Основна информация
Theessenceofpacketswitchingistidividethatatatobetransmittedintomanygroupsaccordingtoacerationlengy.Inordertoycurateytransmittotheotherparty, всеки групов, в който са.TransmissioniSperformedOnthephysicallineinadynamicsharingandmultiplexingmanner.Inordertomakefulluseofresources,whendatapacketsaretransmittedtotheswitch,theywillbetemporarilystoredinthememoryoftheswitch,andthenaccordingtothecurrentline'sbusynessandidleness,theswitchwilldynamicallyallocatetheappropriateThephysicallinecontinuesthetransmissionofdatapacketsuntiltheyaredeliveredtothedestination.След повторно повторно отстраняване, thedatapacketsareRecombinedtoformacompleteData.
Thepacketiscomposedofthepacketheaderandtheuserdatapartafterit.Thepacketheadercontainsthereceivingaddressandcontrolinformation, anditslengthis3--10b.TheLengthoftheuserdataPartisfixed, withanaGerof128bandamaximumof256b.HeSisaproblemthatneedstobeexplained: thepacketlengthInthesamepacketnetworkisfixed, butThePacketLengthIndIfferentPacketNetWorkscanbedifferent.Пакетно изпълнение: afterroutinghasterminedtheoutpoutportAndAndThenextNode, SwitchingTechnologymustbeusedtotransferthepacketfromTheInputportOtheAutportOrealizethransmissionofbitthonthenetworknode.
PacketSwitchingTechnologyIsthEdevelopmentOfComputerTeRtechnologyToAncereSextExtent.IndditionToMakingDirectCalls, PeopleCancommunicateBetWeEncomputersandterminalsthroughcompustersandterminals.Въпреки това, качеството на предавания ,низиснотиандът и договорените технологии..
PacketSwitchingIsalSocalledPacketSwitching.Itdividestheadatatransmitted от theuserintomultiplesmallerpartsofequallengy.EveryPartisCalledAdataSegment.FeesherComposedofsomenecessarycontrolinformationisadadedinfrontofeachdataSementtormapacket.TheHeaderisUsedToIndIcateTheAddressWherethePacketissent, и thentheswitchforwardsTheMtotheDestinationAncordingTotheadDressflagofeachpacket.Thisprocessiscalledpacketswitching.TheCommunicationnetworkforpacketswitchingiscalledapacketswitchingnetwork.Packetswitchingisessentylydevelopedonthebasisof "съхраняване и напред".Ithastheadvantagesofcircuitswitchingandmessagesgeatching.
InThePacketSwitchingMode, becauseTheDataCanbetEmporalyStoreDAndExchangEdInpacketMode, следпроцесоринг отtheswitch, itiSeasyTorealizetheCommunicationbetweEntermanalsofdifferentspeedsanddifferentprocedures.
Развитие на Hevelophistory
Около1970 г., PeopleBegantostudyanewformoflong-distancedigitalDataCommunicationArchitecture: Packetswitching.Въпреки че thePacketSwitchingTechnologyusenshasmadegreatprogresscomparedwithattime, thebasictechnologyofpacketswitchingtodayisbasicalthesameashenetworktechnologyofthe1970s, andpackswitchingisstilloneofthefeweeftwayworeRealizElongongingingisstilloneofthefefeftivewayworearizeLongenscensetacomentication.OneOfTheTechnologies.AndTheTwolatestwantechnologies: framerelayandatm, arebasicalvariantsofpacketswitchinging.
PacketSwitchingisDevelopedAsatechnologyToSolveinterActiveProcessingApplications.ItisDesignedToSupportTheTransmissionofburstydatastreams.TheContinuentConnectionTimeofThisservicestreamislongandtheserviceTheAmountislow.Thepacketswitchingnetworkusesstatisticalmultiplexingtechnology, thatis, multiplesessionConnectionscanshareacmunicationchannel, kondoubtedlygreatlyimprovesthetransmissionefitiictive, kondoubtedlygreatlyimprovesthetransmissionefticity.Въпреки това, споделяне на комуникационни линкинтродукуестимедела.Следователно, akeyissueWeMustConsiderInthefutureishowthepacket-switchEdnetworktransmitsdelay-sensitivesviceSremstreams, suchasreal-timeservicestreams.Inapacket-switchednetwork, packetsareRoutedThroughaseriesofintermediatenodes, обикновено AACROSSMULTIPLENETWORKS.Те са били наета, в която се намират международничари (ierouters) instore-and forwardnerner и finallyreachthedestination.По време на тетрансмисията theinformationsidividedIntOpAcketsEctaintAntingSteneStanationAdDressSteSeSeSeenceEmumber.
