12081220+金雨鑫+Problem-3-4

Problem31.Flowcontrolenablesareceivertoregulatetheflowofdatafromasendersothatthereceiver’sbuffersdonotoverflow.2.Inadatalinkcontrolprotocol,errorcontrolisachievedbyretransmissionofdamagedframesthathavenotbeenacknowledgedorforwhichtheothersiderequestsaretransmission.3.Toachievethenecessarycontrolonsendingdataoveradatacommunicationslink,alayeroflogicisaddedabovethephysicallayer,thislogicisreferredtoasdatalinkcontroloradatalinkcontrolprotocolWhenadatalinkcontrolprotocolisused,thetransmissionmediumbetweensystemsisreferredtoasadatalink4.Someoftherequirementsandobjectivesforeffectivedatacommunicationbetweentwodirectlyconnectedtransmitting-receivingstations:FramesynchronizationFlowcontrol,Errorcontrol,Addressing,Controlanddataonsamelink,Linkmanagement.5.Stop-and-waitflowcontrolisthesimplestformofflowcontrol.Inthismethod,thereceiverindicatesitsreadinesstoreceivedataforeachframe,themessageisbrokenintomultipleframes.ThesenderwaitsforanACK(acknowledgement)aftereveryframeforspecifiedtimecalledtimeout.Itissenttoensurethatthereceiverhasreceivedtheframecorrectly.ItwillthensendthenextframeonlyaftertheACKhasbeenreceived.6.thetransmissiontimeisthetimeittakesforastationtotransmitaframe,andthepropagationdelayisthetimeittakesforabittotravelfromsendertoreceiver.7.bitlengthofalinkasfollows:B=R*d/VWhereV=velocityofpropagation,inm/sd=length,ordistance,ofthelinkinmetersR=datarateofthelink,inbpsB=lengthofthelinkinbits;thisisthenumberofbitspresentonthelink8.weexpressaasa=B/L,whereListhenumberofbitsintheframe(lengthoftheframeinbits).Thenaislessthan1meansthepropagationtimeislessthanthetransmissiontimeInthiscase,theframeissufficientlylongthatthefirstbitsoftheframehavearrivedatthedestinationbeforethesourcehascompletedthetransmissionoftheframe.Ifaislargerthan1,thenTheessenceoftheproblemdescribedsofaristhatonlyoneframeatatimecanbeintransit.Efficiencycanbegreatlyimprovedbyallowingmultipleframestobeintransitatthesametime9.Sliding-windowflowcontrolallowsthesendercansendandthereceivercanacceptnframeswithouthavingtowaitforanacknowledgement.Asequencenumberisassignedtoframesinordertohelpkeeptrackofthoseframeswhichdidreceiveanacknowledgement.Thereceiveracknowledgesaframebysendinganacknowledgementthatincludesthesequencenumberofthenextframeexpected.Thisacknowledgementannouncesthatthereceiverisreadytoreceivenframes,beginningwiththenumberspecified.Boththesenderandreceivermaintainwhatiscalledawindow.Thesizeofthewindowislessthanorequaltothebuffersize.10.ForSliding-windowflowcontrolpiggybackingmeansEachdataframeincludesafieldthatholdsthesequencenumberofthatframeplusafieldthatholdsthesequencenumberusedforacknowledgment.Thus,ifastationhasdatatosendandanacknowledgmenttosend,itsendsbothtogetherinoneframe,savingcommunicationcapacity.Ofcourse,ifastationhasanacknowledgmentbutnodatatosend,itsendsaseparateacknowledgmentframe,suchasRR(RequestReady)orRNR(RequestNotReady)..11.Errorcontrolreferstomechanismstodetectandcorrecterrorsthatoccurinthetransmissionofframes.Therearetwotypesoferrors:LostframeandDamagedframe.12.ARQmeansautomaticrepeatrequesttheeffectofARQistoturnanunreliabledatalinkintoareliableone.ThreeversionsofARQhavebeenstandardized:Stop-and-waitARQ，Go-back-NARQ，Selective-rejectARQ13.ThethreestationtypesofHDLCare:Primarystation，Secondarystation，Combinedstation.ThetwolinkconfigurationsareUnbalancedconfiguration，Balancedconfiguration.ThethreedatatransfermodesareNormalresponsemode(NRM):，Asynchronousbalancedmode(ABM):，Asynchronousresponsemode(ARM):.14.HDLCheadercontainsframeflag,address,controlfields.Flagfieldsdelimittheframeatbothendswiththeuniquepattern01111110.Withtheuseofbitstuffing,arbitrarybitpatternscanbeinsertedintothedatafieldoftheframe.Thispropertyisknownasdatatransparency.Problems41.Multiplexingallowsseveraltransmissionsourcestosharealargertransmissioncapacity.Thetwocommonformsofmultiplexingarefrequencydivisionmultiplexing(FDM)andtimedivisionmultiplexing(TDM).2.Synchronoustimedivisionmultiplexingcanbeusedwithdigitalsignalsoranalogsignalscarryingdigitaldata.Inthisformofmultiplexing,datafromvarioussourcesarecarriedinrepetitiveframes.Eachframeconsistsofasetoftimeslots,andeachsourceisassignedoneormoretimeslotsperframe.Theeffectistointerleavebitsofdatafromthevarioussources.StatisticaltimedivisionmultiplexingprovidesagenerallymoreefficientservicethansynchronousTDMforthesupportofterminals.Timeslotsarenotpreassignedtoparticulardatasources.Rather,userdataarebufferedandtransmittedasrapidlyaspossibleusingavailabletimeslots.3.Amultiplexingsystemcontainsmultiplexer,thelinkanddemultiplexer.4.Forwavelengthdivisionmultiplexing(WDM),thelightstreamingthroughthefiberconsistsofmanycolors,orwavelengths,eachcarryingaseparatechannelofdata.5.ForTDM,thetimedomainisdividedintoseveralrecurrenttimeslotsoffixedlength,oneforeachsub-channel.6.Withpulsestuffin,theoutgoingdatarateofthemultiplexer,excludingframingbits,ishigherthanthesumofthemaximuminstantaneousincomingrates.Theextracapacityisusedbystuffingextradummybitsorpulsesintoeachincomingsignaluntilitsrateisraisedtothatofalocallyg