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Introduction
HybridfiberCoaxialcable(HybridFiberCoax,referredtoasHFC)isabroadbandaccessnetworkthatcombinesopticalfiberandcoaxialcable.Opticalfiberreplacesordinarycable.AsthebackboneofthecableTVnetwork,itcanprovide30Mbpssharedbandwidthforhigh-speedInternet.Access,VODandanalogordigitalcableTVanddigitalvideoprograms.ThetransmissionmediumfromtheHeadEndcomputerroomtothefibernodeneartheuserisopticalfiber,andtheterminalequipmentfromtheopticalfibernodetotheuserisRG-62cable,soitiscalledhybridfibercoaxial.cable.
FeaturesofHFC
ThefrequencybandwidthofHFCnetworkcanmeettheneedsofintegratedservicesandhigh-speeddatatransmission.Becausethebandwidthofthecoaxialcablecanreach1GHz,inadditiontotheoriginalanalogvideosignal,therearealsoabundantspectrumresourcesthatcanbeusedtotransmitdigitalvideosignalsandtwo-waydata.
ThetransmissionrateofHFCnetworkishigh.WhentheCableModemisusedfortwo-waycommunicationontheHFCnetwork,thedownlinkratecanreach30Mbit/s,andtheuplinkratecanreach10Mbit/s.
TheflexibilityandscalabilityoftheHFCnetworkisbetter.TheHFCnetworkcanbecompatiblewithtraditionaltelephoneservicesandanalogvideoservicesinbusiness,whilesupportingInternetaccess,digitalvideo,VOD,andotherfutureinteractiveservices.Intermsofstructure,HFCnetworkhasstrongflexibilityandcansmoothlytransitiontoFTTH.
NetworkstructureofHFCnetwork
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.
Theintegratedserviceunit(ISU)providestheinterfacewiththetelecommunicationserviceterminalandthecableTVterminal.ISUisdividedintosingleuserunit(H-ISU)andmulti-userunit(M-ISU),andthenetworkstructurecanhavebothH-ISUandM-ISU.ISUhasoptionaltwo-wireanalogvoiceinterface,2B+Dinterface,N×64kbit/s(N=1～31)interface,2048kbit/sinterfaceandotherhigh-speedinterfaces.Inaddition,italsoprovidesmonitoringinterface.TheuseraccessestheHFCnetworkthroughuserterminalequipment,whichincludesusernetworkinterface(UNI),set-topbox(SYB)andcablemodem(CableModem).
cablemodem
Cablemodemisamodemsuitableforcabletransmissionsystem.Itsmainfunctionistomodulatedigitalsignaltoradiofrequencysignalanddemodulatethedigitalinformationinradiofrequencysignal..Inaddition,thecablemodemalsoprovidesastandardEthernetinterface,whichcancompletepartofthefunctionsofbridges,routers,networkcardsandhubs.Therefore,itsstructureismuchmorecomplicatedthantraditionalmodems.
Cablemodemsarebasicallythesameinprincipleastraditionalmodems.Theybothmodulatedigitalsignalsintoanalogsignalsfortransmissioninafrequencyrangeofthecable,andthendemodulatethemintodigitalsignalswhenreceiving.ThedifferenceisthatcablemodemdemodulatesthroughacertaintransmissionfrequencybandofcableTVratherthanthroughtelephonelines.Inaddition,themediumusedbyordinarymodemsisexclusivelyusedbyusers,whilecablemodemsbelongtoasharedmediumsystem,andtheremainingidlefrequencybandscanstillbeusedtotransmitcableTVsignals.
CablemodemisalsosimilartoADSL,providingasymmetrictwo-waychannels.Thecarrierfrequencyrangeadoptedbytheuplinkchannelisbetween5MHzand42MHz,andatransmissionrateof128Kbpsto10Mbpscanbeachieved.Thecarrierfrequencyofthedownlinkchannelrangesfrom42to750MHz,andcanachieveatransmissionrateof27to36Mbps.
Cablemodemhasthecharacteristicsofhighcostperformance,asymmetricprivatelineconnection,notlimitedbyconnectiondistance,usuallydoesnotoccupybandwidth(onlythebandwidthisoccupiedwhendownloadingandsendingdata),anditisnotwrongtowatchTVonline..
Thetransmissionrateofcablemodeminonechannelisupto27～36Mbps.ThebandwidthofeachcableTVchannelis8MHz,andthebandwidthoftheHFCnetworkis750MHz,sotheentirebandwidthcansupportnearly90channels.IntheHFCnetwork,therearecurrentlyabout33channels(550-750MHzrange)reservedfordatatransmission,andtheentirebandwidthisconsiderable.