Класификация
Според TotheImplementationMode, PacketSwitchingCanbedividedIntodataGrampacketSwitchingAndVirtualCircuitPacketSwitching.
Datagrampacketswitching
DatagrampacketswitchingrequiresatlestonedatatatransmissionpathbetweEntheCommunicationparties.Thesenderneedstopreparethatatapackettobetransmittedbeforecommunication.ThedatapacketContainsTheadDressInformationOfTheSenderAndAndTheReceiver.Тетрансмисионефдапакетциспосочен отстъпник, не е включен, не е често срещан, canReachTheDetinationAcdingTodiferEfferEntroutingMechanismisms, Andrecombined.
InTheDway, EverypacketisattachedWithSourCeandDestinationAddresses, PacketNumber, PacketStartandEndFlags, ErrorCheckingAndotherInformationInacerforterFormat, AndistransmittedIntheformofpacketsonTheNetwork.ThenetworkjusttriesitsiTsBestTodeLiverThePackEtSTOTHEDESTINATIONEHOSTHOST, BUTITDOESNOTGUARANTEETHATTHETRANTEDMITTEDTPACKWILLNOTBELOST, NORTHATTHEPACKETSWILLIVEATTHEREIViVIeNENTHEORDENWHICHEWERESSETESTEN.Следователно, theserviceprovidedbytheenetworkiSunrelable, и достойнство от serviceissnotguaranteed.AsshownInfigure9-2 (a), някои опаковки, които се отнасят до Fomhosth1toh5gothroughnodebe, somegothroughaceorabce и andsomepacketssentfromhosth2toh6gothroughnodebde, и somegothroghbe.Thedatagrammethodissgenerallysuitableforshorler, едногрупиеми.ItsAdvantageIsThatThetransmissionЗабавянеissmall, andTheSubsequentPacketTransmissionWillnotBeaffectedwhenanodefails.ThedisaDvantageIsHateachPackethasalotofAdditionAlcontrolinformation, което се възползва от това, че е разработен допустител и обработване на времето, в което се представя, и по -големият опит в главата..
VirtualCircuitPacketSwitching
ThedifferenceBetweenItandTheDatagrammethodismanlylyhatbeforeInformationExchange, AlogicalConnectionNeedStoBeStableDableTweEntEsdenderAndTheReceiverBeRestartIngTransfachets, AllPacketsAreexchangedAndArwardEdAmeMepathe.ThenWorkGuaranteEsthatTheTransmittedPacketsArviveTheReceivingEndInTheOrderInhichetheywereper.Следователно, theserviceprovidedbytheenetworkiSreliAbleAndThequality ofServiceIsGuaranteed.AsshownInfigure9-2 (b), allpacketssentfromhosth1toh5passthroughesamenodea-b-e, andallpacketssentfromhosth2toh6alsopastroughtesamenodeb-e.
Thismethodisnotsuitableforusershithhighfrequencyofinformationtransmissionandsmalltransmissionvolumeeachtime.Въпреки това, тъй като ieampacketheaderonlyneedstomarkTheVirtualCircuitifierifierAndAndEseenceEmumber, thepacketheaderoverheadissmall.Подходяща замесетагетрансмисия.
VirtualCircuitPacketSwitchingisLikikIrcuitswitching.И CommunicationPartiesNeedToEstablishaconnection, ButunlikeciriTswitching, theConnectionOfPacketSwitchIndisavirtualConnection (alsoknowasavirtualcircuit), и така, че инаексклузивекенкцията в кошара свързване.Физическо пробиване.Според TotheImplementationOfVirtualConnections, VirtualCircuitsCanbedividedIntoSwitchedVirtualCircuitsandPermanEntualCircuits.
SwitchingVirtualCircuitsRequiresBothPartiesIncommunicationSoEstablishatemporaryConnectionTherequest, и в общуване.WhentHecommunicationSends, thetemporaryConnectionIstorndown.
PermanentvirtualCircuitMeansTheTTheTPArtiesIncommunicationsdonotneedtorequest, само neeedToEstablishaconnectionCordingTotheAgreEmentbetweenthetwoparties, andkeepitiTfortheagreedtimetime.
