Basicinformation
Theessenceofpacketswitchingistodividethedatatobetransmittedintomanygroupsaccordingtoacertainlength.Inordertoaccuratelytransmittotheotherparty,eachgroupismarkedwithmanydifferentdatagroups.Transmissionisperformedonthephysicallineinadynamicsharingandmultiplexingmanner.Inordertomakefulluseofresources,whendatapacketsaretransmittedtotheswitch,theywillbetemporarilystoredinthememoryoftheswitch,andthenaccordingtothecurrentline'sbusynessandidleness,theswitchwilldynamicallyallocatetheappropriateThephysicallinecontinuesthetransmissionofdatapacketsuntiltheyaredeliveredtothedestination.Afterreachingthedestination,thedatapacketsarerecombinedtoformacompletedata.
Thepacketiscomposedofthepacketheaderandtheuserdatapartafterit.Thepacketheadercontainsthereceivingaddressandcontrolinformation,anditslengthis3--10B.Thelengthoftheuserdatapartisfixed,withanaverageof128Bandamaximumof256B.Thereisaproblemthatneedstobeexplained:thepacketlengthinthesamepacketnetworkisfixed,butthepacketlengthindifferentpacketnetworkscanbedifferent.Packetswitching:Afterroutinghasdeterminedtheoutputportandthenextnode,switchingtechnologymustbeusedtotransferthepacketfromtheinputporttotheoutputporttorealizethetransmissionofbitsthroughthenetworknode.
Packetswitchingtechnologyisthedevelopmentofcomputertechnologytoacertainextent.Inadditiontomakingdirectcalls,peoplecancommunicatebetweencomputersandterminalsthroughcomputersandterminals.However,thequalityoftransmissionlinesisnothighandthenetworktechnologymeansInasimplersituation,anexchangetechnologyemergedatthehistoricmoment.
Packetswitchingisalsocalledpacketswitching.Itdividesthedatatransmittedbytheuserintomultiplesmallerpartsofequallength.Eachpartiscalledadatasegment.Aheadercomposedofsomenecessarycontrolinformationisaddedinfrontofeachdatasegmenttoformapacket.Theheaderisusedtoindicatetheaddresswherethepacketissent,andthentheswitchforwardsthemtothedestinationaccordingtotheaddressflagofeachpacket.Thisprocessiscalledpacketswitching.Thecommunicationnetworkforpacketswitchingiscalledapacketswitchingnetwork.Packetswitchingisessentiallydevelopedonthebasisof"store-and-forward".Ithastheadvantagesofcircuitswitchingandmessageswitching.
Inthepacketswitchingmode,becausethedatacanbetemporarilystoredandexchangedinpacketmode,afterprocessingbytheswitch,itiseasytorealizethecommunicationbetweenterminalsofdifferentspeedsanddifferentprocedures.
Developmenthistory
Around1970,peoplebegantostudyanewformoflong-distancedigitaldatacommunicationarchitecture:packetswitching.Althoughthepacketswitchingtechnologyusedhasmadegreatprogresscomparedwiththattime,thebasictechnologyofpacketswitchingtodayisbasicallythesameasthenetworktechnologyofthe1970s,andpacketswitchingisstilloneofthefeweffectivewaystorealizelong-distancedatacommunication.Oneofthetechnologies.AndthetwolatestWANtechnologies:framerelayandATM,arebasicallyvariantsofpacketswitching.
Packetswitchingisdevelopedasatechnologytosolveinteractiveprocessingapplications.Itisdesignedtosupportthetransmissionofburstydatastreams.ThecontinuousconnectiontimeofthisservicestreamislongandtheserviceTheamountislow.Thepacketswitchingnetworkusesstatisticalmultiplexingtechnology,thatis,multiplesessionconnectionscanshareacommunicationchannel,whichundoubtedlygreatlyimprovesthetransmissionefficiency.However,sharingthecommunicationlinkintroducestimedelay.Therefore,akeyissuewemustconsiderinthefutureishowthepacket-switchednetworktransmitsdelay-sensitiveservicestreams,suchasreal-timeservicestreams.Inapacket-switchednetwork,packetsareroutedthroughaseriesofintermediatenodes,usuallyacrossmultiplenetworks.Theyareforwardedbetweenaseriesofpacketswitches(ierouters)inastore-and-forwardmanner,andfinallyreachthedestination.Duringthetransmission,theinformationisdividedintopacketscontainingthedestinationaddressandsequencenumber.
