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xDigital Subscriber Line



ThesynonymxDSLgenerallyreferstothexdigitalsubscriberline.ThegeneraltermforvarioustypesofDSL(DigitalSubscribeLine),includingADSL,RADSL,VDSL,SDSL,IDSLandHDSL.xDSLisanewtransmissiontechnologythatusesahigherfrequencyandcorrespondingmodulationtechnologyontheexistingcoppertelephoneline,thatis,theuseofsignalprocessingtechnologythataddsorobtainsmoredigitaldataintheanaloglinetoobtainhightransmissionSpeed​​(theoreticalvaluecanreach52Mbps).ThebiggestdifferencebetweenvariousDSLtechnologiesisreflectedinthedifferenceinsignaltransmissionrateanddistance,andthedifferenceinsymmetrybetweentheupstreamchannelandthedownstreamchannel.

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-Speed​​DigitalSubscriberLine)

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-Speed​​DigitalSubscriberLine)

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.

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