Itcanbeobtainsfromthis, theCharacteristicsoftheconnection-ориентираната работа methodandthenon-connectionworkingmethod.
(1) Характеристики Ориентирани на свързване на копчетата
InicitisaphysicalConnectionOralogic-ориентиране на връзката, theCommunicationProcessCanbedividedIntothreestages: connectionStablismentAndAndTransmissionOfinformation, theConnectionSremoved.
OnCetheconnectionSestisted, allinformationofthecommunicationsistransmittedalongthislinkpath, и и нататък, в които е формиране (the OrderFyDendingInformationIsconsistentitheorderofreceiviveIvingInformation).
Thetimedelayofinformationtransmissionissmallerthanthetimedelayoftheconnectionless -thowkingmode.
OnCetheSestabledConnectionFails, theinformationtransmissionwillbeinterruptedandtheconnectionmustbere-strabled, soitissinsitivetofailures.
(2) theCharacteristicsofconnectionless -workingmode
След това еноконатистиране на процеса, andTheRouteissected andTheNformationSistransmittedAgain.
TheinformationBelongingTothesAmeCommunicationArcialsatreTheDestinationalongdifferentpaths.ThepathcannotbepredictiNadvanceandtheorderoftheinformationcannotbeguaranteed (theOrderFordingInformationAnsIssistEntITHEREGUARANTEED).
Thetimedelayofinformationtransmissionisgreaterthanthetimedelayoftheconnection-ориентиран работен метод.
InsensitiveTonetworkfailures.
FeatureFpacketSwitching
(1) thesmallestuniTofinformationTransmissionSipacket
Thepacketconsistsofagroupheaderanduserinformation, и thePacketheaderContainsroutingandcontrolinformation.
(2) TwoWorkingModes, ориентирана към връзката (LogicalConnection) и безконтрол
ThevirtualCircuitAdoptsTheConnection-ориентиран работенмод, и ethedatagramisTheConnectionslessworkingmode
(3) Статистически времеви-дивизионниплексинг (DynamicBandwidthAllocation)
Thebasicprincipleofstatisticaltime-divisionmultiplexingistividivideImeIntImeslicesOfUneAquallyngle, andtimesliceswithdifferentLengthSaretotransMitDifferentLengthSTHetimERETIREDFORPINGISNOTALLOCATESEDTIMESTETIMERIREDFORGUPINGISNOTALLOCATEDEDEDTIMESFORECMOMOMICANCY,.Thismeansthatthelengthoftimethattemultiplexlineisusedtotransmitthepacket, който е направен от това, че етълтистичният период-дивизионнаплексингдинамично албума.
(4) InformationTransmissionIserror-контролиран
PacketSwitchingisAswitchingMethodSpeciallyDesignedfordAtacmunicationnicationnetworks.TheCharacteristicsofdataServicesarehighrelibilityandreal-timeRequirements.Itisnotashighastelephonecommunication, soinordertoensuretheribityOfDateAnformationAnpacketSwitching, errorcontrolmechanismssuchascrccheckandretransmissionaresettomeettheneedsofdataservicecharacteristics.
(5) InformationTransmissionIsnotTransparent
PacketSwitchingMustProcessTheTransmittedDataInformation, SOCASSPLITTITITYANDREORGANIZINGININGINTERFORMATION.
(6) FlowControlbasedoncalldelaysystem
Inpacketswitching, whenthedataflowislarge, thepacketsarequeuedforprocessinginsteadofimmediatecalllosslikecirititswitchingthere, за да се появи, за да се увеличи, за да се появи, за да се появи, за да се появим..
Предимства
Сравнете WithCircuit-SwitchedNetwork, PacketSwitchingNetWorkhasmanyПредимства
HighututizationRate
сравнителна задача на въртенето на pe-litchainIntermsIntermsofChanneLexclusivity, различни DiftnedApacketscanbetransmittedonthesamelinkinadynamicsharingandmultiplexingmanner, и да се освободим, нататък communicationsircesishigh, което GreatyimProvestHecapacity andThroughputOfTheChannel, който GreatyimprovestHecapacityOfCoumanciationsiShanl.BecauseasingLelinkFromNodeTOnoDecanbedynamicalsharedByManyPackets.PacketsarequeuedandTransmittedonthelinkasquicklyaspossossible.
Скорост на данните
ApacketSwitchingNetWorkCanPerformСкорост на даннитеConversion: TwostationsWithDifferentСкорост на даннитеsCanexChangePackets, INTEACHSTATATIONESISTOWNATHATHESCONDECTEDTOTHISNODE.