Classification
Accordingtotheimplementationmode,packetswitchingcanbedividedintodatagrampacketswitchingandvirtualcircuitpacketswitching.
Datagrampacketswitching
Datagrampacketswitchingrequiresatleastonedatatransmissionpathbetweenthecommunicatingparties.Thesenderneedstopreparethedatapackettobetransmittedbeforecommunication.Thedatapacketcontainstheaddressinformationofthesenderandthereceiver.Thetransmissionofdatapacketsisindependentofeachother,doesnotaffecteachother,canreachthedestinationaccordingtodifferentroutingmechanisms,andrecombined.
Inthisway,eachpacketisattachedwithsourceanddestinationaddresses,packetnumber,packetstartandendflags,errorcheckingandotherinformationinacertainformat,andistransmittedintheformofpacketsonthenetwork.Thenetworkjusttriesitsbesttodeliverthepacketstothedestinationhost,butitdoesnotguaranteethatthetransmittedpacketswillnotbelost,northatthepacketswillarriveatthereceivingendintheorderinwhichtheyweresent.Therefore,theserviceprovidedbythenetworkisunreliable,andthequalityofserviceisnotguaranteed.AsshowninFigure9-2(a),somepacketssentfromhostH1toH5gothroughnodeABE,somegothroughACEorABCE,andsomepacketssentfromhostH2toH6gothroughnodeBDE,andsomegothroughBE.Thedatagrammethodisgenerallysuitableforshorter,single-groupedmessages.Itsadvantageisthatthetransmissiondelayissmall,andthesubsequentpackettransmissionwillnotbeaffectedwhenanodefails.Thedisadvantageisthateachpackethasalotofadditionalcontrolinformation,whichincreasesthelengthandprocessingtimeofthetransmissioninformation,andincreasestheadditionaloverhead.
Virtualcircuitpacketswitching
Thedifferencebetweenitandthedatagrammethodismainlythatbeforeinformationexchange,alogicalconnectionneedstobeestablishedbetweenthesenderandthereceiverbeforestartingTransferpackets,allpacketsareexchangedandforwardedalongthesamepath,andthelogicalconnectionisremovedafterthecommunicationends.Thenetworkguaranteesthatthetransmittedpacketsarriveatthereceivingendintheorderinwhichtheyweresent.Therefore,theserviceprovidedbythenetworkisreliableandthequalityofserviceisguaranteed.AsshowninFigure9-2(b),allpacketssentfromhostH1toH5passthroughthesamenodeA-B-E,andallpacketssentfromhostH2toH6alsopassthroughthesamenodeB-E.
Thismethodisnotsuitableforuserswithhighfrequencyofinformationtransmissionandsmalltransmissionvolumeeachtime.However,sinceeachpacketheaderonlyneedstomarkthevirtualcircuitidentifierandsequencenumber,thepacketheaderoverheadissmall.Suitableforlongmessagetransmission.
Virtualcircuitpacketswitchingislikecircuitswitching.Bothcommunicationpartiesneedtoestablishaconnection,butunlikecircuitswitching,theconnectionofpacketswitchingisavirtualconnection(alsoknownasavirtualcircuit),andthereisnoexclusiveconnectionintheconnection.Physicalwiring.Accordingtotheimplementationofvirtualconnections,virtualcircuitscanbedividedintoswitchedvirtualcircuitsandpermanentvirtualcircuits.
Switchingvirtualcircuitsrequiresbothpartiesincommunicationtoestablishatemporaryconnectionthroughrequest,andthencommunicate.Whenthecommunicationends,thetemporaryconnectionistorndown.
Permanentvirtualcircuitmeansthatthetwopartiesincommunicationdonotneedtorequest,onlyneedtoestablishaconnectionaccordingtotheagreementbetweenthetwoparties,andkeepitfortheagreedtime.
Itcanbeobtainedfromthis,thecharacteristicsoftheconnection-orientedworkingmethodandthenon-connectionworkingmethod.
