计算机网络7-DataLinkControlppt课件.pptx

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1、DataandComputerCommunicationsNinthEditionbyWilliamStallingsChapter7–DataLinkControlProtocolsDataandComputerCommunications,NinthEditionbyWilliamStallings,(c)PearsonEducation-PrenticeHall,2011DataLinkControlProtocols“Greatandenlightenedone,”saidTen-teh,assoonashisstuporwaslifted,“ha

2、sthispersondeliveredhismessagecompetently,forhismindwasstillasearedvisionofsnowandsandandperchancehistonguehasstumbled?”“Bendyourearstothewall,”repliedtheEmperor,“andbeassured.”—KaiLung'sGoldenHours,EarnestBramahDataLinkControlProtocolswhensendingdata,toachievecontrol,alayeroflogi

3、cisaddedabovethePhysicallayerdatalinkcontroloradatalinkcontrolprotocoltomanageexchangeofdataoveralink:framesynchronizationflowcontrolerrorcontroladdressingcontrolanddatalinkmanagementFlowControlensuresendingentitydoesnotoverwhelmreceivingentitypreventbufferoverflowinfluencedby:tra

4、nsmissiontimetimetakentoemitallbitsintomediumpropagationtimetimeforabittotraversethelinkassumptionis:allframesaresuccessfullyreceivedwithnoframeslostorarrivingwitherrorsModelofFrameTransmissionStopandWaitsimplestformofflowcontrolworkswellforamessagesentinafewlargeframesstopandwait

5、becomesinadequateiflargeblockofdataissplitintosmallframesbysourcesourcetransmitsframedestinationreceivesframeandreplieswithacknowledgement(ACK)sourcewaitsforACKbeforesendingnextframedestinationcanstopflowbynotsendingACKStopandWaitItalldependsonthevalueofa=B/L=(Rd)/(VL)B:lengthofli

6、nk(inbits)L:messagelength(inbits)R:datarate(inbps)d:distance(inmeters)V:velocityofpropagation(m/s)StopandWaitLinkUtilizationSlidingWindowsFlowControlallowsmultiplenumberedframestobeintransitreceiverhasbufferWlongtransmittersendsuptoWframeswithoutACKACKincludesthenumberofnextframee

7、xpectedsequencenumberisboundedbysizeoffield(k)framesarenumberedmodulo2kgivingmaxwindowsizeofupto2k–1receivercanACKframeswithoutpermittingfurthertransmission(RNR:ReceiveNotReady)mustsendanormalacknowledgetoresumeifhavefull-duplexlink,canpiggybackACKsSlidingWindowDiagramSlidingWindo

8、wExampleReceiveReady3ErrorControl