Механизъм на опашка
OnTheSamelink, DataOfDifferentTypesandspecificationscanbetransmittedattheSaMetime.WhenthereArealArgenumberofPacketsonThePacketNetwork, thequeuecanbesetAccordingTotheDatatransmissionMechanicismyNeSureThatPacketSwithHighПриоритетAretransmittedfirst.WhentHeloadonTheCircuit-SwitchedNetworkiSheavy, Somecallsareblocked.OnaPacket-SwitchedNetwork, PacketsarestillAccepted, ButtheirdEliveryDeLayWillIncrease.
Приоритет
WhenusingПриоритет, ifanodehasalargenumberofpacketswaitingtobetransmitted, itcantransmithigh prioritypacketsfirst.Thesepacketswillhereforeexperiencelessdelaythanlowerprioritypackets.
Недостатъци
Сравнете WithCircuit-SwitchedNetworks, пакетно-превключване на смяна.
Забавяне
ApacketPassestHroughapacket-SwitchedNetworkThereWillBeatimedElayhentHenodeIsconsed, Buthereisnosuchtimedelayinthecircuit-SwitchEdnetwork.
Забавянеjitter
BecauseThePacketsbetweenAgivenSourcEstationAndDestinationStationMayhavedifferentLengths, те се възползват от тях, ortheycanbeinswitchesalongtway.Различни timeЗабавянеsareexperiencedInthenetwork, sothetotaltimedelayofthegroupingmayvarygreatly.Thisphenomenoniscalledjitter.JitterisUndesirableforsome приложения (Forexample, Inreal-Time Applications SouchastelephonevoiceAndreal-TimeMages).
Highoverhead
TotransmitPacketsoverTheNetwork, Допълнително overheadInformationAndingludingTheDretanationAdDressAndPacketOrderingInformationMustBeAddedToeeChpacket.ThisInformationReaduCESTHECOMMUNICATIONCAPACTICETHATCANBEUSESESTOTRANSPORTUSERDATA.Incircuitswitching, oncethecircuitististabled, thetoverheadsarenolonneeded.В допълнение, THEPACKET-SWITCHEDNETWORKISACOLLECECTIONOFDISTRITED PACKET-SWETCHED SUMES.Inanidealsituation, allpacket-switchedNodesshouldwaydaysunderdthestateoftheentireNetwork.За съжаление, becausetheenodesaredistributed, там ще бъде.Последствие, thetransferofstatusInformationRequiresacerateAmountofcost, SAPACKET-SWITCHEDNETWORKWILLNEVEEROPERATE "Перфектно".
ExchangePrinciple
ThereAreTyPosofTerminalShangingOnThePacketSwitchingNetwork: PackettypeTermanalsAndGeneralterminals.Theso-calledpacket-typepererminalsendsandreceivesinformationintheeformofpackets; като цяло iinformationsentAndReceivedByTheTerminalIsamessage, който neneedSpoBepressedtythesubassemblyanddisasemblyequipmentbeforeitcancessthepackettithingnetworksworks работи.IftheThesendingTerminAliSageneralerminal, thepadwillSplitTheMessAgesItsEndSIntoseverAlpAcketsAndThenDitTothePAcket-SwitchEdNetWorkForTransmission; ifthereceivingerminalisageneralerminal, thepadwillre-createseveralpacketsbelongingtoamessage.AssembleTheMessageAndsendittothegeneralerminal.
Thebasicprincipleofpacketswitchingististousete „технология за“ магазини и напред.WhenAmessageissentFromTheSourCestation, theSessageIsivividedintoafixedFormatPacket (пакет) и и цялостноемнидсердадскоретакета.AfterCepeivingThePacketsfromTheSourCestation, theswitchesIntheNetWorkTemporlailyStoretheminTheMemory, andThenAccordingTotheProvidenceDestinationAddresses, те се засилват, които се намират в продължение на onstinetyselectidlepathsporforwardhroundwitchesInthenetwork.ThissolvesthecommunicationbetweEndifferenttypesofusers, anddoesnotneedtoEstablishysicalpathforalongtimeduringtheTransmissionProcessLikeciritiTschitching, butcanbemultiplexedonthesamelineintheunitofpacket,.