(1)Characteristicsofconnection-orientedworkingmethods
Whetheritisaphysicalconnectionoralogic-orientedconnection,thecommunicationprocesscanbedividedintothreestages:connectionestablishmentandtransmissionofinformation,Theconnectionisremoved.
Oncetheconnectionisestablished,allinformationofthecommunicationistransmittedalongthislinkpath,andtheorderoftheinformationisguaranteed(theorderofsendinginformationisconsistentwiththeorderofreceivinginformation).
Thetimedelayofinformationtransmissionissmallerthanthetimedelayoftheconnectionlessworkingmode.
Oncetheestablishedconnectionfails,theinformationtransmissionwillbeinterruptedandtheconnectionmustbere-established,soitissensitivetofailures.
(2)Thecharacteristicsofconnectionlessworkingmode
Thereisnoconnectionestablishmentprocess,andtherouteisselectedandtheinformationistransmittedagain.
Theinformationbelongingtothesamecommunicationarrivesatthedestinationalongdifferentpaths.Thepathcannotbepredictedinadvanceandtheorderoftheinformationcannotbeguaranteed(theorderofsendinginformationisinconsistentwiththeorderofreceivinginformation).
Thetimedelayofinformationtransmissionisgreaterthanthetimedelayoftheconnection-orientedworkingmethod.
Insensitivetonetworkfailures.
Featuresofpacketswitching
(1)Thesmallestunitofinformationtransmissionispacket
Thepacketconsistsofagroupheaderanduserinformation,andthepacketheadercontainsroutingandcontrolinformation.
(2)Twoworkingmodes,connection-oriented(logicalconnection)andconnectionless
Thevirtualcircuitadoptstheconnection-orientedworkingmode,andthedatagramistheconnectionlessworkingmode
(3)Statisticaltime-divisionmultiplexing(dynamicbandwidthallocation)
Thebasicprincipleofstatisticaltime-divisionmultiplexingistodividetimeintotimeslicesofunequallength,andtimesliceswithdifferentlengthsaretotransmitdifferentlengthsThetimerequiredforgroupingisnotallocatedafixedtimesliceforeachcommunication,butisusedondemand.Thismeansthatthelengthoftimethatthemultiplexlineisusedtotransmitthepacket,whichshowsthatthestatisticaltime-divisionmultiplexingdynamicallyallocatesbandwidth.
(4)Informationtransmissioniserror-controlled
Packetswitchingisaswitchingmethodspeciallydesignedfordatacommunicationnetworks.Thecharacteristicsofdataservicesarehighreliabilityandreal-timerequirements.Itisnotashighastelephonecommunication,soinordertoensurethereliabilityofdatainformationinpacketswitching,errorcontrolmechanismssuchasCRCcheckandretransmissionaresettomeettheneedsofdataservicecharacteristics.
(5)Informationtransmissionisnottransparent
Packetswitchingmustprocessthetransmitteddatainformation,suchassplittingandreorganizinginformation.
(6)Flowcontrolbasedoncalldelaysystem
Inpacketswitching,whenthedataflowislarge,thepacketsarequeuedforprocessinginsteadofimmediatecalllosslikecircuitswitchingTherefore,itsflowcontrolisbasedoncalldelay.
Advantages
Comparedwithcircuit-switchednetwork,packetswitchingnetworkhasmanyadvantages
highutilizationrate
comparedtocircuit-switchedpairchainIntermsofchannelexclusivity,differentdatapacketscanbetransmittedonthesamelinkinadynamicsharingandmultiplexingmanner,andtheutilizationrateofcommunicationresourcesishigh,whichgreatlyimprovesthecapacityandthroughputofthechannel.Becauseasinglelinkfromnodetonodecanbedynamicallysharedbymanypackets.Packetsarequeuedandtransmittedonthelinkasquicklyaspossible.
Datarate
Apacketswitchingnetworkcanperformdatarateconversion:twostationswithdifferentdataratescanexchangepackets,becauseeachstationusesitsowndataTherateisconnectedtothisnode.
Queuingmechanism
Onthesamelink,dataofdifferenttypesandspecificationscanbetransmittedatthesametime.Whentherearealargenumberofpacketsonthepacketnetwork,thequeuecanbesetaccordingtothedatatransmissionMechanismtoensurethatpacketswithhighpriorityaretransmittedfirst.Whentheloadonthecircuit-switchednetworkisheavy,somecallsareblocked.Onapacket-switchednetwork,packetsarestillaccepted,buttheirdeliverydelaywillincrease.
Priority
Whenusingpriority,ifanodehasalargenumberofpacketswaitingtobetransmitted,itcantransmithigh-prioritypacketsfirst.Thesepacketswillthereforeexperiencelessdelaythanlowerprioritypackets.
Disadvantages
Comparedwithcircuit-switchednetworks,packet-switchednetworksalsohavesomedisadvantages.
Delay
Apacketpassesthroughapacket-switchednetworkTherewillbeatimedelaywhenthenodeisconnected,butthereisnosuchtimedelayinthecircuit-switchednetwork.
Delayjitter
Becausethepacketsbetweenagivensourcestationanddestinationstationmayhavedifferentlengths,theycantakedifferentpaths,ortheycanbeinswitchesalongtheway.Differenttimedelaysareexperiencedinthenetwork,sothetotaltimedelayofthegroupingmayvarygreatly.Thisphenomenoniscalledjitter.Jitterisundesirableforsomeapplications(forexample,inreal-timeapplicationssuchastelephonevoiceandreal-timeimages).
Highoverhead
Totransmitpacketsoverthenetwork,additionaloverheadinformationincludingthedestinationaddressandpacketorderinginformationmustbeaddedtoeachpacket.Thisinformationreducesthecommunicationcapacitythatcanbeusedtotransportuserdata.Incircuitswitching,oncethecircuitisestablished,theseoverheadsarenolongerneeded.Inaddition,thepacket-switchednetworkisacollectionofdistributedpacket-switchednodes.Inanidealsituation,allpacket-switchednodesshouldalwaysunderstandthestateoftheentirenetwork.Unfortunately,becausethenodesaredistributed,thereisalwaysadelaybetweenachangeinthestateofonepartofthenetworkandtheotherpartofthenetworkknowingthechange.Inaddition,thetransferofstatusinformationrequiresacertainamountofcost,soapacket-switchednetworkwillneveroperate"perfectly".
Exchangeprinciple
Therearetwotypesofterminalshangingonthepacketswitchingnetwork:packettypeterminalsandgeneralterminals.Theso-calledpacket-typeterminalsendsandreceivesinformationintheformofpackets;generally,theinformationsentandreceivedbytheterminalisamessage,whichneedstobeprocessedbythesubassemblyanddisassemblyequipmentbeforeitcanaccessthepacketswitchingnetwork.Ifthesendingterminalisageneralterminal,thePADwillsplitthemessageitsendsintoseveralpacketsandthensendittothepacket-switchednetworkfortransmission;ifthereceivingterminalisageneralterminal,thePADwillre-createseveralpacketsbelongingtoamessage.Assemblethemessageandsendittothegeneralterminal.
Thebasicprincipleofpacketswitchingistousethe"store-and-forward"technology.Whenamessageissentfromthesourcestation,themessageisdividedintoafixedformatpacket(Packet),andthedestinationaddressisaddedtothepacket.Afterreceivingthepacketsfromthesourcestation,theswitchesinthenetworktemporarilystoretheminthememory,andthenaccordingtotheprovideddestinationaddresses,theycontinuouslyselectidlepathsforforwardingthroughotherswitchesinthenetwork,andfinallysendthemtothedestinationaddress.Thissolvesthecommunicationbetweendifferenttypesofusers,anddoesnotneedtoestablishaphysicalpathforalongtimeduringthetransmissionprocesslikecircuitswitching,butcanbemultiplexedonthesamelineintheunitofpacket,soitgreatlyimprovesTheutilizationrateofthelineisimproved.
