通信及基础工程20

Unit13TextBPacketSwitchingPacketSwitchingPacketswitchingisadigitalnetworkingcommunicationsmethodthatgroupsalltransmitteddataintosuitablysizedblocks,calledpackets,whicharetransmittedviaamediumthatmaybesharedbymultiplesimultaneouscommunicationsessions.Packetswitchingincreasesnetworkefficiency,robustnessandenablestechnologicalconvergenceofmanyapplicationsoperatingonthesamenetwork.TextTextWordsNotesDiscussionHomePacketsarecomposedofaheaderandpayload.Informationintheheaderisusedbynetworkinghardwaretodirectthepackettoitsdestinationwherethepayloadisextractedandusedbyapplicationsoftware.TextTextWordsNotesDiscussionHomeStartinginthelate1950s,AmericancomputerscientistPaulBaran[1]developedtheconceptDistributedAdaptiveMessageBlockSwitchingwiththegoaltoprovideafault-tolerant,efficientroutingmethodfortelecommunicationmessagesaspartofaresearchprogramattheRANDCorporation[2],fundedbytheUSDepartmentofDefense.Thisconceptcontrastedandcontradictedthen-establishedprinciplesofpre-allocationofnetworkbandwidth,largelyfortifiedbythedevelopmentoftelecommunicationsintheBellSystem.(tobecontinued)TextTextWordsNotesDiscussionHome(Continued)ThenewconceptfoundlittleresonanceamongnetworkimplementersuntiltheindependentworkofBritishcomputerscientistDonaldDavies[3]attheNationalPhysicalLaboratory(UnitedKingdom)inthelate1960s.Daviesiscreditedwithcoiningthemodernnamepacketswitchingandinspiringnumerouspacketswitchingnetworksinthedecadefollowing,includingtheincorporationoftheconceptintheearlyARPANETintheUnitedStates.TextTextWordsNotesDiscussionHome1.ConceptAsimpledefinitionofpacketswitchingis:

