Подробно обяснение
Eachnodereceivestheentiremessage,checkstheaddressofthetargetnode,andthenforwardsittothenextnodewhenappropriateaccordingtothetrafficsituationinthenetwork.Aftermanystore-and-forwards,itfinallyreachesthetarget,sosuchanetworkiscalledstore-and-forwardnetwork.Theexchangenodemusthaveenoughstoragespace(usuallyadisk)tobufferthelongmessagesreceived.
Theswitchingnodequeuesthemessagesreceivedinalldirections,comparesitwiththenextforwardingnode,andthenforwardsitout,allofwhichbringdelaysinqueuing.Theadvantageofmessageexchangeisthatnodedicatedlinkisestablished,butthelineutilizationrateishigher,whichisduetothewaitingtimedelayincommunication.
Системата за електронна поща (имейл) е подходяща за обмен на съобщения.
Предимства и недостатъци
Messageexchangeisbasedonmessagesastheunitofdataexchange.Messagescarryinformationsuchasdestinationaddressandsourceaddress.Storeandforwardareusedattheswitchingnode.Thetransmissionmethodhasthefollowingadvantagesanddisadvantages:
Предимства
①Themessageexchangedoesnotneedtoestablishadedicatedcommunicationlineforbothpartiesinadvance,andthereisnoconnectionestablishmentdelay,andtheusercanSendmessagesatanytime.
②Becauseoftheuseofstore-and-forwardtransmission,ithasthefollowingadvantages:a.Itiseasytosetupcodeinspectionanddataretransmissionfacilitiesinthemessageexchange,andtheexchangenodealsohaspathselection.Whenacertaintransmissionpathfails,anotherpathisselectedtotransmitdata,whichimprovesthereliabilityoftransmission;b.Itiseasytorealizecodeconversionandratematchinginstoreandforward,andeventhesendingandreceivingpartiescannotbeintheavailablestateatthesametime.Thisfacilitatescommunicationbetweencomputersofdifferenttypes,specificationsandspeeds;c.Providesmulti-targetservices,thatis,amessagecanbesenttomultipledestinationaddressesatthesametime,whichisdifficulttoachieveincircuitswitching;d.AllowedEstablishthepriorityofdatatransmissionsothatpacketswithhigherpriorityareconvertedfirst.
③Thecommunicationpartiesdonotoccupyacommunicationlinefixedly,butpartlyoccupythisphysicalpathatdifferenttimes,thusgreatlyimprovingtheutilizationrateofthecommunicationline.
Недостатъци
①Becausethedatahastogothroughtheprocessofstorageandforwardingafterenteringtheswitchingnode,itcausesforwardingdelay(includingreceivingmessages,checkingcorrectness,queuing,sendingTime,etc.),andthegreatertheamountofnetworkcommunication,thegreaterthedelaycaused,sothereal-timeperformanceofmessageexchangeispoor,anditisnotsuitablefortransmittingreal-timeorinteractiveservicedata.
②Обменът на съобщения е приложим само за цифрови сигнали.
③Sincethemessagelengthisnotlimited,andeachintermediatenodemustreceivetheentiremessagecompletely,whentheoutputlineisnotidle,itmayalsostoreseveralcompletemessagestowaitForwardingrequireseachnodeinthenetworktohavealargerbuffer.Inordertoreducecostsandreducethecapacityofthenode'sbuffermemory,sometimesthemessageswaitingtobeforwardedarestoredonthedisk,whichfurtherincreasesthetransmissiondelay.
Превключване на вериги
Becausecircuitswitchingneedstoestablishaphysicalpathbetweenthetwocommunicationpartiesbeforecommunication(bytheswitchingequipmentandlinkbetweenthetwocommunicationpartiessectionbysection)Connected),soithasthefollowingadvantagesanddisadvantages.
①Becausethecommunicationlineisdedicatedtobothusersofthecommunication,andthedataisdirect,thetimedelayofdatatransmissionisverysmall.
②Oncethephysicalpathbetweenthetwocommunicationpartiesisestablished,thetwopartiescancommunicateatanytimewithstrongreal-timeperformance.
③Whenthetwopartiescommunicate,thedataistransmittedintheorderofsending,andthereisnoproblemofout-of-sequence.
