《通信工程专业英语（第二版）》课件Unit 13

Unit13TextAStoredProgramControlStoredProgramControlStoredprogramcontrol(SPC)isatelecommunicationstechnologyusedfortelephoneexchangescontrolledbyacomputerprogramstoredinthememoryoftheswitchingsystem.SPCwastheenablingtechnologyofelectronicswitchingsystems(ESS)developedintheBellSystem[1]inthe1950s.TextTextWordsNotesDiscussionHomeEarlyexchangessuchasStrowger,panel,rotary,andcrossbarswitcheswereconstructedpurelyfromelectromechanicalswitchingcomponentswithanalogcontrolelectronics,andhadnocomputersoftwarecontrol.StoredprogramcontrolwasinventedbyBellLabs[2]scientistErnaSchneiderHooverin1954whoreasonedthatcomputersoftwarecouldcontroltheconnectionoftelephonecalls.TextTextWordsNotesDiscussionHomeSPCwasintroducedinelectronicswitchingsystemsinthe1960s.The101ESSPBXwasatransitionalswitchingsystemintheBellSystemtoprovideexpandedservicestobusinesscustomersthatwereotherwisestillservedbyanelectromechanicalcentralofficeswitch,whiletheWesternElectric1ESSswitchandtheAXEtelephoneexchangebyEricssonwerelarge-scalesystemsinthepublicswitchedtelephonenetwork.SPCenabledsophisticatedcallingfeatures.AsSPCexchangesevolved,reliabilityandversatilityincreased.Theadditionoftimedivisionmultiplexing(TDM)decreasedsubsystemsizesanddramaticallyincreasedthecapacityofthetelephonenetwork.Bythe1980s,SPCtechnologydominatedthetelecommunicationsindustry.TextWordsNotesDiscussionHome1.IntroductionTheprinciplefeatureofstoredprogramcontrolisoneormultipledigitalprocessingunits(storedprogramcomputers)thatexecuteasetofcomputerinstructions(program)storedinthememoryofthesystembywhichtelephoneconnectionsareestablished,maintained,andterminatedinassociatedelectroniccircuitry.TextWordsNotesDiscussionHomeAnimmediateconsequenceofstoredprogramcontrolisautomationofexchangefunctionsandintroductionofavarietyofnewtelephonyfeaturestosubscribers.TextWordsNotesDiscussionHomeAtelephoneexchangemustruncontinuouslywithoutinterruptionatalltimes,byimplementingafault-tolerantdesign.Earlytrialsofelectronicsandcomputersinthecontrolsubsystemsofanexchangeweresuccessfulandresultedinthedevelopmentoffullyelectronicsystems,inwhichtheswitchingnetworkwasalsoelectronic.AtrialsystemwithstoredprogramcontrolwasinstalledinMorris,Illinoisin1960.Itusedaflying-spotstorewithawordsizeof18bitsforsemi-permanentprogramandparameterstorage,andabarrier-gridmemoryforrandomaccessworkingmemory.Theworld’sfirstelectronicswitchingsystemforpermanentproductionuse,theNo.1ESS,wascommissionedbyAT&TatSuccasunna,NewJersey,inMay1965.By1974,AT&Thadinstalled475No.1ESSsystems.Inthe1980sSPCdisplacedelectromechanicalswitchinginthetelecommunicationindustry,hencethetermlostallbuthistoricalinterest.TodaySPCisastandardfeatureinallelectronicexchanges.TextWordsNotesDiscussionHomeTheattemptstoreplacetheelectromechanicalswitchingmatricesbysemiconductorcrosspointswitcheswerenotimmediatelysuccessful,particularlyinlargeexchanges.Asaresult,manyspacedivisionswitchingsystemsusedelectromechanicalswitchingnetworkswithSPC.Nonetheless,privateautomaticbranchexchanges(PABX)andsmallerexchangesdouseelectronicswitchingdevices.TextWordsNotesDiscussionHome2.TypesStoredprogramcontrolimplementationsmaybeorganizedintocentralizedanddistributedapproaches.Earlyelectronicswitchingsystems(ESS)developedinthe1960sand1970salmostinvariablyusedcentralizedcontrol.AlthoughmanypresentdayexchangedesigncontinuetousecentralizedSPC,withadventoflowcostpowerfulmicroprocessorsandVLSIchipssuchasprogrammablelogicarray(PLA)andprogrammablelogiccontrollers(PLC),distributedSPCbecamewidespreadbytheearly21stcentury.TextWordsNotesDiscussionHome(1)CentralizedcontrolIncentralizedcontrol,allcontrolequipmentisreplacedbyacentralprocessingunit.Itmustbeabletoprocess10to100callspersecond,dependingontheloadtothesystem.Multiprocessorconfigurationsarecommonplaceandmayoperateinvariousmodes,suchasinload-sharingconfiguration,insynchronousduplex-mode,oroneprocessormaybeinstand-bymode.TextWordsNotesDiscussionHome1)StandbymodeStandbymodeofoperationisthesimplestofadual-processorconfiguration.Normallyoneprocessorisinstandbymode.Thestandbyprocessorisbroughtonlineonlywhentheactiveprocessorfails.Animportantrequirementofthisconfigurationisabilityofstandbyprocessortoreconstitutethestateofexchangesystemwhenittakesoverthecontrol;meanstodeterminewhichofthesubscriberlinesortrunksareinuse.TextWordsNotesDiscussionHomeInsmallexchanges,thismaybepossiblebyscanningthestatussignalsassoonasthestandbyprocessorisbroughtintoaction.Insuchacaseonlythecallswhicharebeingestablishedatthetimeoffailurearedisturbed.Inlargeexchangesitisnotpossibletoscanallthestatussignalswithinasignificanttime.Heretheactiveprocessorcopiesthestatusofsystemperiodicallyintosecondarystorage.Whenswitchoveroccurstherecentstatusfromthesecondarymemoryisloaded.Inthiscaseonlythecallswhichchangestatusbetweenlastupdateandfailureareaffected.Thesharedsecondarystorageneednottobeduplicatedandsimpleunitlevelredundancywouldsuffice.1ESSswitchwasaprominentexample.TextWordsNotesDiscussionHome2)SynchronousduplexmodeInsynchronousduplexmodeofoperationhardwarecouplingisprovidedbetweentwoprocessorswhichexecutesamesetofinstructionsandcomparetheresultscontinuously.Ifmismatchoccursthenthefaultyprocessorisidentifiedandtakenoutofservicewithinafewmilliseconds.Whensystemisoperatingnormally,thetwoprocessorshavesamedatainmemoriesatalltimesandsimultaneouslyreceiveinformationfromexchangeenvironment.Oneoftheprocessoractuallycontrolstheexchange,buttheotherissynchronizedwiththeformerbutdoesnotparticipateintheexchangecontrol.(tobecontinued)TextWordsNotesDiscussionHome(Continued)Ifafaultisdetectedbythecomparatortheprocessorsaredecoupledandacheck-outprogramisrunindependentlytofindfaultyprocessor.Thisprocessrunswithoutdisturbingthecallprocessingwhichissuspendedtemporarily.Whenoneprocessoristakenoutthentheotherprocessoroperatesindependently.Whenthefaultyprocessorisrepairedandbroughtinservicethenmemorycontentsoftheactiveprocessorarecopiedintoitsmemoryandthetwoaresynchronizedandcomparatorisenabled.TextWordsNotesDiscussionHomeItispossiblethatacomparatorfaultoccursonlyduetotransientfailurewhichisnotshownevenwhencheckoutprogramisrun.Insuchcasethreepossibilitiesexists:·Continuewithbothprocessors·Takeouttheactiveprocessorandcontinuewiththeother.·Continuewithactiveprocessorbutremoveotherprocessorfromservice.TextWordsNotesDiscussionHomeScheme1isbasedontheassumptionthatthefaultistransientoneandmaynotappearagain.Inscheme2and3theprocessortakenoutissubjectedtoextensivetestingtoidentifyamarginalfailureinthesecases.TextWordsNotesDiscussionHome3)Load-sharingmodeInload-sharingoperation,anincomingcallisassignedrandomlyorinapredeterminedordertooneoftheprocessorswhichthenhandlesthecallrightthroughcompletion.Thus,boththeprocessorsareactivesimultaneouslyandsharetheloadandtheresourcesdynamically.Boththeprocessorshaveaccesstotheentireexchangeenvironmentwhichissensedaswellascontrolledbytheseprocessors.Sincethecallsarehandledindependentlybytheprocessors,theyhaveseparatememoriesforstoringtemporarycalldata.Althoughprogramsandsemi-permanentdatacanbeshared,theyarekeptinseparatememoriesforredundancypurposes.TextWordsNotesDiscussionHomeTheChineseandUSnetworkswillbothusedarkfiberstosendquantumkeys.Butthesefibers“arenotalwaysavailableandcanbeprohibitivelyexpensive”,saysAndrewShields,aquantumphysicistatToshibaResearchEuropeinCambridge,UK.Onewaytosidesteptheproblemistopiggybackthephotonstreamsontothe‘lit’fibersthattransmitconventionaltelecommunicationsdata.However,thoseconventionaldatastreamsareusuallyaboutamilliontimesstrongerthanquantumstreams,sothequantumdatatendtobedrownedout.TextWordsNotesDiscussionHomeThereisaninterprocessorlinkthroughwhichtheprocessorsexchangeinformationneededformutualcoordinationandverifyingthe'stateofhealth'oftheother.Iftheexchangeofinformationfails,oneoftheprocessorswhichdetectthesametakesovertheentireloadincludingthecallsthatarealreadysetupbythefailingprocessor.However,thecallsthatwerebeingestablishedbythefailingprocessorareusuallylost.Sharingofresourcescallsforanexclusionmechanismsothatboththeprocessorsdonotseekthesameresourceatthesametime.Themechanismmaybeimplementedinsoftwareorhardwareorboth.TextWordsNotesDiscussionHome(2)DistributedcontrolDistributedSPCisbothmoreavailableandmorereliablethancentralizedSSPCTextWordsNotesDiscussionHome1)VerticaldecompositionWholeexchangeisdividedintoseveralblocksandaprocessorisassignedtoeachblock.Thisprocessorperformsallthetaskrelatedtothatspecificblocks.Therefore,thetotalcontrolsystemconsistsofseveralcontrolunitscoupledtogether.Forredundancypurposeprocessormaybeduplicatedineachblocks.TextWordsNotesDiscussionHomeHecalculatesthatitwouldbepossibletosendQKDsignalsalongside40conventionaldatachannels.Opticalfibersusuallycarrybetween40and160telecommunicationschannels,meaningthatquantumcommunicationcouldbecarriedoutwithexistinginfrastructure.TextWordsNotesDiscussionHome2)HorizontalDecompositionInthistypeofdecompositioneachprocessorperformsonlyoneorsomeexchangefunction.TextWordsNotesDiscussionHomememoryn.[计]存储器，内存；记忆，

paneln.仪表板；嵌板rotaryadj.&n.旋转的，转动的；[动力]转缸式发动机crossbarn.横木；横条；横梁，横档electromechanicaladj.[电][机]电动机械的transitionaladj.渐变的，转变的；变迁的WordsWordsTextWordsNotesDiscussionHomeversatilityn.多用途；可转动性executevt.执行；处死，处决parametern.参数；参量；限制因素barrier-gridn.障栅commissionvt.委任，授予；使服役displacevt.移动，移走；替换，取代WordsTextWordsNotesDiscussionHomeprogrammableadj.可设计的，可编程的duplexadj.有两部分的standbyn.&adj.备用品；备用的reconstitutevt.再组成，再构成sufficev.足够；有能力duplicatev.&adj.复制；重复；完全一样的WordsTextWordsNotesDiscussionHomeredundancyn.（机器的）多余度；过多synchronousadj.同时存在[发生]的synchronizevt.使同步；使同时comparatorn.比较仪，比较器；比测器transientadj.短暂的；转瞬即逝的predeterminev.预先裁定；注定

couplev.连在一起，连接decompositionn.分解；腐烂WordsTextWordsNotesDiscussionHome[1]BellSystem：贝尔本人创立的贝尔电话公司曾形成庞大的贝尔系统（BellSystem），并垄断美国的电信事业达百年之久，贝尔系统以AT&T公司为母公司，下属众多子公司和研究所。TextWordsNotesDiscussionNotesHome[2]BellLabs：美国贝尔实验室是晶体管、激光器、太阳能电池、发光二极管、数字交换机、通信卫星、电子数字计算机、蜂窝移动通信设备、长途电视传送、仿真语言、有声电影、立体声录音，以及通信网等许多重大发明的诞生地。自1925年以来，贝尔实验室共获得两万五千多项专利，现在，平均每个工作日获得三项多专利。TextWordsNotesDiscussionHomeQuestionsfordiscussion1.WhatisStoredProgramControl?2.WhatmainfeaturedoesSPCbear?3.HowmanycategoriesdoesSPCfallinto?Whatarethey?4.Whatisstandbymode?TextWordsNotesDiscussionHomeAnswerstoquestionsfordiscussionTextWordsNotesDiscussionHome1.WhatisStoredProgramControl?Storedprogramcontrol(SPC)isatelecommunicationstechnologyusedfortelephoneexchangescontrolledbyacomputerprogramstoredinthememoryoftheswitchingsystem.SPCwastheenablingtechnologyofelectronicswitchingsystems(ESS)developedintheBellSysteminthe1950s.TextWordsNotesDiscussionHome2.WhatmainfeaturedoesSPCbear?ThemainfeatureofSPCisoneormultipledigitalprocessingunits(storedprogramcomputers)thatexecuteasetofcomputerinstructions(program)storedinthememoryofthesystembywhichtelephoneconnectionsareestablished,maintained,andterminatedinassociatedelectroniccircuitry.TextWordsNotesDiscussionHome3.HowmanycategoriesdoesSPCfallinto?Whatarethey?SPCfallsintotwocategories:centralizedSPCanddistributedSPC.Morespecifically,storedprogramcontrolimplementationsmaybeorganizedintocentralizedanddistributedapproaches.Earlyelectronicswitchingsystems(ESS)developedinthe1960sand1970salmostinvariablyusedcentralizedcontrol.AlthoughmanypresentdayexchangedesigncontinuetousecentralizedSPC,withadventoflowcostpowerfulmicroprocessorsandVLSIchipssuchasprogrammablelogicarray(PLA)andprogrammablelogiccontrollers(PLC),distributedSPCbecamewidespreadbytheearly21stcentury.TextWordsNotesDiscussionHome4.Whatisstandbymode?Standbymodeofoperationisusuallyreferredtoasthesimplestofadual-processorconfiguration.Normally,oneprocessorisinstandbymode.Thestandbyprocessorisbroughtonlineonlywhentheactiveprocessorfails.Animportantrequirementofthisconfigurationisabilityofstandbyprocessortoreconstitutethestateofexchangesystemwhenittakesoverthecontrol;meanstodeterminewhichofthesubscriberlinesortrunksareinuse.TextWordsNotesDiscussionHomeThankyou!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di