Store and forward

Store-and-forwardprocess

Store-and-forwardisatraditionalforwardingmethod,andoneoftheearliestusedforwardingmethods.Thefigureisastructurediagramofadataframe.Thedataenterstheswitchfromlefttorightinthefigure,thatis,firstentersfromthestartfieldof"Preamble".First,theswitchstartsthereceivingprocessandstartstoreceivetheframe,startingfromthe"Preamble"fieldandendingwiththefinalCRC.Whenthecompleteframeisreceived,thereceivedpacketisputintothebuffer,andthentheswitchstartstheforwardingprocess,accordingtothereceivedframeTheDMACshownisthetargetMACaddresstodeterminetheforwardingstrategy.IfitexistsintheMACaddresstable,itisforwardedtothecorrespondingport,andifitdoesnotexist,itissenttoallports.

Storeandforwardfeatures

(1)Errorcheckingfunction

ThefunctionofCRCistocheckthepreviousdatatopreventerrors.Sincestore-and-forwardonlystartstheforwardingprocessaftertheentireframeisreceived,whentheCRCfieldisreceived,errorcheckingcanbeperformed.TheswitchperformsCRCcalculationonthereceiveddata,andthecalculatedvalueisthesameasthereceivedone.ComparethevalueoftheCRCfield.Ifthetwoarethesame,itmeansthatthedatahasnotbeencorrupted,andiftheyaredifferent,itmeansthatthedatahasbeencorrupted.

(2)Automaticcaching

(3)Policyfunction

AlsothefunctionofACLaccesscontrollist,theaccesscontrollistismainlyusedtocontroldatathroughpolicyForcontrol,thecontrollevelinvolvedinACLrangesfromthesecondlayertotheseventhlayerofOSI.Sincestore-and-forwardstorestheentireframe,itisconceivablethatiftheswitchhastheabilitytoprocessmultiplelayersofdata,itcanexecuteACLAfterall,thetargetreferencedbyACLalreadyexistsinthereceivedframe.

Store-and-forwardswitchingmethod

Store-and-forwardtechnologyisdividedintomessageswitchingtechnologyandpacketswitchingtechnology.

(1)Messageexchangetechnology

Whenthemessageexchangetechnologyisusedinthesituationwherethetwocommunicatingpartiesareadjacentnodes,thesourcedirectlydeliversthemessagetothedestination.

Whenthecommunicationpartiesofthemessageexchangetechnologyarenotinthesamephysicalnetwork,thepreparedmessageistransmittedtothereceiverthroughtheintermediatenodethroughacertainroutingmechanism.Atthistime,theintermediatenodenolongeronlyplaystheroleofconnection,italsohastheabilitytostoreandprocessdata.

Inthemessageexchangesystem,duetothelargeamountofdataprocessedbyoneexchange,itisnecessarytoputforwardhigherrequirementsonthestorageandprocessingcapabilitiesofcommunicationprocessing,whichgreatlyincreasesthecommunicationcost.Moreover,thissystemisalsopronetoblockage,anditsflexibilityandreliabilitywillbereduced.Atthesametime,thepossibilityoferrorsinthedataexchangeprocessisalsogreat,anddetectionisdifficult.

(2)Packetswitchingtechnology

Thepacketswitchingtechnologyistodividethepayloaddatainamessage,addcertaincontrolinformationandthemessagenumbertoformaAftergrouping,adataexchangemodeinwhichtransmissioniscarriedoutinunitsofpackets.

Theintermediatenodeinthisswitchingsystemisnolongerjustasimpleconnectionlink,butneedstojudgewhetherthepacketisrightorwrong,andmakerouteselection.

Inordertoavoidnetworkcongestioninpacketswitchingtechnology,acertainflowcontrolmechanismisadopted.Thegroupingandreassemblyofmessageswilltakealotoftime,butitonlyoccursatthereceivingandsendingplaces,andtheprocessingefficiencyofthegroupingishigh,sotheoverallefficiencyisalsohigh.However,becausesomecommunicationcontroldataneedstobeaddedinthecommunicationprocess,itwillcausealotofrepetitiveadditionaloverhead;duringthetransmissionprocess,thepacketmaybelost,broken,disordered,etc.,andrelevantpersonnelneedtoadoptcertainstrategiestosolvetheseproblems.problem.