Packetswitchingisaswitchingtechnologythatcombinestheadvantagesofcircuitswitchingandmessageswitching.Thecircuitswitchingprocessissimilartomakingaphonecall.Whenauserneedstosenddata,thecallingpartyneedstomakeacall,andthecalledpartycompletestheswitchingnetworkbeforeestablishingaphysicalconnectiondatapathwithit.Whentheconnectionneedstobedisconnected,itisdonebyeitherofthetwocommunicationparties..Itscharacteristicisthatitissuitableforsendinglargequantitiesofinformationatonetime.Duetothelongtimetoestablishaconnection,theefficiencyislowwhentransmittingshortmessages.Anditisfullycompatiblewiththecommunicationpartiesintermsofinformationtransmissionrate,encodingformat,communicationprotocol,etc.,whichlimitsthecommunicationbetweenusersofdifferentrates,differentencodingformats,anddifferentcommunicationprotocols.Thebasicprincipleofmessageexchangeistousethe"store-and-forward"technology.Whensendingamessagefromthesourcestation,thedestinationaddressisaddedtothemessage,andthentheswitchinthenetworkwilltemporarilystorethemessagefromthesourcestationinthememoryafterreceivingit.,Andthenaccordingtotheprovideddestinationaddress,continuetoselectidlepathsforforwardingthroughotherswitchesinthenetwork,andfinallysendittothedestinationaddress.Thissolvesthecommunicationbetweendifferenttypesofusers,anddoesnotneedtoestablishaphysicalpathforalongtimeinthetransmissionprocesslikecircuitswitching,butcanbemultiplexedonthesamelineintheunitofmessage,sogreatlyImprovetheutilizationrateoftheline.However,thismethodhasalongtimedelayandlargetimedelaychanges.Itisnotsuitableforreal-timeandconversationalcommunication,butissuitableforservicessuchase-mail,computerfiles,andpublictelegrams.
Packetswitchingstillusesthe"store-and-forward"technology,butitisnotexchangedintheunitofmessagelikemessageexchange,butthemessageisdividedintofixedformatpackets(Packet)forexchange,Transmission,generally1kbittothousandsofbits,eachpacketisattachedwithsourceanddestinationaddresses,packetnumber,packetstart,endflag,errorcheckandotherinformationaccordingtoacertainformat,andistransmittedinthenetworkinpacketform.WhenthesourceDTEtransmitsthepackettothelocalpacketswitchPSEintheformofabitstring,thelocalPSEreceivestheforwardinginformationrequiredbyeachpacket,regardlessofwhetheritisconnectedtothedestinationaddressdevice,firststoresit,andthenchecksthedestinationaddressandsavesitinthePSEFindthesendingpathspecifiedbythedestinationaddressintheroutingtable,andthePSEwillforwardthepacketatthemaximumallowablesendingrate.Similarly,eachtransitPSEstoresandforwardseachpacketinthiswayuntilthepacketisdeliveredtothedestinationpSE,andthenthePSEisdeliveredtothedestinationaddressDTE(seeFigure8-7).Whatistransmittedintheabove-mentionedwayisthedatagramwayinpacketswitching.Generallyapplicabletoshortersinglepacketmessages.Itsadvantagesarehightransmissionreliabilityandlowtransmissiondelay.BecausethememorycapacityonthePSEisreduced,theeconomyisimproved.Thedisadvantageisthateachpacketaddsmorecontrolinformation,whichincreasesthelengthandprocessingtimeofthetransmissioninformation.Greatadditionaloverhead.
Anothermodeofpacketswitchingiscalledvirtualcircuitmode.Themaindifferencebetweenitandthedatagrammodeisthatbeforeinformationexchange,thesourceDTEsendsaspecificcallrequestpackettothelocalPSE,whichcontainsthedestination.TheaddressandlogicalchannelidentifieroftheDTEareforwardedbythePSE.IfthecallisacceptedbythedestinationDTE,thecorrespondingresponse"callacceptance"isanswered,andthenetworksendsa"callconnection"tothesourceDTE.Atthistime,thecallisestablishedandalogicalpathcalledavirtualcircuitisestablishedbetweenthetwoDTEs.,Informationcanbetransmittedonthisvirtualcircuituntiltheendofthedataexchange,thevirtualcircuitisremoved,andthecorrespondinglogicalchannelidentifierisreleased.Therefore,thevirtualcircuitmethodhasthreestagesofvirtualcircuitestablishment,datatransmissionandremovalduringeachcommunication,whichissimilartothecircuitswitchingmethod,butthetransmissioninthenetworkisthepacketswitchingmethod.Thismethodisnotsuitableforuserswithhighfrequencyofinformationtransmissionandsmalltransmissionvolumeeachtime.However,sinceeachpacketheaderonlyneedstobemarkedwithavirtualcircuitidentifierandsequencenumber,thepacketheaderhasasmalloverheadandissuitableforlongmessagetransmission.