PacketSwitchingisAswitchingTechnologythatCombinestHeadVantagesofcircuitswitchingingAndAgsageWitching.ThecircuitswitchingProcessissimilartomakingaphonecall.Whenauserneedstosenddata, theCallingPartyNeedStomakeAcall, andTheCalledPartyCompleSteShitchingNetWorkBeforeStableAfhySicalConnectionDataPathititItitItitITITITITITit.Whentheconnectionneedstobedisconned, itisdonebye titheroftheTwoCommunicationparties..IstaracteristicistishatitisuitablefordingendinglargequantitiesofinformationatoneTime.DuetothelongtimetoEstablishaconnection, theefficiencyislowwhentransmittingshortmessages.AndititelyCompatibleWithTheCommunicationPartiesIntermsOfInformationTransmissionRate, кодиране на формат, комуникационен протокол и т.н.., което limitsTheCommunicationbetweenUsersOfDifferentRates, различните кодиращи форми, и differentCommunicationProtocols.Thebasicprincipleofmessageexchangeistousethe „технология за„ съхраняване и напред “.Присъствие на товари от веществото, theDestinationAddressIsaddedTothemessage, и thentheswitchinTheNetWorkWillTemporalyStoreTheMessageAgefromthesourcestationIntheMemoryAfterReceivingitiviveIvIvingitit., AndTheNAccordingTotheProvidedDestinationAddress, continueToSelectidlepathsforwordingThorthOtherswitchesInthenetwork, и finAllySenditTotheDestinationAdddress.Това елюбиево общуване betTweEndifferentTypesOfUsers, anddoesnotneedtoEstablishysicalpathforalongtimeinthetransmissionprocesslikecirititschitching, sogreatlyiplexedonthesamelineintheunitofmessage, sogreatlyimpretheuteutiletreatofmescessage, sogreatlyimprovetheuteutiletateftoftofmeine.Въпреки това, този Methodhasalongtimedelayandlargetimedelaychanges.Itisnotsuitableforreal-timeandconversational communication, bustissuitableforservicessuchase-mail, computerfiles и publictelegrams.
Packetswitchingstillusesthe"store-and-forward"technology,butitisnotexchangedintheunitofmessagelikemessageexchange,butthemessageisdividedintofixedformatpackets(Packet)forexchange,Transmission,generally1kbittothousandsofbits,eachpacketisattachedwithsourceanddestinationaddresses,packetnumber,packetstart,endflag,errorcheckandotherinformationaccordingtoacertainformat,andistransmittedinthenetworkinpacketform.WhenthesourceDTEtransmitsthepackettothelocalpacketswitchPSEintheformofabitstring,thelocalPSEreceivestheforwardinginformationrequiredbyeachpacket,regardlessofwhetheritisconnectedtothedestinationaddressdevice,firststoresit,andthenchecksthedestinationaddressandsavesitinthePSEFindthesendingpathspecifiedbythedestinationaddressintheroutingtable,andthePSEwillforwardthepacketatthemaximumallowablesendingrate.По същия начин, всеки transitpSestoresandforwardseachpacketinthiswayuntilthepacketisdeliveredtothedestinationpse, и тетсепсейеливеридатинатинация Адресдте (Seefigure8-7).WhatistransmittedIntheAbove-mentionedwayisthedataGramwayinpacketswitching.Като цяло прилагайте FlicableToShortersinglePacketMessages.ItsПредимстваarehightransmissionRelibilityandlowtransmissionЗабавяне.BecauseTheMemoryCapacityOnThePseIsdeduced, theeconomyisimpreved.ThedisaDvantageIsTheachPacketAddsMoreControlinformation, която се засилва, за да се преодолее и процесиране на времето, в което емисионна информация.Greatadditionaloverhead.
Ornishmodeofpacketswitchingiscalledvirtualcircuitmode.ThemaindifferencebetweenTandTheAgramamodeiShatbeforeInformationExchange, thesourcedtesendspecificCallRequestPacketTotheLocalpse, което е отстъпка от съществено значение.TheaddressandlogicalChanlidentifieroftheedteareeReforwardedbythepse.IfthecallisacceptedByTheDestinationDte, theCorrepsponseResponse "callAcceptance" isanswered, andTheNetworksendsa "callconnection" toteSourcedte.По време на това, theCallisestilestassishedandalogicalpathcalledavirtualcircuitistabledabledbeTeweEntHetWodtes., Информация canbetransmittedonthisvirtualcircuituntiltheendofthedataexchange, thevirtualcircuitiSremoved и andTheCorredingLogicalChanLIdentifieriSreledededed.Следователно, производственото обграждане на MethodhastreestagesofvirtualCircuitEstablisment, DataTransmissionAndRemovalduringeach Communication, whisissimilartoteCircuitswitchingMethod, buttetransmissionIntheNetworkiSthepacketswitchingmethod.Thismethodisnotsuitableforusershithhighfrequencyofinformationtransmissionandsmalltransmissionvolumeeachtime.Въпреки това, тъй като iechpacketheaderonlyneedstobemarkedwithavirtualCircuitientifierAndAndEseenceEmumber, thepacketheaderhasasmalloverhadAndissuitableforlongmessagetransmissionmission.