Accordingtothedifferentimplementationmechanisms,packetswitchingtechnologycanbedividedinto:datagramswitchingtechnologyandvirtualcircuitswitchingtechnology.

①Datagramexchangetechnology

Datagramexchangetechnologyissimilartomessageexchangetechnology.Thedifferenceisthattheformertransmitsdatapacketseverytime,whilethelattertransmitstheentirepacket.Arts.

Thisisa"connectionless"dataexchangetechnology.Ithasacertainroutingalgorithmtodynamicallyselectthetransmissionpathofthedatagramintheprocessofdatatransmission,andthisisadisorderedtransmission.

②Virtualcircuitswitchingtechnology

Virtualcircuitswitchingtechnologyisdifferentfromdatagramswitchingtechnology.Itisaconnection-orientedswitchingtechnology.Itiscalleda"virtualcircuit"becauseintheprocessofdataexchange,aspecialdynamiclogiccircuitisestablishedbetweenthecommunicatingparties.Differentfromthephysicalcircuitinthecircuitswitchingsystem,thislogicalcircuitisnotexclusivelyused,butissimilartothestrategyofchannelmultiplexingtechnologytomultiplexintermediatenodesfordataexchange.Butbecausethetimeforeachdataexchangeisveryshort,wecanregardallpackettransmissionsassimultaneous.Everysuccessfulvirtualcircuitneedstobeassignedanidentificationnumberandcodedintothegrouptoavoidconfusion.Packetsaretransmittedinorderthroughthededicatedchannelestablished,soeachpacketnolongercontainscontroldata,reducingauxiliaryoverhead.

Virtualcircuitswitchingtechnologyisaswitchingtechnologywithstrongreliabilityandhighsystemefficiency.Itistoestablishadedicatedlogicalconnectionintheworkingprocess,andeachnodecanestablishmultiplevirtualcircuitconnectionswithanynode.Becausethereisnoauxiliaryinformation,thesystemoverheadissmall,andtheintermediatenodehasacertainerrordetectionmechanism.

Thepacketsindatagramswitchingtechnologyandvirtualcircuitswitchingtechnologyneedtoselectthecommunicationpathaccordingtoacertainroutingalgorithm,buteachpacketoftheformermustfindarouteseparately,whileinthevirtualcircuitswitchingtechnology,onecommunicationAllgroupsoftheprocessonlyneedtoberoutedonce.Obviously,routingcalculationsbasedondatagramsaremorefrequentthanthosebasedonvirtualcircuits.

Technicalprinciple

Store-and-forwardtechnologyrequirestheswitchtodecidehowtoforwardafterreceivingalldatapackets,whilepass-throughforwardingistostartforwardingdatabeforetheswitchreceivestheentireframeInthisway,theexchangedelaycanbeeffectivelyreduced.However,iftheswitchhasstarteddataforwardingbeforeithascompletelyreceivedandcheckedthecorrectnessofthedatapacket,soinanenvironmentwherethecommunicationqualityisnothigh,theswitchwillforwardallcompletedatapacketsanderrordatapackets,whichisactuallyBringsalotofjunkcommunicationpacketstotheentireswitchingnetwork.Therefore,thecut-throughforwardingtechnologyissuitableforanetworkenvironmentwithbetternetworklinkqualityandfewererrordatapackets.

AdvantagesandDisadvantages

Advantages:Thereliabilityisverygood,becauseitstoresthedataframeoftheinputportintheswitchbufferfirst,andthenperformsCRCcheck.Ifanerrorisdetectedintheframe,theframeisdiscarded,otherwisethedestinationaddressoftheframeistakenout,theoutputportisobtainedbylookinguptheMACaddresstable,andthenthedataframeisforwarded;andthestore-and-forwardswitchingmethodalsosupportsthecommunicationbetweenportsofdifferentspeedsConversiontofacilitatetheworkoftheprotocolbetweenthehigh-speedportandthelow-speedport.

Disadvantages:Accordingtosomecharacteristicsofstoreandforward,wecanseethatthecharacteristicsthatstoreandforwardcanprovidearestillveryattractiveandperfect,butthefatalweaknessofstoreandforwardisspeed.Thedataprocessingtimedelayofstore-and-forwardswitchingmodeisrelativelylong,andthemainreasonisthattheinputandoutputterminalsmustundergoserial-to-parallelconversion.Suchacumbersomeprocesswillaffecttheresponsespeedandcausethephenomenonofhighdelay.