Exchangeprotocol
Thepublicpacketswitcheddatanetworkisanimportantpubliccommunicationplatformthatrealizeslong-distancedatatransmissionbetweendifferenttypesofcomputers,andisawide-areaconnectioncommonlyusedinternationallyWay.TheX.25protocolformulatedbytheITU-TSS,thetelecommunicationsstandarddepartmentoftheInternationalTelecommunicationUnion,isaninternationalstandardsupportedandabidedbybymanytelecommunicationsorganizationsandmanufacturersintheworld.X.25networkisapublicdatanetworkwidelyusedinternationally.X.25wasintroducedbyTyltmetin1970,anditwasthefirst-generationpacketswitchingsystem.TheX.25networkwasdevelopedtotransmitdata,soitisnotindirectcompetitionwithtelephoneserviceproviders.X.25packetswitchingtechnologyemergedtomeettheneedsofinteractiveservices.Interactiveprocessingappearedinthelate1960s.Itisaburstydatastreamwithalongconnectiontimebutlowdatavolume.X.25providesatechnologythatallowsmultiplesessionstosharethesamecommunicationchannel.
WhenX.25networkproposedX.25technology,thenetworkwasmainlyasimulatedenvironment.Amoreseriousproblemwithanalognetworksisthatnoisewillbeamplifiedwhenpassingthroughsomeamplifiers,whichwillresultinaveryhigherrorrate.Therefore,avalue-addedserviceprovidedbyX.25istoimplementerrorcontrolfunctionswithinthenetwork.Sincepacketswitchingisastore-and-forwardtechnology,ateachintermediatenode,packeterrorsmustbedetected.Ifeverythingisnormalforthepacket,theintermediatenodewillsendanacknowledgmentmessagetotheoriginalsendingnode;ifthereisanerrorinthepacketreceivedbytheintermediatenode,thenodewillsendamessagetorequestretransmission.Therefore,atanynodeofthepacketpath,ifitisfoundthattheaccumulationofnoisecausesanerror,theerrorwillbecorrected,sothatthedatastreamcanbetransmittedmoreaccurately.JustlikethePSTNnetwork,theX.25packet-switchednetworkisalsohierarchical.
IntheX.25packetswitchingnetwork,packetswitchesaredividedintotwocategories:(1)Onetypeofswitchisclosetotheuserend,infact,theyarelocatedattheaccesspointofthenetwork.Thefunctionsofthistypeofswitchincluderouting,forwardingpackets,anderrordetectionandcorrection.Inaddition,inordertosupporttheaccessofdifferenttypesofcomputers,tocompletetheconversionbetweendifferentprotocols,ortheconversionbetweendifferentworkingspeedsanddifferentencodingmethods,someadditionalfunctionsareneeded,andthesefunctionsareattheaccesspoint.Theswitchisdone.Inotherwords,becauseitcanperformthenecessaryconversionworkaccordingtoyourrequirements,andusethisconversionaspartofthenetworkbusiness,sothroughtheX.25network,youcanrealizetheconnectionbetweendifferenttypesofdevices.(2)Thesecondtypeofswitchesareinsidethenetwork.Theydonotprovidetheaforementionedhigh-levelvalue-addedfunctions,butonlycompletepacketrouting,forwarding,anderrorcontrolanddetection.Non-standardX.25terminalsneedtobeconnectedtoanX.25networkthroughapacketdisassemblydevice(PAD).PADcompletestheconversionoftheprotocolandgeneratesthepacketsspecifiedbytheX.25standard,sothatthedatacanbetransmittedthroughtheX.25network.ThesePADscanbeplacedontheusersideoronthenetworkside.X.25isessentiallyastandardaccessprotocolbetweenuserequipmentandpacketswitchingnetworksestablishedbyITU-T.Itdefinestheinterfacethroughwhichtheterminalinthepacketmodeconnectstothepublicdatanetworkthroughadedicatedcircuit.TherearealsosomecommonlyusedX-seriesprotocols,theyare:
(1)X.28isthestandardprotocolbetweenterminalequipmentandPAD;
(2)X.29isThestandardprotocolbetweenPADandthenetwork;
(3)X.75isagatewayprotocolfortheinterconnectionoftwoormorepacket-switchednetworks,oneofwhichcanbeadedicatedpacketdatanetwork,andtheotherOnecanbeapublicpacketdatanetwork,ortheycanbetwodifferentoperatornetworks,andsoon.
