Basicconcepts
Withthedevelopmentofdatacommunicationservices,relativelyfixedusertrafficisrelativelylarge,andrequiresstabledelayandreal-timeperformance.high.Drivenbymarketdemand,thedigitaldatanetwork(DDN),whichisasemi-permanentconnectionbetweenpermanentconnectionandswitchedconnection,hasemerged
Thefoundationofdigitaldatanetworkisdigitaltransmissionnetwork.Thedigitaltransmissionnetworkmustbeestablishedbasedonopticalcables,digitalmicrowaves,anddigitalsatellitecircuits.Inthepast,itwasdifficulttoestablishadigitaltransmissionnetworkwithtraditionalopenwires,cables,coaxialcables,analogmicrowaves,andshortwaves.Comparedwithtraditionalanalogchannels,usingdigitalchannelstotransmitdatasignalshasaseriesofadvantagessuchashightransmissionquality,highspeed,andhighbandwidthutilization.
DDNprovidesuserswithasemi-permanentdigitalconnection,withoutcomplexsoftwareprocessingalongtheway,sothedelayissmall,avoidingtheshortcomingsoflongandunfixedtransmissiondelayinthepacketswitchingnetwork;DDNThedigitalcross-connectdevicecanbeconnectedtothelineoftherequiredbandwidthwithintheagreedtimeaccordingtotheneedsoftheuser.Thechannelcapacityallocationandconnectionarecarriedoutunderthecontrolofthecomputer,whichhasgreaterflexibility.
Transmissionrate
Theratecanbe2.4,4.8,9.6,19.2,64,n×64kb/soreven2Mb/s.
Advantages
Comparedwiththetraditionalanalogdatanetwork,thedigitaldatanetworkhasthefollowingadvantages:
(1)Goodtransmissionquality.Generally,thebiterrorrateoftheanalogchannelisbetween1×10-5and1×10-6,andthequalitydecreaseswiththeincreaseofdistanceandthenumberoftransfers,whilethedigitaltransmissionissegmentedregenerationwithoutnoiseaccumulation.Thebiterrorrateoftheopticalcablewillbebetterthan1×10-8;
(2)Highutilizationrate.Thetypicalrateofapulsecodemodulation(PCM)digitalvoicecircuitis64kb/s.Whenusedfordatatransmission,itcanactuallybeupto48kb/sor56kb/s.Throughsynchronousmultiplexing,59.6kb/sormorecanbetransmitted.Manylow-speeddatacircuits,anda300～3400Hzstandardanalogvoicechannelcanusuallyonlytransmit9.6kb/srate,evenifacomplexmodem(MODEM)isused,itcanonlyreach14.4kb/sand28.8kb/s;
(3)Noexpensivemodemisrequired.Forusers,theyonlyneedamodemwithsimplefunctionsforbasebandtransmission,andthepriceisonlyaboutone-third.
Systemcomposition
Adigitaldatanetworkismainlycomposedoffourparts:
(1)Localtransmissionsystem,whichreferstotheslaveterminalThetransmissionsystembetweentheuserandthelocalofficeofthedigitaldatanetwork,thatis,theuserline,generallyusesanordinarylocaltelephonesubscriberline,andcanalsouseadataequipment(DOV)multiplexedonthetelephoneline.
(2)Crossconnectionandmultiplexingsystem,multiplexingisthetimedivisionofdatastreamsofmultipleusersbelow64kb/sTheprincipleofmultiplexingiscombinedintoa64kb/saggregatedatasignal,usuallycalledthezero-ordergroupsignal(DS0),andthenmultipleDS0signalsarefurthermultiplexedintoaprimarygroupaccordingtothearchitectureofthedigitalcommunicationsystem,whichis2.048Mb/sorhigherordersignal.Cross-connectionistocross-connecttheuserdatasignalofacertainformatwiththeinputofthezero-ordermultiplexerortheoutputofonemultiplexerwiththeinputofanothermultiplexertoachieveasemi-permanentfixedconnection.Howtocross-routeTheoperatorofthenetworkmanagementcenterimplements;
(3)Interofficetransmissionandsynchronizationclocksystem,mostoftheinterofficetransmissionisrealizedbyexistingdigitalchannels.ItisextremelyimportantthatallnodesinaDDNnetworkmaintainclocksynchronization.Usuallyadoptthewholenetworksynchronousclocksystemofthedigitalcommunicationnetwork,forexampleadoptthecesiumatomicclock,itsprecisioncanreachn×10-12,connectseveralrubidiumclocks,itsprecisionshouldbeconsistentwiththemasterclock.Itcanalsobeimplementedwithamulti-useandmulti-satelliteglobalpositioningsystem(GPS);
(4)Networkmanagementsystem,whetheritisanationalbackbonenetworkoraregionalnetworkapplicationSetupanetworkmanagementcentertoimplementcentralizedmanagementofthetransmissionchannelsontheInternet,theaddition,deletion,modification,monitoring,maintenance,andschedulingofuserparameters.
Generalstructure
Basedontheabove-mentioneddigitaldatatransmissionsystem,itcanformadigitaldatatransmissionnetwork.
(1)DDNnodetype
Fromtheperspectiveofnetworkingfunctions,DDNnodescanbedividedinto2Mnodes,accessnodesandusernodes.Kindoftype.
2MnodesareusedforbackbonenodesontheInternettoperformtheconversionfunctionofnetworkservices,andprovide2Mbit/sinterface(E1),forN*64kbit/ssignalsaremultiplexedandcross-connected;
TheaccessnodemainlyprovidesaccessfunctionsforvariousDDNservices,andmultiplexessub-ratesignalslessthan64kbit/s.Useandcross-connect,andprovideframerelayservicesandcompressedvoice/G3faxuserstoaccessthenetwork;
UsernodesmainlyprovideinterfacesforDDNuserstoaccessthenetwork,andperformnecessaryprotocolconversion.
(2)DDNnetworkstructure
DDNisgenerallyahierarchicalnetwork.Accordingtothebusinessconditionsofthenetwork,theDDNnetworkcanbesetupwithasecondarytrunknetworkandalocalnetwork.
IftheDDNnetworkisseparatedvertically,itsfunctionalhierarchicalstructurecanbedividedintothreelayers,namelythetransmissionlayer,theaccesslayerandtheuserlayer.Thetransportlayerisresponsibleforthetransmissionofdigitalsignalsfromtheaccesslayer;theaccesslayerusesabandwidthmanagertorealizetheuser'smultipleserviceaccess,providingdigitalcross-connectionandmultiplexingfunctions;theuserlayerreferstotheuserterminalequipmentthatentersthenetworkanditsThefunctionofthelink.
DDNnetworkservicecategory
DDNnetworkmainlyprovidesuserswithdedicatedcircuits,includingpoint-to-pointorpoint-to-multipointdigitaldedicatedcircuitswithspecifiedratesanddedicatedcircuitswithspecificrequirements.Andframerelayservicesandcompressedvoice/G3faxservices.Framerelayserviceistoprovidepermanentvirtualcircuitservicebyintroducingframerelayservicemodule;andtoprovidecompressedvoice/G3faxservicebyintroducingvoiceservicemodule.