Circuit switched network

Classification

Therearetwomaintypesofcircuit-switchednetworks,publicswitchedtelephonenetwork(PSTN)andintegratedservicesdigitalnetwork(ISDN),whichwillbeintroducedseparatelybelow.

PublicSwitchedTelephoneNetwork(PSTN)

PublicSwitchedTelephoneNetwork(PSTN)isacircuit-switchednetworkbasedonanalogtechnology.Itisacollectionofallcircuit-switchedtelephonenetworkssincetheinventionofthetelephone,andisthemostpopularandlowest-costpubliccommunicationnetwork.Itmainlyprovidesvoicecommunicationservices,butalsoprovidesdatacommunicationservices,suchasdataexchange,fax,videotext,etc.

Fromafunctionalpointofview,PSTNiscomposedofinternationalexchanges,long-distanceexchanges,centralexchanges,endexchanges,andusers.Fromtheperspectiveofequipmentcomposition,PSTNiscomposedofthreeparts:switch,transmissioncircuit(subscriberlineandinter-officetrunkcircuit)anduserterminalequipment(suchastelephone).

NetworkinterconnectioncanbeachievedthroughPSTN,butPSTNisbasedonanalogtechnologyandprovidesanalogchannelsforaccessdevices.WhendigitaldevicessuchascomputersareconnectedviaPSTN,modemsneedtobeusedatbothendstorealizetheconversionbetweendigitalsignalsandanalogsignals.

DatacommunicationusingPSTNischeap,butitsdatatransmissionrateislow,anditcannotprovideflowcontrol,errorcontrol,etc.,sothetransmissionqualityispoor.Itcanonlybeusedinoccasionswherethecommunicationrateisnothigh,andthereisnostoreandforwardfunctioninthemiddle,whichmakesitdifficulttoachievevariablespeedtransmission.BecausethePSTNhasawidedistributionrangeandlowcost,itisasolutionforearlyhomeusersandsmallnetworkswithlowrequirementstoaccesstheInternet.

IntegratedServicesDigitalNetwork(lSDN)

TheconceptofIntegratedServicesDigitalNetwork(ISDN)originatedinthe1970sandmaturedinthe1980s.Atthattime,variouspublicnetworkscoexistedatthesametime,providingdifferentservices,resultinginarelativelyindependentstateoffragmentation.Forexample,telephonenetworksprovidevoiceservices,usertelegraphnetworksprovidetextcommunicationservices,andcircuit-switchedandpacket-switchednetworksprovidedatatransmissionservices.ThepurposeofIsDNistouseasinglenetworktoprovideavarietyofdifferentservicestothepublic,notonlycanprovidevoiceservices,butalsocanprovidevariousnon-voiceservicessuchasdata,imageandfax.IntegratedServicesDigitalNetwork(ISDN)isdividedintonarrowbandandbroadband,respectivelycalledN．ISDN(NarrowISDN)andB．ISDN(BroadbandISDN),ISDNreferstoN．ISDN.

IntegratedServicesDigitalNetwork(ISDN)isatypicalcircuit-switchednetwork.ItisacommunicationnetworkdevelopedonthebasisofIntegratedTelephoneNetwork(IDN),whichcanprovideend-to-enddigitalconnectionstosupportIntegratedservicessuchasvoice,data,graphics,andvideo.ISDNusesasinglenetworkinterface,whichcanbeusedtorealizesimultaneousdigitalcommunicationconnectionsformultipleterminals.

ThebiggestdifferencebetweenISDNandothernetworksisthatitcanprovideend-to-enddigitaltransmission.Theso-calledend-to-enddigitaltransmissionmeansthatthetransmissionfromoneuserterminaltoanotherisalldigital.Inthetraditionaltelephonenetwork,thetransmissionfromtheuserterminaltotheswitchisanalog.Iftheuserwantstocommunicatedata,itneedstouseamodemtoperformdigital-to-analogconversionandthentransmitontheuserline.Atthereceivingend,italsoneedstouseamodemforsignalconversion..ISDNhaschangedthestateofthetraditionaltelephoneanaloguserloop,makingfulldigitalizationareality,anduserscanobtainexcellentdigitalperformance.Simplyput,inthemodernPSTN,thebackbonenetworkhasbeenfullydigitalized,buttheuserterminalsofthePSTNarestillanalogdevices,andISDNisanetworkcreatedaftertheequipmentandthetransmittedinformationaredigitizedonthebasisofthePSTN.

Routeselection

Inlargecircuit-switchednetworkssuchasAT&Tlong-distancetelephonenetworks,mostcircuitconnectionsneedtopassthroughmorethanoneswitch.Whenacalloccurs,thenetworkmustestablisharoutefromthecallingusertothecalleduserthroughthenetwork.Thisroutepassesthroughmultipleswitchesandtrunks.Theroutingstrategyhastwomainrequirementsforthenetworkarchitecture:efficiencyandresilience.Firstofall,peoplehopetominimizetheequipment(switchesandtrunks)involvedinthenetworkunderthepremiseofbeingabletohandletheexpectedload.Thisloaddemandisusuallyexpressedbythetermbusy-hourtrafficload,whichreferstotheaverageloadusedinthebusiesthourofthewholeday.Fromafunctionalpointofview,thistrafficloadmustbehandled.Fromacostperspective,wewanttobeabletousetheleastamountofequipmenttohandlethisload.Butthereisanotherrequirement,namelyadaptability.Althoughthescaleofthenetworkmaybebasedonthebusyandtimelytrafficvolume,itisverylikelythattherewillbeshort-termtrafficpeaksthatexceedtheaveragetrafficlevel(forexample,duringpeakperiods).Thereisalsoasituationthatswitchesandtrunksoccasionallyfailandcannotbeusedforashortperiodoftime(mayunfortunatelyhappentobeinthesamepeakperiod).Wehopethatundertheaboveconditions,thelevelofserviceprovidedbythenetworkisalsoacceptable.

