無線通信原理與應用第三章-網(wǎng)絡技術基礎

CommunicationNetworksChapter3TypesofCommunicationNetworksTraditionalTraditionallocalareanetwork(LAN)Traditionalwideareanetwork(WAN)Higher-speedHigh-speedlocalareanetwork(LAN)Metropolitanareanetwork(MAN)High-speedwideareanetwork(WAN)SpeedandDistanceofCommunicationsNetworksCharacteristicsofWANsCoverslargegeographicalareasCircuitsprovidedbyacommoncarrierConsistsofinterconnectedswitchingnodesTraditionalWANsprovidemodestcapacity64000bpscommonBusinesssubscribersusingT-1service–1.544MbpscommonHigher-speedWANsuseopticalfiberandtransmissiontechniqueknownasasynchronoustransfermode(ATM)10sand100sofMbpscommonCharacteristicsofLANsLikeWAN,LANinterconnectsavarietyofdevicesandprovidesameansforinformationexchangeamongthemTraditionalLANsProvidedataratesof1to20MbpsHigh-speedLANSProvidedataratesof100Mbpsto1Gbps

DifferencesbetweenLANsandWANsScopeofaLANissmallerLANinterconnectsdeviceswithinasinglebuildingorclusterofbuildingsLANusuallyownedbyorganizationthatownstheattacheddevicesForWANs,mostofnetworkassetsarenotownedbysameorganizationInternaldatarateofLANismuchgreaterTheNeedforMANsTraditionalpoint-to-pointandswitchednetworktechniquesusedinWANsareinadequateforgrowingneedsoforganizationsNeedforhighcapacityandlowcostsoverlargeareaMANprovides:ServicetocustomersinmetropolitanareasRequiredcapacityLowercostandgreaterefficiencythanequivalentservicefromtelephonecompany

SwitchingTermsSwitchingNodes:IntermediateswitchingdevicethatmovesdataNotconcernedwithcontentofdataStations:EnddevicesthatwishtocommunicateEachstationisconnectedtoaswitchingnodeCommunicationsNetwork:Acollectionofswitchingnodes

SwitchedNetworkObservationsofFigure3.3

Somenodesconnectonlytoothernodes(e.g.,5and7)SomenodesconnecttooneormorestationsNode-stationlinksusuallydedicatedpoint-to-pointlinksNode-nodelinksusuallymultiplexedlinksFrequency-divisionmultiplexing(FDM)Time-divisionmultiplexing(TDM)Notadirectlinkbetweeneverynodepair

TechniquesUsedinSwitchedNetworksCircuitswitchingDedicatedcommunicationspathbetweentwostationsE.g.,publictelephonenetworkPacketswitchingMessageisbrokenintoaseriesofpacketsEachnodedeterminesnextlegoftransmissionforeachpacketPhasesofCircuitSwitchingCircuitestablishmentAnendtoendcircuitisestablishedthroughswitchingnodesInformationTransferInformationtransmittedthroughthenetworkDatamaybeanalogvoice,digitizedvoice,orbinarydataCircuitdisconnectCircuitisterminatedEachnodedeallocatesdedicatedresourcesCharacteristicsofCircuitSwitchingCanbeinefficientChannelcapacitydedicatedfordurationofconnectionUtilizationnot100%DelaypriortosignaltransferforestablishmentOnceestablished,networkistransparenttousersInformationtransmittedatfixeddataratewithonlypropagationdelay

ComponentsofPublicTelecommunicationsNetworkSubscribers-devicesthatattachtothenetwork;mostlytelephonesSubscriberline-linkbetweensubscriberandnetworkAlsocalledsubscriberlooporlocalloopExchanges-switchingcentersinthenetworkAswitchingcentersthatsupportsubscribersisanendofficeTrunks-branchesbetweenexchanges

HowPacketSwitchingWorks

Dataistransmittedinblocks,calledpacketsBeforesending,themessageisbrokenintoaseriesofpacketsTypicalpacketlengthis1000octets(bytes)PacketsconsistsofaportionofdataplusapacketheaderthatincludescontrolinformationAteachnodeenroute,packetisreceived,storedbrieflyandpassedtothenextnode