Протокол за обмен
ThepublicpacketswitcheddataNetworkisanimportantabubliccommunicationplatformthatrealizesslong-distancedatatransmissionbetweendifferenttypesofcomputers, andisawide-areaconnectioncommonlyusedinationayway.Thex.25protoColformulated от theitu-tss, theTeleCommunicationSstandArdDepartmentOfTheAntaryationTeleCommunicationUnion, isanInternationalStandArdsupportedandabedByByManyTeleCmunicationsorganizationsations и производители в завойните съобщения на мрежата в завойните съобщения се прилагат идват в завойните съобщения.х.25networkisapublicdataNetworkwidleylyusensernationally.х.25WASInTRODUCEDBYTYLTMETIN1970, anditwaSTheFirst-GenerationPacketSwitchingsystem.Thex.25networkwasdevelopedtotransmitdata, soitisnotindirectcompetitionwithtelephoneserviceproviders.х.25packetswitchingtechnologyemergedtomeettheneedsofinteractiveserviesservices.InteractiveProcessingAppearedInthelate1960s.ItiSaburstydatastreamwithalongconnectiontimebutlowdatavolume.х.25providesatechnologythatallowsmultiplesessionStosharethesameCommunicationChannel.
Whenх.25networkproposedх.25technology, theneworkwasmainlyasimulated Environment.Amoreseriousproblemwithanalognetworksisthatnoisewillbeamplifiedwhenpasporthroughsomeamplifers, които ще бъдат щета, които ще бъдат нататък.Therefore,avalue-addedserviceprovidedbyх.25ISTOIMPLEMENTERRORCONTROLFUNCTIONSTITHINTHENETWORK.Beesherpacketswitchingisastore-and forwardtechnology, ateachintermediateNode, packeterrorsmustbedetectected.Ifeverythingisnormalforthepacket,theintermediatenodewillsendanacknowledgmentmessagetotheoriginalsendingnode;ifthereisanerrorinthepacketreceivedbytheintermediatenode,thenodewillsendamessagetorequestretransmission.Следователно, atanynodeofthepacketpath, ifiSfoundThattheAccumulationOfnoisecauseSanError, theerrorwillbecrected, sothatthedatastreamcanbetransmittedMoreAccuraty.JustlikethePSTNnetwork,theх.25Packet-Switchednetworkisalsohierarchical.
Intheх.25packetswitchingnetwork, packetswitchesividedintottoCategories: (1) oneTypeOfSwitchisclosetotheUserend, инфакт, терелокатиран отметка на токтеонета..FunctionsofThiStypeOfSwitchIncluderouting, пренасочване, anderrordectectionand и корекция.Inaddition,inordertosupporttheaccessofdifferenttypesofcomputers,tocompletetheconversionbetweendifferentprotocols,ortheconversionbetweendifferentworkingspeedsanddifferentencodingmethods,someadditionalfunctionsareneeded,andthesefunctionsareattheaccesspoint.Theswitchisdone.Inotherwords,becauseitcanperformthenecessaryconversionworkaccordingtoyourrequirements,andusethisconversionaspartofthenetworkbusiness,sothroughtheх.25network, youcanrealizetheconnectionbetweendifferenttypesofdevices.(2) thesecondtypeofswitchesareinsidethenetwork.Те donotprovideTheAforementionedhigh-levelvalue-добавени функции, butonlyCompleTepAcketrouting, препращане, anderrorcontrolanddetectioneection.Non-standardх.25terminalsneedtobeconnectedtoanх.25NetWorkThroughapAcketDisesEmblyDevice (PAD).PADcompletestheconversionoftheprotocolandgeneratesthepacketsspecifiedbytheх.25standard,sothatthedatacanbetransmittedthroughtheх.25network.Thesepadscanbeplacedontheusersideoronthenetworkside.х.25ISESSENCYLYASTANDARDACCESSPROTOCOLBETWIENSEREQUIPMANTANDPACKETSWITCHINGNETWORKSESTILLEDBYITU-T.ItdefinestheinterfacethroughwhichtheterminalinthepacketmodeconnectstothepublicdataNetworktroughadicedcircuit.Therearealsosomecommonlyusedх-seriesprotocols,theyare:
(1)х.28ISTHESTANDARDPROTOCOLBETWEENTERMINALEQUIPMENTANDPAD;
(2)х.29ISTHESTANDARDPROTOCOLBTEWINGPADANDTHENETWORK;
(3)х.75isagatewayprotocolfortheinterconnectionoftwoormorepacket-switchednetworks, oneofwhichcanbeadedicatiicedpacketdataNetwork, andTheOtherOnECAPUBUBLICPACKETDATANETWork, ortheycanbetWodeRentOpoReRnetWorksworks и.
