Sofar,themodulationanddemodulationtechnologyadoptedbyxDSLhasnotyetformedarelativelycentralizedunifiedstandard.Nomatterwhetheritisinternationallyordomestically,ithasnotbeendevelopedandpromotedonalargescale,anditisstillonlyusedinspecialoccasions.Forexample,largededicatedlineusersrequirehigh-speed(above2Mbps)accesstonearbytelecomoffices.SincexDSLisbasedontheexistingcoppertelephonelineandadoptsbettermodulationanddemodulationtransmissioncodepatterns,xDSLhasthefollowingcharacteristics:
1.Itworksindifferentfrequencybandsfromvoice,basicallyDoesnotaffectthenormaluseofthephone,thefrequencybandoccupiedbythevoiceis0-4KHz;thexDSLmodulationfrequencybandis4.4KHz-1MHz.
2.Ahigherdatatransmissionratecanbeprovidedwithinacertaindistancecenteredontheofficeandthehome;
3,thespecificconditionsofthecopperwire,theweatherandthefragmenteddialoguemaychangeAffectthetransmissionperformance;
Accordingtotheabovecharacteristics,itcanbeseenthattheadvantagesofusingxDSLare:userscanobtainhighercommunicationbandwidthatarelativelylowcost;reducetheloadonthecentralofficeswitchesofenterprisesandinstitutions;allowServiceprovidersprovideuserswithnewnetworkservices,etc.However,xDSLrequirestelecomserviceproviderstohavelargerandmoreeffectivebackbonedatanetworksupport;newservicemodelsandpricepointsneedtobedefined.
Classificationandapplication
The"x"inxDSLrepresentsanycharacterorstring.Accordingtodifferentmodulationmethods,theobtainedsignaltransmissionrateanddistancearedifferent,aswellastheuplinkchannelandthedownlinkchannel.ThesymmetryofxDSLisdifferent,andxDSLcanbedividedintothefollowingtypes:
ADSL(AsymmetricDigitalSubscriberLine)
ADSLisatechnologywithasymmetricupstreamanddownstreamtransmissionrates.Onatelephoneline,thedownstreamratefromthetelecommunicationsnetworkprovidertotheusercanreach1.5Mbpsto8Mbps,whiletheupstreamrateinthereversedirectionis16Kbpsto640Kbps.ThemaximumtransmissiondistanceofADSLis5.5Km,whichismainlysuitableforuserremotecommunicationandcentralofficeconnection.TheabovecharacteristicsmakeADSLtechnologyanidealwayforNetSurfing,VideoonDemand(VOD)andRemoteLAN.FormostInternetandIntranetapplications,theamountofdatadownloadedbyusersismuchgreaterthantheamountofupload.TheadvantageofADSLtechnologyisthatitappearsinastandardform,onlyusesapairoftelephonelines,andhasalongtransmissiondistance;itsdisadvantageisthatthecurrentmodemisexpensiveandinconvenienttopromote,andthetransmissionrateanddistancearemutuallyrestricted.
RADSL(RateAdaptiveDigitalSubscriberLine)
Inadditiontoadaptingtolineconditions,RADSLprovidesbasicallythesametransmissionrateanddistancerangeasADSL.RADSLcandynamicallyadjusttheuser'saccessrateaccordingtothequalityofthetwisted-paircopperwireandthedistanceofthetransmissiondistance.Thisallowsuserstoconnectdifferentcopperwiresatdifferentratestomaximizetheuseofexistingcommunicationresources.
HDSL(High-SpeedDigitalSubscriberLine)
ThetransmissionrateprovidedbyHDSLtechnologyissymmetrical,thatis,itprovidesequalbandwidthforupstreamanddownstreamcommunications,andthetransmissionratecanreachT1/E1Generally,twopairsoftelephonelinesareusedforfull-duplexcommunication,andtheeffectivetransmissiondistanceisonly5km.ItstypicalapplicationistoreplacetheexistingT1methodtoconnectremoteoffices.Comparedwithgeneralbasebandmodems,HDSListhemostmatureofvariousDSLtechnologies,withgoodinterconnectivity,longertransmissiondistance,andlowerequipmentprices.Therefore,HDSLtechnologyhasbeenusedinsometelecommunicationscompaniesandcampusintranets.AlthoughtheeffectivetransmissiondistanceofHDSLisonly5km,itcanbeextendedbyinstallingsignalrepeaters.BecauseHDSLusestwopairsoftelephonelinesfortwo-waytransmission,itisverysuitableforconnectingPBXsystems,digitallocalloops,Internetserviceproviders,andcampusnetworks.ThedisadvantageofHDSListhattheuserneedsasecondtelephoneline,andtherearecurrentlyfewermanufacturersofproductsavailable.
VDSL(VeryHigh-SpeedDigitalSubscriberLine)
VDSL,likeADSL,isalsoatechnologywithasymmetricupstreamanddownstreamtransmissionrates.VDSLusesatelephonelinetoobtainadownlinktransmissionrateof13Mbps-52Mbps,anuplinkrateof1.5Mbps-2.3Mbps,andatransmissiondistanceofnotmorethan1.5km.Itismainlyusedforvideoandmultimediaandotherrelatedoccasions.ItcanbeseenthatthebiggestadvantageofVDSListhatitcanobtainextremelyhighdatatransmissionrates;butitstransmissiondistanceisshort,thetransmissionrateisunstable,andthereisnostandard.
SDSL(SinglePairLine/SymmetricalDigitalSubscriberLine)
SDSLcanbesaidtobeabranchofHDSL.SDSLonlyusesonetelephoneline(ieapairofcopperwires)forfullduplexCommunication,supportingthetransmissionrateofuptoT1/E1symmetricaluplinkanddownlinkchannels,andthetransmissiondistancecanreach3kmatthesametime.ThecharacteristicsofSDSLtechnologyarebasicallythesameasthoseofHDSL,anditsstandardsareunderdevelopment.
