讲义教程分析26switch principle and techniques

SwitchprincipleandCircuitswitchingisamethodology[ˌmeθəˈdɔlədʒi:]（方法论）ofimplementing municationsnetworkinwhichtwonetworknodesestablishadedicated（的）communicationschannel(circuit)throughthenetworkbeforethenodesmaycommunicate.Thecircuitguaranteesthefullbandwidthofthechannelandremainsconnectedforthedurationofthecommunicationsession.Thecircuitfunctionsasifthenodeswerephysicallyconnectedaswithanelectricalcircuit.Packetswitchingisadigitalnetworkingcommunicationsmethodthatgroupsalltransmitteddata–regardlessofcontent,type,orstructure–intosuitablysizedblocks,calledpackets.Twomajorpacketswitchingmodesconnectionlesspacketswitching,alsoknownasdatagramconnection-orientedpacketswitching,alsoknownasvirtualcircuitInthefirstcaseeachpacketincludescompleteaddressingorroutinginformationThepacketsareroutedindividually,sometimesresultingindifferentpathsandout-of-orderdelivery.Inthesecondcaseaconnectionisdefinedandpreallocated（预分配）ineachinvolvednodeduringaconnectionsetupphasebeforeanypacketistransferred.Thepacketsincludeaconnectionidentifierratherthanaddressinformationandaredeliveredinorder.SomeconnectionlessprotocolsareEthernet,IP,andUDP;connectionorientedpacket-switchingprotocolsincludeX.25,Framerelay,MultiprotocolLabelSwitching(MPLS),andInconnection-orientednetworks,eachpacketislabeledwithaconnectionIDratherthananaddress.Addressinformationisonlytransferredtoeachnodeduringaconnectionset-upswitchingtableineachnetworknodethroughwhichtheconnectionpasses.Thesignallingprotocolsusedallowtheapplicationtospecifyitsrequirementsandthenetworktospecifywhatcapacityetc.isavailable,andacceptablevaluesforserviceparameterstobenegotiated.Routingapacketisverysimple,asitjustrequiresthenodetolookuptheIDinthetable.Thepacketheadercanbesmall,asitonlyneedstocontaintheIDandanyinformation(suchaslength,timestamp,orsequencenumber)whichisdifferentfordifferentpackets.Inconnectionlessnetworkseachpacketislabeledwithadestinationaddresssourceaddress,andportnumbers;itmayalsobelabeledwiththesequencenumberofthepacket.Thisprecludestheneedforadedicatedpathtohelpthepacketfinditswaytoitsdestination,butmeansthatmuchmoreinformationisneededinthepacketheader,whichisthereforelarger,andthisinformationneedstobelookedupinpower-hungry(耗电的)content-addressablememory（可寻址器）.Eachpacketisdispatchedandmaygoviadifferentroutes;potentially,thesystemhastodoasmuchworkforeverypacketastheconnection-orientedsystemhastodoinconnectionset-up,butwithlessinformationastotheapplication'srequirements.Atthedestination,theoriginalmessage/dataisreassembledinthecorrectorder,basedonthepacketsequencenumber.Thusavirtualconnection,alsoknownasavirtualcircuitorbytestreamisprovidedtotheend-userbyatransportlayerprotocol,althoughintermediatenetworknodesonlyprovidesaconnectionlessnetworklayerserviceTheerlang(symbolE[1])isadimensionlessunit（无量纲单）thatisusedinephonyasameasureofofferedloadorcarriedloadonservice-providingelementssuchasephonecircuitsorephoneswitchingequipment.Forexample,asinglecordcircuithasthecapacitytobeusedfor60minutesinonehour.Ifonehundredsix-minutecallsarereceived,thenthetotaltrafficinthathourissixhundredminutesor10TrafficTrafficmeasurementsof ephoneOfferedtraffic(inerlangs)isrelatedtothecallarrivalrate,λ,andtheaveragecall-holdingtime,h,by:SpacedivisionTheyprovideaseparatephysicalconnectionbetweeninputsandoutputs.sothedifferentsignalsareseparatedinspace.Crossbarswitchissaidtobenonblocking(无阻塞)。CLOSTimeswitch(T型交换机TimeswitchconsistsofSM(话音器)andCM（控制器Spaceswitch(S型交换机SignallingSystemNo.7(SS7)isasetofephonysignalingprotocolswhichareusedtosetupmostoftheworld'spublicswitchedephonenetworkephonecalls.Themainpurposeistosetupandteardownephonecalls.Otherusesincludenumbertranslation,localnumberportability,prepaidbillingmechanisms（预计费机制）,shortmessageservice(SMS),andavarietyofothermassmarketservices.ItisusuallyreferencedasSignallingSystemNo.7orSignallingSystem#7,orsimplyabbreviatedtoSS7.InNorthAmericaitisoftenreferredtoasCCSS7,anabbreviationforCommonChannelSignallingSystem7.InsomeEuropeancountries,specificallytheUnitedKingdom,itissometimescalledC7(CCITTnumber7)andisalsoknownasnumber7andCCIS7(CommonChannelInterofficeSignaling7).InGermanyitisoftencalledasN7(SignalisierungssystemNummer7).ThereisonlyoneinternationalSS7protocoldefinedbyITU-TinitsQ.700-seriesmendations.[1]Therearehowever,manynationalvariantsoftheSS7protocols.MostnationalvariantsarebasedontwowidelydeployednationalvariantsasbyANSIandETSI,whichareinturnbasedontheinternationalprotocoldefinedbyITU-T.Eachnationalvarianthasitsownuniquecharacteristics.SomenationalvariantswithratherstrikingcharacteristicsaretheandJapanese(TTC)nationalvariants.TheInternetEngineeringTask(IETF)hasalsodefinedlevel2,3,and4protocolsthatarecompatiblewithSS7:MessageTransferPart(MTP)level2(M2UAandMessageTransferPart(MTP)level3SignallingConnectionControlPart(S)butuseaStreamControlTransmissionProtocol(SCTP)transportmechanism.ThissuiteofprotocolsiscalledSIGTRANSS7SS7protocolSS7protocolsbyOSIINAP,MAP,IS-TCAP,CAP,ISUP,MTPLevel3+DataMTPLevelMTPLeveldedicatedcommunicationschannel通信信道Circuitswitching电路交Packetswitching分组交 datagramswitching数据报交virtualcircuitswitching虚电路交换preallocateconnectionset-upphase连接建立阶段erlang话务量的单位Offeredtraffic流入话务量callarrivalrate平均呼叫次数 averagecall-holdingtime呼叫平均占用时长nonblocking无阻塞Timeswitch T型交换机 Spaceswitch S型交换机SignallingSystemNo.7 7号信令ThesummaryofswitchprincipleandSwitchmodeis