanbeinsertedintothedatafieldoftheframewithbitstuffing.Originalpattern:1111111100111111111101111001111111Afterbitstuffing:111110111001111101111101111001111101(2)AddressfieldTheaddres