IDSL(basedonISDNDigitalSubscriberLine)
IDSLcanbeconsideredasanextensionofISDNtechnology,itisusedtoprovideuserswithbasicrateBRI(128Kbps)ISDNservices,butItstransmissiondistancecanreach5km,anditsmainapplicationsareremotecommunicationandremoteofficeconnection.
Modulationtechnology
ThekeytoxDSListhemodulationanddemodulationtechnologyituses.Generally,high-speeddigitalsignalprocessingtechnologyandbetter-performancetransmissionpatternsareusedtoobtainhightransmissionratesandLongdistance.Atpresent,ADSLmodulationanddemodulationtechnologyhasnotyetformedaunifiedstandard,butthemaintechnologiescanbedividedintotwotypes:
1,CAPmodulation(CarrierlessAmplitudeandPhaseModulation,nocarrieramplitudeandphasemodulation)
2,DMTmodulation(DiscreteMultiToneModulation,DiscreteMultiToneModulation)
NowitisdifficulttojudgewhichofCAPandDMTmodulationtechnologywillgetgreaterdevelopment,perhapsthefiercemarketeconomyandCompetitionwilltellustheanswer.
Accessmethod
ThefollowingtakesADSLasanexampletoillustratethexDSLaccessmethod.OtherxDSLtechnologiesaresimilartoADSL.
ADSLequipmentconnectionisdividedintotwoends:userequipmentandserviceproviderequipment.Amongthem,theuserequipmentincludesPOTS(PlainOldTelephoneService)splitterandADSLmodem(MODEM).ThePOTSsplitterisusedtoseparatethevoiceanddatatransmission,sothatthevoiceistransmittedtothetelephone,andthedataissenttotheADSLMODEM,sothatthevoiceanddatacanbetransmittedatthesametime.ADSLMODEMisusedtoconvertthedatatobereceivedandsentintocorrespondingtransmissioncodestoobtainhigh-speedandlong-distancetransmission.ItmustbecompatiblewiththeMEDEMattheaccessendofthenetworkservice.Itcanbeseenfromthisthattheuserterminalaccessisverysimple.
ServiceproviderequipmentmainlyincludesDSLaccessmultiplexer(DSLAM)andotherequipment.DSLAMequipmentiscomposedofcentralnetworkpointADSLMODEMandserviceaccessmultiplexerSAM.ItsmainfunctionistogatherADSLlinesofmanyuserstogether,redirectvoicetoPSTNandtransmitdatatohigh-speedlines.
Developmentstatusandtrends
In1987,theBellcoreFederationcomposedofPROCproposedtheoriginalxDSLtechnology.Atthattime,xDSLwasdesignedtoprovideVODontwisted-paircopperwires.AndinteractiveTVapplications.ButthenthecooperationbetweenBellandcableCATVappearedcracks,soxDSLtechnologyalsostagnated.Duetotherapidpopularizationofinformationnetworksandthehighcostofopticalfibersthatarenotsuitableforlarge-scaleuse,thexDSLtechnologyhasregainedtheattentionoftheindustry.IntheUnitedStates,the"TelecommunicationReformAct"approvedbythegovernmentin1996allowedfreecompetitioninthetelecommunicationsfield,whichgreatlypromotedthedevelopmentofxDSLtechnology.AlthoughxDSLtechnologieshavemanyadvantages,manyproblemsneedtobesolvedbeforetheycanbewidelyusedincommercialapplications.Firstofall,themostimportantissueistheneedtoformarelativelycentralized,unifiedandopenstandard;secondly,thereareissuesofinteroperability,security,andinterference.Finally,whethernetworkserviceproviderscansuccessfullyswitchfromexistingtechnologytoxDSLtechnologyandothernon-technicalfactorsarealsoissuesthataffectthedevelopmentofxDSL.
Inthenexttenyears,xDSLtechnologyisexpectedtobeusedtoextendwide-areabroadbandnetworkssuchasATMtransmissiontomillionsofhouseholdsonexistingtelephonelines.Inordertoachievethisambitiousgoal,organizationssuchasANSI,ETSI,ADSLForum,ATMForum,andDAVICarejointlyformulatingrelevantstandards.Ofcourse,thereisstillalongwaytogotoachievetransmissionstandardswithhighperformanceandpriceratios.
AmongvariousxDSLtechnologies,ADSLtechnologyiscurrentlythemostattractive.Inordertoobtainahightransmissionrate,ADSLusestwomethods,frequencydivisionmultipleaccess(FDM)andechocancellation,todividetheavailablebandwidthofatelephonelineintomultiplechannels.ExperimentsforthedevelopmentofADSLtechnologyaremainlyconcentratedincountriesnearNorthAmericaandthePacificOcean,andarecurrentlyinthestageoflarge-scaleexperiments.ADSLMODEMhasbeensuccessfullytestedbymorethan30companies,andthousandsoflineshavebeenusedfortestinginNorthAmericaandEurope.Thekeyissueatpresentistoformaunifiedinternationalstandard;andtosolvetheproblemsarisingfromthemixingofcommercialapplicationsandconsumerapplications,suchasmanagementandbillingissues.
Accordingtoestimatesbysomeexperts,ADSLproductsandserviceswillbeintroducedattheendofthisyearorearlynextyear,andADSLtechnologywillbeusedonalargescalewithintwoyears.ADSLapplicationswillincludeInternetaccess,remotedatabaseaccess,video-on-demand,homeshopping,interactivegames,anddistanceeducation.Asusers,wewillwaitandsee.