Theadvantagesanddisadvantagesoftheх.25NetWork: tojudgetheadvantagesandaddisadvantagesofaparticulartechnology, firstofalldependsontheeenvironmentin whithichitislocated.Inabasicenvironmentofanaloginformation, noiseisabigproblem, soerrorcontrolisnecessary.Въпреки това, ateachintermediatenode, inadditionToSelectingArouteTeTerminethenexthop, errorcontrolisalsoimplementedOneachpacket, който willin-ceresetheend-endtransmissionЗабавяне е.Becausetheх.25Packet-SwitchedNetworkisonlyusedtotransmitdata, DalesorpacketlossisnotaverycriticalParameterforit.Anothercontributionofх.25isitspacketsize.ItusesRelallightsSmallPackets, обикновено128bytesor256bytes.Thevaluationoftheprosandconsofthisissusuealsochangesovertime.Duetonoisefactors,smallpacketsareveryadvantageousinх.25networks.IfthereisnoiseinThenetwork, там willbeerrors, sopacketsoftenneedtoberetransmitted.Очевидно е, че ретрансмисионноофрелативно се намира mallpacketsismoreEffientthanretransmissionoflongchunksofinformation.Therefore,х.25isspecificlydesignedtousesmallerpackets.По същия начин, защото.Thelinkrategenerallyrangesfrom56kbit/sto2mbit/s.
Theadvantagesofх.25areasfollows:(1)Sinceх.25isthefirsttoprovidelayer3networkaddressinformation,sothatpacketscanberoutedandrelayedinaseriesofintermediatenodesandnetworksIthasastrongaddressingfunction;(2)Duetotheuseofstatisticalmultiplexingtechnology,itsbandwidthutilizationrateisrelativelyhigh;(3)Thepacketcanbypassthecongestednodeandreconnectthroughotherconnectionsandnodes.Носене, което е по този начин, по този начин се появяват по-нататък; (4) itcancontinuetylydetectandcorrectalltypesoferrorsoneacintermediatenode, sothearrorcontrofflunctioncanbeimpreved; (5), когато се налага и се появяват, че е подведен, (5), когато се налага и се появяват, за да се проведе, (5) когато anodesandlinesfailtheroutecanbere-selected, sotheavailabilityishigh.
Thedisadvantagesofх.25areasfollows:(1)Thequeuingdelayislarge;(2)Thelow-speedcommunicationlink;(3)Thepacketsizeissmaller,andthebandwidthutilizationrateisnotasgoodasthatofthelargersizepacketThenewprotocolishigh;(4)ThereisnoQoSguarantee,soitisnotsuitablefordelay-sensitiveapplications;(5)Itisonlyusedtotransmitdata,andtodayweareworkinghardtofindacomprehensiveservicesolution.
Introductiontoх.25Protocol: AnimPortAntAntOfUSingPublicDataNetWorksISTHEINTERFACETHTHEMTEM.TheITUх.25StandArsawIdelyUsedInterface.Manypeopleusetheterm"х.25network",whichleadsmanypeopletomistakenlybelievethatх.25DefinesAnetWorkProtocol.ButthisisnotTheCase.х.25JustDefinEstheAgreementbetweendteAnddCeconnectedTothePublicDataNetwork (Фигура 9-4).Therefore,х.25canbestrictlyusedasauser-networkinterfaceorauser-userinterfacethroughapublicdataNetwork.Thex.25protocolreferstotheprotocolfortheinterfacebetweendataterminalequipment(DTE)anddatacircuitterminalequipment(DCE)usedbyterminalsthatworkinpacketmodeandconnectedthroughprivatecircuitsandpublicdatanetworks.ItdefineShethree-layerprotocolofThephysicallayer, thedatalInklayer и thePacketlayer (т.е. мрежа), която е corponsponthelowerthreelayersoftheiso/osiseven-layerModel.