Theroutingandtransferringofdatabymeansofaddressedpacketssothatachannelisoccupiedduringthetransmissionofthepacketonly,anduponcompletionofthetransmissionthechannelismadeavailableforthetransferofothertraffic.TextTextWordsNotesDiscussionHomePacketswitchingfeaturesdeliveryofvariablebitratedatastreams,realizedassequencesofpackets,overacomputernetworkwhichallocatestransmissionresourcesasneededusingstatisticalmultiplexingordynamicbandwidthallocationtechniques.Astheytraversenetworknodes,suchasswitchesandrouters,packetsarereceived,buffered,queued,andtransmitted(storedandforwarded),resultinginvariablelatencyandthroughputdependingonthelinkcapacityandthetrafficloadonthenetwork.TextTextWordsNotesDiscussionHomePacketswitchingcontrastswithanotherprincipalnetworkingparadigm,circuitswitching,amethodwhichpre-allocatesdedicatednetworkbandwidthspecificallyforeachcommunicationsession,eachhavingaconstantbitrateandlatencybetweennodes.Incasesofbillableservices,suchascellularcommunicationservices,circuitswitchingischaracterizedbyafeeperunitofconnectiontime,evenwhennodataistransferred,whilepacketswitchingmaybecharacterizedbyafeeperunitofinformationtransmitted,suchascharacters,packets,ormessages.TextTextWordsNotesDiscussionHomePacketmodecommunicationmaybeimplementedwithorwithoutintermediateforwardingnodes(packetswitchesorrouters).Packetsarenormallyforwardedbyintermediatenetworknodesasynchronouslyusingfirst-in,first-outbuffering,butmaybeforwardedaccordingtosomeschedulingdisciplineforfairqueuing,trafficshaping,orfordifferentiatedorguaranteedqualityofservice,suchasweightedfairqueuingorleakybucket.Incaseofasharedphysicalmedium(suchasradioor10BASE5),thepacketsmaybedeliveredaccordingtoamultipleaccessscheme.TextTextWordsNotesDiscussionHome2.HistoryInthelate1950s,theUSAirForceestablishedawideareanetworkfortheSemi-AutomaticGroundEnvironment(SAGE)radardefensesystem.Theysoughtasystemthatmightsurviveanuclearattacktoenablearesponse,thusdiminishingtheattractivenessofthefirststrikeadvantagebyenemies.TextTextWordsNotesDiscussionHomeLeonardKleinrockconductedearlyresearchinqueueingtheory[4]whichprovedimportantinpacketswitching,andpublishedabookintherelatedfieldofdigitalmessageswitching(withoutthepackets)in1961;healsolaterplayedaleadingroleinbuildingandmanagementoftheworld'sfirstpacket-switchednetwork,theARPANET.TextTextWordsNotesDiscussionHomeTheconceptofswitchingsmallblocksofdatawasfirstexploredindependentlybyPaulBaranattheRANDCorporationintheUSandDonaldDaviesattheNationalPhysicalLaboratory(NPL)intheUKintheearly-to-mid-1960s.TextTextWordsNotesDiscussionHomeBarandevelopedtheconceptofdistributedadaptivemessageblockswitchingduringhisresearchattheRANDCorporationfortheUSAirForceintocommunicationsnetworks,thatcouldsurvivenuclearwars,firstpresentedtotheAirForceinthesummerof1961asbriefingB-265,laterpublishedasRANDreportP-2626in1962,andfinallyinreportRM3420in1964.ReportP-2626describedageneralarchitectureforalarge-scale,distributed,survivablecommunicationsnetwork.Theworkfocusesonthreekeyideas:useofadecentralizednetworkwithmultiplepathsbetweenanytwopoints,dividingusermessagesintomessageblocks,latercalledpackets,anddeliveryofthesemessagesbystoreandforwardswitching.TextTextWordsNotesDiscussionHomeBaran'sworkwasknowntoRobertTaylorandJ.C.R.LickliderattheInformationProcessingTechnologyOffice,whoadvocatedwideareanetworks,anditinfluencedLawrenceRobertstoadoptthetechnologyinthedevelopmentoftheARPANET.TextTextWordsNotesDiscussionHomeStartingin1965,DonaldDaviesattheNationalPhysicalLaboratory,UK,independentlydevelopedthesamemessageroutingmethodologyasdevelopedbyBaran.Hecalleditpacketswitching,amoreaccessiblenamethanBaran's,andproposedtobuildanationwidenetworkintheUK.Hegaveatalkontheproposalin1966,afterwhichapersonfromtheMinistryofDefence(MoD)toldhimaboutBaran'swork.AmemberofDavies'team(RogerScantlebury)metLawrenceRobertsatthe1967ACMSymposiumonOperatingSystemPrinciplesandsuggesteditforuseintheARPANET.TextTextWordsNotesDiscussionHomeDavieshadchosensomeofthesameparametersforhisoriginalnetworkdesignasdidBaran,suchasapacketsizeof1024bits.In1966,DaviesproposedthatanetworkshouldbebuiltatthelaboratorytoservetheneedsofNPLandprovethefeasibilityofpacketswitching.TheNPLDataCommunicationsNetworkenteredservicein1970.TextTextWordsNotesDiscussionHomeThefirstcomputernetworkandpacketswitchingnetworkdeployedforcomputerresourcesharingwastheOctopusNetworkattheLawrenceLivermoreNationalLaboratorythatbeganconnectingfourControlData6600computerstoseveralsharedstoragedevices(includinganIBM2321DataCellin1968andanIBMPhotostorein1970)andtoseveralhundredTeletypeModel33ASRterminalsfortimesharingusestartingin1968.TextTextWordsNotesDiscussionHomeIn1973,VintCerfandBobKahnwrotethespecificationsforTransmissionControlProtocol(TCP),aninternetworkingprotocolforsharingresourcesusingpacket-switchingamongthenodes.TextTextWordsNotesDiscussionHome3.Connectionlessandconnection-orientedmodesPacketswitchingmaybeclassifiedintoconnectionlesspacketswitching,alsoknownasdatagramswitching,andconnection-orientedpacketswitching,alsoknownasvirtualcircuitswitching.TextTextWordsNotesDiscussionHomeExamplesofconnectionlessprotocolsareEthernet,InternetProtocol(IP),andtheUserDatagramProtocol(UDP).Connection-orientedprotocolsincludeX.25,FrameRelay,MultiprotocolLabelSwitching(MPLS),andtheTransmissionControlProtocol(TCP).TextTextWordsNotesDiscussionHomeInconnectionlessmodeeachpacketincludescompleteaddressinginformation.Thepacketsareroutedindividually,sometimesresultingindifferentpathsandout-of-orderdelivery.Eachpacketislabeledwithadestinationaddress,sourceaddress,andportnumbers.Itmayalsobelabeledwiththesequencenumberofthepacket.Thisprecludestheneedforadedicatedpathtohelpthepacketfinditswaytoitsdestination,butmeansthatmuchmoreinformationisneededinthepacketheader,whichisthereforelarger,andthisinformationneedstobelookedupinpower-hungrycontent-addressablememory.(tobecontinued)TextTextWordsNotesDiscussionHome(Continued)Eachpacketisdispatchedandmaygoviadifferentroutes;potentially,thesystemhastodoasmuchworkforeverypacketastheconnection-orientedsystemhastodoinconnectionset-up,butwithlessinformationastotheapplication'srequirements.Atthedestination,theoriginalmessage/dataisreassembledinthecorrectorder,basedonthepacketsequencenumber.Thusavirtualconnection,alsoknownasavirtualcircuitorbytestreamisprovidedtotheend-userbyatransportlayerprotocol,althoughintermediatenetworknodesonlyprovidesaconnectionlessnetworklayerservice.TextTextWordsNotesDiscussionHomeConnection-orientedtransmissionrequiresasetupphaseineachinvolvednodebeforeanypacketistransferredtoestablishtheparametersofcommunication.Thepacketsincludeaconnectionidentifierratherthanaddressinformationandarenegotiatedbetweenendpointssothattheyaredeliveredinorderandwitherrorchecking.Addressinformationisonlytransferredtoeachnodeduringtheconnectionset-upphase,whentheroutetothedestinationisdiscoveredandanentryisaddedtotheswitchingtableineachnetworknodethroughwhichtheconnectionpasses.(tobecontinued)TextTextWordsNotesDiscussionHome(Continued)Thesignalingprotocolsusedallowtheapplicationtospecifyitsrequirementsanddiscoverlinkparameters.Acceptablevaluesforserviceparametersmaybenegotiated.Routingapacketrequiresthenodetolookuptheconnectionidinatable.Thepacketheadercanbesmall,asitonlyneedstocontainthiscodeandanyinformation,suchaslength,timestamp,orsequencenumber,whichisdifferentfordifferentpackets.TextTextWordsNotesDiscussionHomepacketn.&vt.数据包，信息包；包装，打包robustnessn.[自]鲁棒性；[计]稳健性header