Asamajordesignissue,routingstrategyneedstofindabalancebetweennetworkefficiencyandadaptability.Traditionally,theroutingfunctioninpublictelecommunicationnetworkshasbeenverysimple.Essentially,theswitchesofthenetworkareorganizedintoatreestructure,orhierarchicalstructure.Theestablishmentofapathstartsfromthecallinguser,goesupthetree,reachesthefirstcommonnode,andthengoesdownthetreetoreachthecalleduser.Inordertoimprovetheadaptabilityofthisnetwork,inaddition,sometrunklineswithahigherusagerateareaddedtorunthroughthetreestructuretoconnectthoseexchangeofficeswithlargerbusinessvolume.Generallyspeaking,thisisastaticway.Theadditionalhigh-utilizationtrunklineprovidesredundancyandadditionalcapacity,butitstillhasgreatlimitationsintermsofefficiencyandadaptability.Becausethisroutingmechanismcannotadapttochangesinconditions,thenetworkmustbedesignedtomeetcertaintypicallarge-capacityneeds.Let'stakeanexampletoillustratetheproblemscausedbythisapproach.Thebusyhourbusinessvolumefromeasttowestisnotthesameasthebusyhourbusinessvolumefromnorthtosouth,andtheirrequirementsforthesystemarealsodifferent.Theimpactofthesechangesisdifficulttoanalyze,whichwillleadtoexcessivesystemdesigncapacityandultimatelylowefficiency.Fromanadaptabilitypointofview,thefixedhierarchicalstructureplusauxiliarytrunklinesmayhaveweakerresponsetofailures.Usuallyinthiskindofdesign,theresultofafailureiscongestioninmostofthearea.

Inordertomeetthegrowingneedsofpublictelecommunicationnetworks,almostallsuppliershaveswitchedfromastatichierarchicalapproachtoadynamicapproach.Thedynamicrouteselectionmethodreferstotheinfluenceofthecurrenttrafficvolumewhendecidingwhichroutetochoose.Normally,eachnodeofcircuitswitchingisinapeer-to-peerrelationship,ratherthanahierarchicalrelationship.Allnodescanhandlethesamefunction.Inthisstructure,routingismorecomplicatedandmoreflexible.Itismorecomplicatedbecausethisstructuredoesnotprovideapathorasetof"naturallyformed"pathsbasedonahierarchicalstructure,butitisalsomoreflexible,sotherearemoreroutestochoosefrom.

Developmenttrend

Theconnection-orientedcircuitswitchingtechnologyrepresentedbythetraditionaltelephonenetworkhasthecharacteristicsofreal-time,low-rate,symmetricaltraffic,high-qualityvoicetransmission,andperfectmanagementAswellasbillingandotheradvantages,intheforeseeablefuture,itwillstillbethebasicmeansofprovidingtelephoneservicesinmostcountries.

mycountry’stelephonebusinessisgrowingrapidly,andthescaleofthefixedtelephonenetworkrankssecondintheworld.However,thepopulationbaseislargeandthepenetrationrateisstillverylow.Thereisstillalotofroomfordevelopmentinruralareasandunderdevelopedareas.Thesecond-generationcellularmobilenetworkisalsoanetworkbasedoncircuitswitchingandmainlytransmittingvoiceservices,andisinastageofrapiddevelopment.Drivenbybusiness,mycountry'scircuitswitchingnetworkwillcontinuetodevelopsteadilyforalongperiodoftime.

TheexplosivegrowthofInternetserviceshashadanimpactonthetelephonenetwork.Thecharacteristicsoftheportcallareeasytocausenetworkcongestion.Althoughitcanbesolvedbyexpansion,thecostistoohigh.Moreover,duetotheasymmetryofthemonumentservice,fortheconnection-orientedcircuit-switchednetwork,theexpansionwillcausetheutilizationofnetworkresources.Thefurtherreduction.Thesolutionofportcallbypassisadopted,thatis,theaccessservicemoduleisembeddedintheswitchtodirectlyprocesstheYingcall,anditisdirectlyconnectedtotheInternetbackbonenetworkthroughtheDDNorElchannel,therebyreducingtheprocessingloadofthecircuitswitching.

ThedevelopmentoftheInternetanditsserviceswillinevitablyleadtotheintegrationofPservicesandtraditionaltelecommunicationsservices,suchasoraltelephoneandfax.Forthecurrentvoicecallservicebetweenordinarytelephoneusers,thelocaltelephonenetworkisconnectedtotheInternetthroughaVoIPgateway,anditseemsthatithasbecometheaccessnetworkoftheInternet.