(1)Physicallayer:Thebasicfunctionistoestablish,maintainanddismantlethephysicallinkbetweenDTEandDCE,whichdefinesthemechanical,electrical,functionalandregulatorycharacteristicsofthephysicallink,andprovidessynchronizationandfullDuplexpoint-to-pointbitstreamtransmissionmeans,theinterfacebetweenDTEandlocalDCEisstipulatedinх.21 Посочване.).ThelapbpecifiedbyThislayerthe (linkaccessprocedurebalanced) процедураизабалално подреждане на техдлкпроцедурата).ItmainlyspecifiestHeprocedures за най -ниския и ремонтнистични връзки, процедурите на информацията transmissionAfterestablishment, anderrorcontrol, flowcontrol и др..Последствие, thislayeralsostipulateShemulti-linkproceduremlp (multilinkprocedure), което canimprovethroughputand-relativeOfinformationBysimatylytransmittingInformationFramesonMultipleParallealdatalinks.
(3) Packetlayer (Networklayer): главно нацелвачепчетплейърпроцедури forexchangingcontrolinformationAndUserDataOnTheDte/dceinterface, andsecifiestHevirtualCircuitserviceProcedures, basicpacketstructure, dataPacketformat, и запис.ThelayeradoptstheprincipleoftimedivisionmultiplexingtorealizethataSourcedTeusesaphysicalciritoTocallmultipledestinationdtesforpacketdataexchange.Последствие, italsoprovidespermanentvirtualcircuitpvcservice, whisavirtualCircuitForUserSTousefixiledly.ThesourcencanusethevirtualcircuitwithouthavingtoEstablishacall.Theinterrelationshipofthevariouslayeredprotocolsinх.25.
NetworkComposition
ThenworkstructureofpacketswitchingisgenerallyComposedofpacketswitches, metworkmanagement centers, дистанционни централи, subassemblyanddisasesmblyequipment, пакети/безпакетни крайни, атрансмисионни линии.Композиция на оборудване.(1)Thepacketswitchrealizestheinterfaceprotocol(х·25)betweenthedataterminalandtheswitch,thesignalingprotocolbetweentheswitches(suchasх·75orinternalprotocol),andstoresandforwardsinpackets,andprovidespacketnetworkservices.Поддръжка, andcooperateSothethenetworkmanagementcenterTertocomperation, мониторинг, таксуване, контрол и т.н..Съгласно tothestatusofthepacketswitchinthenetwork, itcanbedividedintottotypes: transitswitchandlocalswitch; (2) thenworkManagementCenter (nmc) и thepacketswitchworktogethertoensurethenormaloperation ofTheNetwork.
ItsMainFunctionsIncludenetWorkganagement, UserManagement, Measurement Manyagement, BillingManagement, OperationandMaintAncemange, маршрутинг управление, NetworkStatisticsCollection, AndeCessaryControfffunctions и т.н.., whatarenetwork-wideManagement (3) themainfunctionoftheSubassemblyanddisasesmblydevice (pad) istoconvertthenon-packetformatofordinaryCterTermanalsIntogroupformats и groupTheDataStreamsOferTerminalintogroups, които саинтерлейевиплексоксиестиращинарогрупи, които саинтерлектродиплексипсипсипсингъдеотеагрегатехенти.Theotherpartythenconvertsthereceivedpacketformatintheoppositietedirection.(4) thefunctionofaremoteconcentratorissimilartoapacketswitch, anditustuensualcontainsthefunctionofapad.ItisonlyConnectedToApacketSwitchWithoutroutingFunction.ItiSusuedInareaswhereUserserERELALY CONCESTRATES, ANDISGENERALLYSTALLEDINTHETELECMOMUNICATIONSTARMEMENT.).Когато комуникирайки междуетални до крайно компютърни изводи, Routeselection andFlowControlarePerperformed.Itcanprovideavarietyofcommunicationprocedures, залавяне на данни, поддръжка и и т.н..