TheabovecontentisreviewedandannouncedbytheNationalScienceandTechnologyTermReviewCommittee
Въведение
HybridfiberCoaxialcable(HybridFiberCoax,referredtoasHFC)isabroadbandaccessnetworkthatcombinesopticalfiberandcoaxialcable.Opticalfiberreplacesordinarycable.AsthebackboneofthecableTVnetwork,itcanprovide30Mbpssharedbandwidthforhigh-speedInternet.Access,VODandanalogordigitalcableTVanddigitalvideoprograms.ThetransmissionmediumfromtheHeadEndcomputerroomtothefibernodeneartheuserisopticalfiber,andtheterminalequipmentfromtheopticalfibernodetotheuserisRG-62cable,soitiscalledhybridfibercoaxial.cable.
Характеристики на HFC
ThefrequencybandwidthofHFCnetworkcanmeettheneedsofintegratedservicesandhigh-speeddatatransmission.Becausethebandwidthofthecoaxialcablecanreach1GHz,inadditiontotheoriginalanalogvideosignal,therearealsoabundantspectrumresourcesthatcanbeusedtotransmitdigitalvideosignalsandtwo-waydata.
ThetransmissionrateofHFCnetworkishigh.WhentheCableModemisusedfortwo-waycommunicationontheHFCnetwork,thedownlinkratecanreach30Mbit/s,andtheuplinkratecanreach10Mbit/s.
TheflexibilityandscalabilityoftheHFCnetworkisbetter.TheHFCnetworkcanbecompatiblewithtraditionaltelephoneservicesandanalogvideoservicesinbusiness,whilesupportingInternetaccess,digitalvideo,VOD,andotherfutureinteractiveservices.Intermsofstructure,HFCnetworkhasstrongflexibilityandcansmoothlytransitiontoFTTH.
Мрежова структура на HFC мрежа
TheHFCnetworkevolvedfromtheCATVnetworkhastwotypes:one-waynetworkandtwo-waynetwork.Amongthem,theone-wayHFCnetworkonlyreplacesthetraditionalCATVnetworkbackbonecableswithopticalfibers,expandingthecoverageoftheaccessnetwork,andbasicallyonlysupportsCATVservices.Thetwo-wayHFCnetworkistransformedonthebasisoftheone-wayHFCnetwork,anditsstructureisshowninFigure1.Thetwo-wayHFCnetworkcanprovideCATV,voice,dataandotherinteractiveservices,andrealizethe"tripleplay"oftelecommunicationnetwork,cabletelevisionnetworkandcomputernetwork.
Thetwo-wayHFCnetworkiscomposedofalocalareanetworkdigitalterminal(HDTorgateway),anopticalfiberfeedernetwork,anopticalnode,acoaxialcabletransmissionanddistributionnetwork,andanintegratedserviceunit.Amongthem,thelocaldigitalterminalhasfourmainfunctions:First,itcompletestheinterfacefunctionsofvariousbusinessnodeequipment,suchasthelocalswitchofPSTN,theprogramsourceofCATV,theVODserver,andtheaccesstothebroadbandnetworkthroughtheedgeswitchofthebroadbandnetwork.Internetserviceprovider’saccessserverinterface,etc.;second,HFCusessub-carriermultiplexingtransmissiontechnology,andthemodulationanddemodulationpartofthecentraldigitalterminalmodulatesvariousservicesignalsontheirrespectivesub-carrierstoachievefrequencyDivisionmultiplexing(FDM),sharingthetransmissionmedium;third,completingthephotoelectricconversionfunction;fourth,providingthemonitoringinterfacefunction.
Opticalfiberfeedernetworkandcoaxialcabletransmissionanddistributionnetworkarethebackbonetransmissionnetworkanduser-sidedistributionnetworkofHFCnetworkrespectively.
Theopticalnodeistheconnectionpointbetweentheopticalfiberbackbonetransmissionnetworkandtheuser-sidecoaxialdistributionnetworkintheHFCnetwork.Itmainlycompletesoptical/electricconversion,multiplexing/demultiplexingofelectricalsignals,andmodulation/demodulation.Inaddition,theopticalnodecontrolsthefrequencybandallocationtorealizemultipleservices,andmanagesthecompetitionontheuplinkchanneltorealizemulti-usersharingofthetransmissionmedium.
Интегрираният обслужващ модул (ISU) осигурява интерфейс с терминала за телекомуникационни услуги и кабелния телевизионен терминал. ISU е разделен на единичен потребител (H-ISU) и многопотребителски модул (M-ISU), а мрежовата структура може да има както H-ISU, така и M-ISU. ISU има допълнителен двужилен аналогов гласов интерфейс, 2B+D интерфейс, N×64kbit/s(N=34) интерфейс /sinterfaceи други високоскоростни интерфейси.Освен това предоставя също интерфейс за наблюдение.Потребителят осъществява достъп до HFC мрежата чрез потребителско терминално оборудване, което включва потребителски мрежов интерфейс (UNI), приемник (SYB) и кабелен модем (CableModem).
кабелен модем
Cablemodemisamodemsuitableforcabletransmissionsystem.Itsmainfunctionistomodulatedigitalsignaltoradiofrequencysignalanddemodulatethedigitalinformationinradiofrequencysignal..Inaddition,theкабелен модемalsoprovidesastandardEthernetinterface,whichcancompletepartofthefunctionsofbridges,routers,networkcardsandhubs.Therefore,itsstructureismuchmorecomplicatedthantraditionalmodems.
Cablemodemsarebasicallythesameinprincipleastraditionalmodems.Theybothmodulatedigitalsignalsintoanalogsignalsfortransmissioninafrequencyrangeofthecable,andthendemodulatethemintodigitalsignalswhenreceiving.Thedifferenceisthatкабелен модемdemodulatesthroughacertaintransmissionfrequencybandofcableTVratherthanthroughtelephonelines.Inaddition,themediumusedbyordinarymodemsisexclusivelyusedbyusers,whileкабелен модемsbelongtoasharedmediumsystem,andtheremainingidlefrequencybandscanstillbeusedtotransmitcableTVsignals.
CablemodemisalsosimilartoADSL,providingasymmetrictwo-waychannels.Thecarrierfrequencyrangeadoptedbytheuplinkchannelisbetween5MHzand42MHz,andatransmissionrateof128Kbpsto10Mbpscanbeachieved.Thecarrierfrequencyofthedownlinkchannelrangesfrom42to750MHz,andcanachieveatransmissionrateof27to36Mbps.
Cablemodemhasthecharacteristicsofhighcostperformance,asymmetricprivatelineconnection,notlimitedbyconnectiondistance,usuallydoesnotoccupybandwidth(onlythebandwidthisoccupiedwhendownloadingandsendingdata),anditisnotwrongtowatchTVonline..
Thetransmissionrateofкабелен модемinonechannelisupto27～36Mbps.ThebandwidthofeachcableTVchannelis8MHz,andthebandwidthoftheHFCnetworkis750MHz,sotheentirebandwidthcansupportnearly90channels.IntheHFCnetwork,therearecurrentlyabout33channels(550-750MHzrange)reservedfordatatransmission,andtheentirebandwidthisconsiderable.