PacketSwitchingPacketSwitchingPacketSwitchingAdvantagesLineefficiencyisgreaterManypacketsovertimecandynamicallysharethesamenodetonodelinkPacket-switchingnetworkscancarryoutdata-rateconversionTwostationswithdifferentdataratescanexchangeinformationUnlikecircuit-switchingnetworksthatblockcallswhentrafficisheavy,packet-switchingstillacceptspackets,butwithincreaseddeliverydelayPrioritiescanbeusedDisadvantagesofPacketSwitchingEachpacketswitchingnodeintroducesadelayOverallpacketdelaycanvarysubstantiallyThisisreferredtoasjitterCausedbydifferingpacketsizes,routestakenandvaryingdelayintheswitchesEachpacketrequiresoverheadinformationIncludesdestinationandsequencinginformationReducescommunicationcapacityMoreprocessingrequiredateachnode

PacketSwitchingNetworks-DatagramEachpackettreatedindependently,withoutreferencetopreviouspacketsEachnodechoosesnextnodeonpacket’spathPacketsdon’tnecessarilyfollowsamerouteandmayarriveoutofsequenceExitnoderestorespacketstooriginalorderResponsibilityofexitnodeordestinationtodetectlossofpacketandhowtorecover

PacketSwitchingNetworks–DatagramAdvantages:CallsetupphaseisavoidedBecauseit’smoreprimitive,it’smoreflexibleDatagramdeliveryismorereliablePacketSwitchingNetworks–VirtualCircuitPreplannedrouteestablishedbeforepacketssentAllpacketsbetweensourceanddestinationfollowthisrouteRoutingdecisionnotrequiredbynodesforeachpacketEmulatesacircuitinacircuitswitchingnetworkbutisnotadedicatedpathPacketsstillbufferedateachnodeandqueuedforoutputoveralinePacketSwitchingNetworks–VirtualCircuitAdvantages:PacketsarriveinoriginalorderPacketsarrivecorrectlyPacketstransmittedmorerapidlywithoutroutingdecisionsmadeateachnodeEffectofPacketSizeonTransmissionEffectofPacketSizeonTransmission

BreakinguppacketsdecreasestransmissiontimebecausetransmissionisallowedtooverlapFigure3.9aEntiremessage(40octets)+headerinformation(3octets)sentatonceTransmissiontime:129octet-timesFigure3.9bMessagebrokeninto2packets(20octets)+header(3octets)Transmissiontime:92octet-times

EffectofPacketSizeonTransmissionFigure3.9cMessagebrokeninto5packets(8octets)+header(3octets)Transmissiontime:77octet-timesFigure3.9dMakingthepacketstoosmall,transmissiontimestartsincreasesEachpacketrequiresafixedheader;themorepackets,themoreheadersAsynchronousTransferMode(ATM)AlsoknownascellrelayOperatesathighdataratesResemblespacketswitchingInvolvestransferofdataindiscretechunks,likepacketswitchingAllowsmultiplelogicalconnectionstobemultiplexedoverasinglephysicalinterfaceMinimalerrorandflowcontrolcapabilitiesreducesoverheadprocessingandsizeFixed-sizecellssimplifyprocessingatATMnodes

ATMTerminologyVirtualchannelconnection(VCC)LogicalconnectioninATMBasicunitofswitchinginATMnetworkAnalogoustoavirtualcircuitinpacketswitchingnetworksExchangesvariable-rate,full-duplexflowoffixed-sizecellsVirtualpathconnection(VPC)BundleofVCCsthathavethesameendpoints

AdvantagesofVirtualPathsSimplifiednetworkarchitectureIncreasednetworkperformanceandreliabilityReducedprocessingandshortconnectionsetuptimeEnhancednetworkservicesCallEstablishmentVirtualChannelConnectionUses

BetweenendusersCancarryend-to-enduserdataorcontrolsignalingbetweentwousersBetweenanenduserandanetworkentityUsedforuser-to-networkcontrolsignalingBetweentwonetworkentitiesUsedfornetworktrafficmanagementandroutingfunctionsVirtualPath/VirtualChannelCharacteristicsQualityofserviceSpecifiedbyparameterssuchascelllossratioandcelldelayvariationSwitchedandsemipermanentvirtualchannelconnectionsCellsequenceintegrityTrafficparameternegotiationandusagemonitoringVirtualchannelidentifierrestrictionwithinaVPCATMCellHeaderFormatGenericflowcontrol(GFC)–4bits,usedonlyinuser-networkinterfaceUsedtoalleviateshort-termoverloadconditionsinnetworkVirtualpathidentifier(VPI)–8bitsattheuser-networkinterface,12bitsatnetwork-net