无线通信与网络chap3

CommunicationNetworksChapter3TypesofCommunicationNetworksTraditionalTraditionallocalareanetwork(LAN)Traditionalwideareanetwork(WAN)Higher-speedHigh-speedlocalareanetwork(LAN)Metropolitanareanetwork(MAN)High-speedwideareanetwork(WAN)SpeedandDistanceofCommunicationsNetworksCharacteristicsofWANsCoverslargegeographicalareasCircuitsprovidedbyacommoncarrierConsistsofinterconnectedswitchingnodesTraditionalWANsprovidemodestcapacity64000bpscommonBusinesssubscribersusingT-1service–1.544MbpscommonHigher-speedWANsuseopticalfiberandtransmissiontechniqueknownasasynchronoustransfermode(ATM)10sand100sofMbpscommonCharacteristicsofLANsLikeWAN,LANinterconnectsavarietyofdevicesandprovidesameansforinformationexchangeamongthemTraditionalLANsProvidedataratesof1to20MbpsHigh-speedLANSProvidedataratesof100Mbpsto1GbpsDifferencesbetweenLANsandWANsScopeofaLANissmallerLANinterconnectsdeviceswithinasinglebuildingorclusterofbuildingsLANusuallyownedbyorganizationthatownstheattacheddevicesForWANs,mostofnetworkassetsarenotownedbysameorganizationInternaldatarateofLANismuchgreaterTheNeedforMANsTraditionalpoint-to-pointandswitchednetworktechniquesusedinWANsareinadequateforgrowingneedsoforganizationsNeedforhighcapacityandlowcostsoverlargeareaMANprovides:ServicetocustomersinmetropolitanareasRequiredcapacityLowercostandgreaterefficiencythanequivalentservicefromtelephonecompanySwitchingTermsSwitchingNodes:IntermediateswitchingdevicethatmovesdataNotconcernedwithcontentofdataStations:EnddevicesthatwishtocommunicateEachstationisconnectedtoaswitchingnodeCommunicationsNetwork:AcollectionofswitchingnodesSwitchedNetworkObservationsofFigure3.3Somenodesconnectonlytoothernodes(e.g.,5and7)SomenodesconnecttooneormorestationsNode-stationlinksusuallydedicatedpoint-to-pointlinksNode-nodelinksusuallymultiplexedlinksFrequency-divisionmultiplexing(FDM)Time-divisionmultiplexing(TDM)NotadirectlinkbetweeneverynodepairTechniquesUsedinSwitchedNetworksCircuitswitchingDedicatedcommunicationspathbetweentwostationsE.g.,publictelephonenetworkPacketswitchingMessageisbrokenintoaseriesofpacketsEachnodedeterminesnextlegoftransmissionforeachpacketPhasesofCircuitSwitchingCircuitestablishmentAnendtoendcircuitisestablishedthroughswitchingnodesInformationTransferInformationtransmittedthroughthenetworkDatamaybeanalogvoice,digitizedvoice,orbinarydataCircuitdisconnectCircuitisterminatedEachnodedeallocatesdedicatedresourcesCharacteristicsofCircuitSwitchingCanbeinefficientChannelcapacitydedicatedfordurationofconnectionUtilizationnot100%DelaypriortosignaltransferforestablishmentOnceestablished,networkistransparenttousersInformationtransmittedatfixeddataratewithonlypropagationdelayComponentsofPublicTelecommunicationsNetworkSubscribers-devicesthatattachtothenetwork;mostlytelephonesSubscriberline-linkbetweensubscriberandnetworkAlsocalledsubscriberlooporlocalloopExchanges-switchingcentersinthenetworkAswitchingcentersthatsupportsubscribersisanendofficeTrunks-branchesbetweenexchangesHowPacketSwitchingWorksDataistransmittedinblocks,calledpacketsBeforesending,themessageisbrokenintoaseriesofpacketsTypicalpacketlengthis1000octets(bytes)PacketsconsistsofaportionofdataplusapacketheaderthatincludescontrolinformationAteachnodeenroute,packetisreceived,storedbrieflyandpassedtothenextnodePacketSwitchingPacketSwitchingPacketSwitchingAdvantagesLineefficiencyisgreaterManypacketsovertimecandynamicallysharethesamenodetonodelinkPacket-switchingnetworkscancarryoutdata-rateconversionTwostationswithdifferentdataratescanexchangeinformationUnlikecircuit-switchingnetworksthatblockcallswhentrafficisheavy,packet-switchingstillacceptspackets,butwithincreaseddeliverydelayPrioritiescanbeusedDisadvantagesofPacketSwitchingEachpacketswitchingnodeintroducesadelayOverallpacketdelaycanvarysubstantiallyThisisreferredtoasjitterCausedbydifferingpacketsizes,routestakenandvaryingdelayintheswitchesEachpacketrequiresoverheadinformationIncludesdestinationandsequencinginformationReducescommunicationcapacityMoreprocessingrequiredateachnodePacketSwitchingNetworks-DatagramEachpackettreatedindependently,withoutreferencetopreviouspacketsEachnodechoosesnextnodeonpacket’spathPacketsdon’tnecessarilyfollowsamerouteandmayarriveoutofsequenceExitnoderestorespacketstooriginalorderResponsibilityofexitnodeordestinationtodetectlossofpacketandhowtorecoverPacketSwitchingNetworks–DatagramAdvantages:CallsetupphaseisavoidedBecauseit’smoreprimitive,it’smoreflexibleDatagramdeliveryismorereliablePacketSwitchingNetworks–VirtualCircuitPreplannedrouteestablishedbeforepacketssentAllpacketsbetweensourceanddestinationfollowthisrouteRoutingdecisionnotrequiredbynodesforeachpacketEmulatesacircuitinacircuitswitchingnetworkbutisnotadedicatedpathPacketsstillbufferedateachnodeandqueuedforoutputoveralinePacketSwitchingNetworks–VirtualCircuitAdvantages:PacketsarriveinoriginalorderPacketsarrivecorrectlyPacketstransmittedmorerapidlywithoutroutingdecisionsmadeateachnodeEff