n.头球；页眉；数据头；收割台payload

n.有效负荷；收费载重，酬载resonancen.

[力]共振；共鸣；反响implementer

n.实施者；制订人；实现器WordsWordsTextWordsNotesDiscussionHometraversevt.穿过；反对；详细研究billableadj.可收取费用的，可计费的diminishv.使减少；使变小；减少，缩小decentralize

n.&v.分散；分散集权；疏散methodology

n.方法学，方法论datagramn.数据电报WordsTextWordsNotesDiscussionHomeprecludevt.排除；妨碍；阻止identifiern.标识符，认同者；检验人packetn.&vt.数据包，信息包；包装，打包robustnessn.[自]鲁棒性；[计]稳健性header

n.头球；页眉；数据头；收割台WordsTextWordsNotesDiscussionHome[1]PaulBaran（保罗·巴兰）是波兰裔美国工程师。上个世纪60年代初，巴兰在兰德（RAND）公司担任技术工程师。兰德组建于第二次世界大战之后，并扮演着美国军方智囊团的角色。在兰德工作期间，巴兰开发了一套新型通信系统。该系统的主要特色是：如果部分系统被核武器摧毁，整个通信系统仍能够保持运行。TextWordsNotesDiscussionNotesHome[2]RANDCorporation（兰德公司）是美国最重要的以军事为主的综合性战略研究机构。它先以研究军事尖端科学技术和重大军事战略而著称于世，继而又扩展到内外政策各方面，逐渐发展成为一个研究政治、军事、经济科技、社会等各方面的综合性思想库，被誉为现代智囊的“大脑集中营”、“超级军事学院”，以及世界智囊团的开创者和代言人。它可以说是当今美国乃至世界最负盛名的决策咨询机构。TextWordsNotesDiscussionHome[3]DonaldDavies英国计算机科学家。其主要贡献包括：参与了英国第一台计算机的研制；主持了英国第一个实验网的建设，开发了分组交换技术，使电脑能够彼此通信，并且使互联网成为了可能。TextWordsNotesDiscussionHome[4]queuingtheory（排队论），或称随机服务系统理论，是通过对服务对象到来及服务时间的统计研究，得出这些数量指标（等待时间、排队长度、忙期长短等）的统计规律，然后根据这些规律来改进服务系统的结构或重新组织被服务对象，使得服务系统既能满足服务对象的需要，又能使机构的费用最经济或某些指标最优。该理论广泛应用于计算机网络、生产、运输、库存等各项资源共享的随机服务系统。TextWordsNotesDiscussionHomeQuestionsfordiscussion1.Whatisthefeatureofpacketswitching?2.WhatdoesBaranreportin1962focuson?3.Listsomeexamplesofconnectionlessprotocols.TextWordsNotesDiscussionHomeAnswerstoquestionsfordiscussionTextWordsNotesDiscussionHome1.What