TheadvantagesanddisadvantagesoftheX.25network:Tojudgetheadvantagesanddisadvantagesofaparticulartechnology,firstofalldependsontheenvironmentinwhichitislocated.Inabasicenvironmentofanaloginformation,noiseisabigproblem,soerrorcontrolisnecessary.However,ateachintermediatenode,inadditiontoselectingaroutetodeterminethenexthop,errorcontrolisalsoimplementedoneachpacket,whichwillincreasetheend-to-endtransmissiondelay.BecausetheX.25packet-switchednetworkisonlyusedtotransmitdata,delayorpacketlossisnotaverycriticalparameterforit.AnothercontributionofX.25isitspacketsize.Itusesrelativelysmallpackets,generally128bytesor256bytes.Theevaluationoftheprosandconsofthisissuealsochangesovertime.Duetonoisefactors,smallpacketsareveryadvantageousinX.25networks.Ifthereisnoiseinthenetwork,therewillbeerrors,sopacketsoftenneedtoberetransmitted.Obviously,retransmissionofrelativelysmallpacketsismoreefficientthanretransmissionoflongchunksofinformation.Therefore,X.25isspecificallydesignedtousesmallerpackets.Similarly,becauseitisanearlynetwork,itissuitableforworkingonrelativelylow-speedlinks.Thelinkrategenerallyrangesfrom56kbit/sto2Mbit/s.
TheadvantagesofX.25areasfollows:(1)SinceX.25isthefirsttoprovidelayer3networkaddressinformation,sothatpacketscanberoutedandrelayedinaseriesofintermediatenodesandnetworksIthasastrongaddressingfunction;(2)Duetotheuseofstatisticalmultiplexingtechnology,itsbandwidthutilizationrateisrelativelyhigh;(3)Thepacketcanbypassthecongestednodeandreconnectthroughotherconnectionsandnodes.Carryingoutrouting,thusimprovingthecongestioncontrolability;(4)Itcancontinuouslydetectandcorrectalltypesoferrorsoneachintermediatenode,sotheerrorcontrolfunctioncanbeimproved;(5)WhennodesandlinesfailTheroutecanbere-selected,sotheavailabilityishigh.
ThedisadvantagesofX.25areasfollows:(1)Thequeuingdelayislarge;(2)Thelow-speedcommunicationlink;(3)Thepacketsizeissmaller,andthebandwidthutilizationrateisnotasgoodasthatofthelargersizepacketThenewprotocolishigh;(4)ThereisnoQoSguarantee,soitisnotsuitablefordelay-sensitiveapplications;(5)Itisonlyusedtotransmitdata,andtodayweareworkinghardtofindacomprehensiveservicesolution.
IntroductiontoX.25protocol:Animportantpartofusingpublicdatanetworksistheinterfacewiththem.TheITUX.25standardisawidelyusedinterface.Manypeopleusetheterm"X.25network",whichleadsmanypeopletomistakenlybelievethatX.25definesanetworkprotocol.Butthisisnotthecase.X.25justdefinestheagreementbetweenDTEandDCEconnectedtothepublicdatanetwork(Figure9-4).Therefore,X.25canbestrictlyusedasauser-networkinterfaceorauser-userinterfacethroughapublicdatanetwork.TheX.25protocolreferstotheprotocolfortheinterfacebetweendataterminalequipment(DTE)anddatacircuitterminalequipment(DCE)usedbyterminalsthatworkinpacketmodeandconnectedthroughprivatecircuitsandpublicdatanetworks.Itdefinesthethree-layerprotocolofthephysicallayer,thedatalinklayer,andthepacketlayer(ie,network),whichcorrespondtothelowerthreelayersoftheISO/OSIseven-layermodel.
