Introduction
Theprincipleofmessageexchangeisthatwhenthesender’sinformationarrivesatthecomputerusedformessageexchange,itisfirststoredintheexternalmemory,andthecentralprocessingunitanalyzestheheaderanddeterminestheforwardingroute.,Andselecttheoutputcircuitcorrespondingtothisrouteforqueuing,waitingforoutput.Oncethecircuitisidle,themessageisimmediatelytakenoutfromtheexternalmemoryandsentout,whichimprovestheutilizationofthiscircuit.Althoughthemessageexchangeimprovestheutilizationofthecircuit,themessagewillhavealargerdelayafterbeingstoredandforwarded.Packetswitchingisalsoastore-and-forwardswitchingmethod,butdifferentfrommessageswitching,itdividesmessagesintopacketsofacertainlength,andstoresandforwardstheminunitsofpackets.Thisnotonlyhastheadvantageofmessageexchangemodetoimprovecircuitutilization,butalsoovercomesthedisadvantageoflargetimedelay.
Classification
CircuitSwitching
Whenuserswanttotransmitdata,theswitchingcenterestablishesatemporarydatacircuitbetweenusers.Afterthecircuitisconnected,bothuserscantransmitdata,anditwillbeoccupieduntilthetransmissioniscompletedandthecircuitisremoved.Thetimedelayintroducedbycircuitswitchingisverysmall,andtheswitchdoesnotprocessthedata,soitissuitablefortransmittingreal-timeandlarge-scaledata.
Features
Circuitswitchingcanensurethatafterthephysicalchannelisestablished,thephysicalchannelbecomesadedicatedcircuit.Therefore,thereisnolongeradelayintransmittinginformation.However,becausethecomputertransmitsinformationintermittently,onlyashorttimeisactuallyusedtotransmitinformationduringtheentiretimeoccupiedbythephysicalpath,whichcausesawasteofcommunicationlines.Inaddition,thecallprocessofcircuitswitchingtoestablishapathistoolongforcomputercommunication.
Messageexchange
Generally,itisrealizedbycomputer.Thesenderuserfirstcompilesthedatatobesentintoamessage,andsendsittothelocalswitchingcentertogetherwithauxiliarydatasuchastherecipientaddress,wheretheyarestoredintactandprocessedappropriately.Whentheoutputportofthelocalswitchingcenterisfree,themessageisforwardedtothenextswitchingcenter,andfinallythemessageisdeliveredtotheuserbytheswitchingcenteratthereceivingend.
Features
Intheexchange,themessageisstoredandforwardedfromthesourcetothedestination.Theadvantagesare:①hightransmissionreliability,itcaneffectivelyuseerrorcheckingandretransmissiontechnology;②highlineutilization,itcanconcentratemultiplelow-speedcircuitsintohigh-speedcircuittransmission,andallowsmultipleuserstoshareachannel③Itisflexibletouse,itcancarryoutpreprocessingworksuchascodeconversionandrateconversion,soitcantransmitdatabetweenterminalsofdifferenttypes,rates,andprocedures.
However,messageexchangeisnotsuitableforconversationalandreal-timeservices.Generally,messageexchangesaredividedintodifferentprioritylevelsfortransmissionaccordingtotheimportanceandurgencyofthetransmitteddata.
Packetswitching
Splitthedataintoseveralshort-length(generallynomorethan128characters)packets.Inadditiontothedatainformation,eachpacketalsoincludescontrolinformation.TheyExchangeasawholewithintheswitch.Thetransmissionpathofeachpacketintheswitchingnetworkcanbedifferent.Packetswitchingalsousesstore-and-forwardtechnology,andperformserrorchecking,retransmission,andreturnresponseoperations.Finally,thereceivingendreassemblesallreceivedpacketsintodatainorder.
Features
Comparedwiththemessageexchange,theadvantagesofpacketswitchingare:
①Inthemessageexchange,thetotaltransmissionDelayisthesumofthedelaysofreceivingandforwardingtheentiremessageoneachnode.Inpacketswitching,afterapacketissenttoanode,thenextpacketcanbesent,sothatthetotaldelayisreduced;
②Thebuffercapacityrequiredbyeachnodeisreduced,whichisconducivetoimprovingtheutilizationofthenode’sstorageresources;
③Whenthereisatransmissionerror,justretransmitoneormoreItisnotnecessarytoretransmittheentiremessage,whichcanimprovethetransmissionefficiency.Thedisadvantageofpacketswitchingisthateachpacketneedstobeaddedwithsomecontrolinformation,whichwillreducethetransmissionefficiency,especiallyforlongmessages.Generalpacketswitchingprovidestwobasicservices,virtualcircuitanddatagram.
Hybridswitching
Thesimultaneoususeofcircuitswitchingandpacketswitchinginacomputernetworkiscalledhybridswitching.Thehybridmethodistodividethetransmissionchannelintodifferentbandwidths,allocateapartofthebandwidthforcircuitswitching,andallocateanotherpartofthebandwidthforpacketswitching.Theso-calledbandwidthherereferstothefrequencybandwidthallowedtotransmitinformationonatransmissionchannel,thatis,thehighestfrequencythroughwhichasignalcanpassthroughthechannel.