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Problem51.Circuitswitchingisusedinpublictelephonenetworksandisthebasisforprivatenetworksbuiltonleasedlinesandusingon-sitecircuitswitches.Circuitswitchingwasdevelopedtohandlevoicetrafficbutcanalsohandledigitaldata,althoughthislatteruseisofteninefficient.2.Withcircuitswitching,adedicatedpathisestablishedbetweentwostationsforcommunication.Switchingandtransmissionresourceswithinthenetworkarereservedfortheexclusiveuseofthecircuitforthedurationoftheconnection.Theconnectionistransparent:Onceitisestablished,itappearstoattacheddevicesasiftherewereadirectconnection.3.Packetswitchingwasdesignedtoprovideamoreefficientfacilitythancircuitswitchingforburstydatatraffic.Withpacketswitching,astationtransmitsdatainsmallblocks,calledpackets.Eachpacketcontainssomeportionoftheuserdatapluscontrolinformationneededforproperfunctioningofthenetwork.Akeydistinguishingelementofpacket-switchingnetworksiswhethertheinternaloperationisdatagramorvirtualcircuit.4.Thedevicesattachedtothenetworkarereferredtoasstations.Theswitchingdeviceswhosepurposeistoprovidecommunicationarereferredtoasnodes.5.Forswitchednetworks,somenodesconnectonlytoothernodes.Theirsoletaskistheinternal(tothenetwork)switchingofdata.Othernodeshaveoneormorestationsattachedaswell;inadditiontotheirswitchingfunctions,suchnodesacceptdatafromanddeliverdatatotheattachedstations.6.Communicationviacircuitswitchingimpliesthatthereisadedicatedcommunicationpathbetweentwostations.Thatpathisaconnectedsequenceoflinksbetweennetworknodes.Oneachphysicallink,alogicalchannelisdedicatedtotheconnection.Communicationviacircuitswitchinginvolvesthreephases,whichcanbeCircuitestablishment，Datatransfer，Circuitdisconnect.7.Apublictelecommunicationsnetworkcanbedescribedusingfourgenericarchitecturalcomponents:Subscribers，Subscriberline，Exchanges，Trunks.8.ThreeelementsofCircuitswitchnodearedigitalswitch，networkinterface，controlunit9.Insoftswitchterminology,thephysicalswitchingfunctionisperformedbymediagateway(MG).10.Packetswitchingfeaturesdeliveryofvariablebitratedatastreams(sequencesofpackets)overacomputernetworkwhichallocatestransmissionresourcesasneededusingstatisticalmultiplexingordynamicbandwidthallocationtechniques.Whentraversingnetworkadapters,switches,routers,andothernetworknodes,packetsarebufferedandqueued,resultinginvariabledelayandthroughputdependingonthenetwork'scapacityandthetrafficloadonthenetwork.11.Packetswitchinghasfouradvantagesovercircuitswitching:Lineefficiencyisgreater，Apacket-switchingnetworkcanperformdata-rateconversion，Usuallydonotrejectpackets，Prioritiescanbeused.12.Switchingconcernedwiththreetypesofdelay:Propagationdelay，Transmissiontime，Nodedelay.13.ATMisdesignedtounifytelecommunicationandcomputernetworks.ATMusesasynchronoustime-divisionmultiplexing,andencodesdataintosmall,fixed-sizedpacketscalledcells.ThisdiffersfromapproachessuchastheInternetProtocolorEthernetthatusevariablesizedpacketsandframes.ATMusesaconnection-orientedmodelinwhichavirtualcircuitmustbeestablishedbetweentwoendpointsbeforetheactualdataexchangebegins.