(1)Physicallayer:Thebasicfunctionistoestablish,maintainanddismantlethephysicallinkbetweenDTEandDCE,whichdefinesthemechanical,electrical,functionalandregulatorycharacteristicsofthephysicallink,andprovidessynchronizationandfullDuplexpoint-to-pointbitstreamtransmissionmeans,theinterfacebetweenDTEandlocalDCEisstipulatedinX.21recommendations.(2)Datalinklayer:ThephysicallinkbetweentheDTEandthelocalpacketswitchPSE(PacketSwitchedEquipment)providesthepacketlayerwithwaitingforretransmissionanderrorcontrolpackettransmissionservices,sothereliabilityishigh.TheLAPBspecifiedbythislayerThe(LinkAccessProcedureBalanced)procedureisabalancedsubsetoftheHDLCprocedure.Itmainlyspecifiestheproceduresfortheestablishmentandremovalofdatalinks,theproceduresforinformationtransmissionafterestablishment,anderrorcontrol,flowcontrol,etc.Inaddition,thislayeralsostipulatesthemulti-linkprocedureMLP(MultiLinkProcedure),whichcanimprovethethroughputandreliabilityofinformationbysimultaneouslytransmittinginformationframesonmultipleparalleldatalinks.
(3)Packetlayer(networklayer):mainlydescribesthepacketlayerproceduresforexchangingcontrolinformationanduserdataontheDTE/DCEinterface,andspecifiesthevirtualcircuitserviceprocedures,basicpacketstructure,datapacketformat,andOptionaluserbusinessfunctions,etc.ThislayeradoptstheprincipleoftimedivisionmultiplexingtorealizethatasourceDTEusesaphysicalcircuittocallmultipledestinationDTEsforpacketdataexchange.Inaddition,italsoprovidespermanentvirtualcircuitPVCservice,whichisavirtualcircuitforuserstousefixedly.ThesourceDTEcanusethevirtualcircuitwithouthavingtoestablishacall.TheinterrelationshipofthevariouslayeredprotocolsinX.25.
Networkcomposition
Thenetworkstructureofpacketswitchingisgenerallycomposedofpacketswitches,networkmanagementcenters,remoteconcentrators,subassemblyanddisassemblyequipment,packetterminals/non-packetterminals,andtransmissionlines.Equipmentcomposition.(1)Thepacketswitchrealizestheinterfaceprotocol(X·25)betweenthedataterminalandtheswitch,thesignalingprotocolbetweentheswitches(suchasX·75orinternalprotocol),andstoresandforwardsinpackets,andprovidespacketnetworkservices.Supports,andcooperateswiththenetworkmanagementcentertocompleterouting,monitoring,billing,control,etc.Accordingtothestatusofthepacketswitchinthenetwork,itcanbedividedintotwotypes:transitswitchandlocalswitch;(2)Thenetworkmanagementcenter(NMC)andthepacketswitchworktogethertoensurethenormaloperationofthenetwork.
Itsmainfunctionsincludenetworkmanagement,usermanagement,measurementmanagement,billingmanagement,operationandmaintenancemanagement,routingmanagement,networkstatisticscollection,andnecessarycontrolfunctions,etc.,whicharenetwork-widemanagement(3)Themainfunctionofthesubassemblyanddisassemblydevice(PAD)istoconvertthenon-packetformatofordinarycharacterterminalsintogroupformats,andgroupthedatastreamsofeachterminalintogroups,whichareinterleavedandmultiplexedingroupsontheaggregatechannel.Theotherpartythenconvertsthereceivedpacketformatintheoppositedirection.(4)Thefunctionofaremoteconcentratorissimilartoapacketswitch,anditusuallycontainsthefunctionofaPAD.Itisonlyconnectedtoapacketswitchwithoutroutingfunction.Itisusedinareaswhereusersarerelativelyconcentrated,andisgenerallyinstalledinthetelecommunicationsdepartment.(5)Providethebasicservicesofthenetwork:switchingvirtualcircuitsandpermanentvirtualcircuitsandothersupplementaryservices,suchasclosedusergroups,networkuseridentification,etc.Whencommunicatingbetweenend-to-endcomputers,routeselectionandflowcontrolareperformed.Itcanprovideavarietyofcommunicationprocedures,dataforwarding,maintenanceandoperation,faultdiagnosis,billingandsomenetworkstatistics,etc.