④Circuitswitchingisnotonlysuitablefortransmittinganalogsignals,butalsosuitablefortransmittingdigitalsignals.
⑤Превключващото оборудване (превключватели и т.н.) и управлението на превключването на вериги са относително прости.
Недостатъци:
①Средното време за установяване на връзка на превключване на вериги е твърде дълго за компютърна комуникация.
②Afterthecircuit-switchedconnectionisestablished,thephysicalpathisexclusivelyoccupiedbybothpartiesinthecommunication.Evenifthecommunicationlineisidle,itcannotbeusedbyotherusers,sothechannelutilizationislow.
③Whenthecircuitisswitched,thedataisdirect,anditisdifficultforterminalsofdifferenttypes,specifications,andspeedstocommunicatewitheachother,anditisalsodifficulttocarryouterrorcontrolinthecommunicationprocess.
Превключване на пакети
Превключване на пакетиstillusesstore-and-forwardtransmission,butalongmessageisfirstdividedintoseveralshorterpackets,andthenthesepackets(carryingsourceanddestination)Addressandnumberinformation)aresentoutonebyone.Therefore,inadditiontotheadvantagesofmessages,packetswitchinghasthefollowingadvantagesanddisadvantagescomparedwithmessageswitching:
①Speedsupthetransmissionofdatainthenetwork.Becausethepacketsaretransmittedonebyone,thestorageoperationofthenextpacketcanbeparalleledwiththeforwardingoperationofthepreviouspacket.Thispipelinetransmissionmodereducesthetransmissiontimeofthemessage.Inaddition,thebufferrequiredtotransmitapacketismuchsmallerthanthebufferrequiredtotransmitamessage,sotheprobabilityofwaitingfortransmissionandthewaitingtimeduetoinsufficientbuffersareinevitablymuchless.
②Simplifiedstoragemanagement.Becausethelengthofthepacketisfixed,andthesizeofthecorrespondingbufferisalsofixed,themanagementofthememoryintheswitchingnodeisusuallysimplifiedtothemanagementofthebuffer,whichisrelativelyeasy.
③Reducetheerrorprobabilityandtheamountofretransmissiondata.Becausethepacketisshorter,theerrorprobabilitywillinevitablydecrease,andtheamountofdataretransmittedeachtimeisalsogreatlyreduced.Thisnotonlyimprovesthereliability,butalsoreducesthetransmissiondelay.
④Becauseoftheshortpacketsize,itismoresuitableforadoptingprioritystrategiestofacilitatethetimelytransmissionofsomeurgentdata.Therefore,packetswitchingisobviouslymoresuitableforburstdatacommunicationbetweencomputers.
Недостатъци:
①Althoughpacketswitchinghaslesstransmissiondelaythanmessageswitching,itstillhasstore-and-forwarddelay,anditsnodeswitchmusthavestrongerprocessingcapabilities.
②Превключване на пакетиisthesameasmessageswitching.Eachpacketmustbeaddedwithsource,destinationaddressandgroupnumberinformation,whichincreasestheamountofinformationtransmittedbyapproximately5%to10%,toacertainextentThecommunicationefficiencyisreduced,theprocessingtimeisincreased,thecontroliscomplicated,andthetimedelayisincreased.
③Whenpacketswitchingusesdatagramservices,out-of-sequence,lossorduplicatepacketsmayoccur.Whenthepacketsarriveatthedestinationnode,itisnecessarytosortthepacketsbynumber,whichincreasesthetrouble.Ifthevirtualcircuitserviceisadopted,althoughthereisnodisorderproblem,therearethreeprocessesofcallestablishment,datatransmissionandvirtualcircuitrelease.
Inshort,iftheamountofdatatobetransmittedislargeandthetransmissiontimeismuchlongerthanthecalltime,circuitswitchingismoreappropriate;whentheend-to-endpathconsistsofmanylinks,packetItismoreappropriatetoexchangeandtransmitdata.Fromtheperspectiveofimprovingthechannelutilizationoftheentirenetwork,messageswitchingandpacketswitchingarebetterthancircuitswitching.Amongthem,packetswitchinghasashorterdelaythanmessageswitching,andisespeciallysuitableforburstdatacommunicationbetweencomputers.