Cable channel

Transmissionmedia

Therearefourmaintypesofwiredchannels.Namely,openwire,symmetricalcable,coaxialcableandopticalfiber.

Openwire

Anopenwirereferstoanoverheadlinethatiserectedinparallelonapole.Ititselfisaconductivebarewireorawirewithaninsulatinglayer.Althoughitstransmissionlossislow,itissusceptibletotheinfluenceofweatherandenvironmentandismoresensitivetoexternalnoiseinterference,soithasbeengraduallyreplacedbycables.

Symmetricalcables

Therearetwotypesofcables,namelysymmetricalcablesandcoaxialcables.Asymmetricalcableismadeofseveralpairsofdoublewirescalledcorewiresplacedinaprotectivesheath.Inordertoreducetheinterferencebetweeneachpairofwires,eachpairofwiresismadeintoatwistedshape,calledatwistedpair.,Thepairsofwiresinthesamecablearealsotwistedtogetheraccordingtoacertainrule.Inatelecommunicationsnetwork,thereareusually25pairsoftwistedpairsinasymmetricalcable,andthecorediameterofthesymmetricalcableis0.4mm~1.4mm,thelossisrelativelylarge,buttheperformanceisrelativelystable.Symmetricalcablesarewidelyusedinuseraccesscircuitsinwiredtelephonenetworks.Eachuserphoneisconnectedtothetelephoneexchangethroughapairoftwisted-pairwires,usually22-26gaugetwisted-pairwires.Twisted-paircableshavealsobeenwidelyusedincomputerlocalareanetworks.TheCat5ecableusedinEthernetiscomposedoffourpairsoftwisted-paircables.

Coaxialcable

Thecoaxialcableiscomposedoftwolayersofconcentriccylindersinsideandoutside.Thetwoconductorsareseparatedbyaninsulator.Theinnerconductorismostlyasolidwire,andtheouterconductorisahollowconductivetubeorametalbraidedmesh.Thereisaninsulatingprotectivelayerontheouterconductor.Asoliddielectricmaterialfireinsulationbracketcanbefilledbetweentheinnerandouterconductorstosupportandinsulate..Sincetheouterconductorisusuallygrounded,itcanplayagoodshieldingrole.Withthewideapplicationofopticalfiber,opticalfiberisusedtoreplacecoaxialcableinthemainlineoflong-distancesignaltransmission.Coaxialcableisalsowidelyusedincabletelevisionbroadcasting(CATV:CableTelevision)toprovideuserswithtelevisionsignals.ThePCMgroupsignalinthetelephoneexchangestillusesthecoaxialcabletotransmitthesignal.Thecoaxialcableisalsousedasthetransmissionmediumfortheintermediatefrequencyandradiofrequencypartofthecommunicationequipment,suchasthefeederbetweenthewirelesscommunicationtransceiverequipmentandtheantenna.

Opticalfiber

Thewiredchannelthattransmitsopticalsignalsisopticalfiber,oropticalfiberforshort.ModernopticalfiberwasimprovedbytheChinesescientistCharlesKuen,whoisconsideredthe"fatherofopticalfiber."In1970,CorningintheUnitedStatesproducedtheworld'sfirstpracticalopticalfiber.Withthecontinuousimprovementofprocessingandmanufacturingtechnology,theattenuationofopticalfibercontinuestodecrease.Themaintransmissionnetworkoftheworld'strunklinesismainlycomposedofopticalfiber.

Thetransmissionofopticalsignalsinopticalfibersisbasedontheprincipleoftotalreflection.Opticalfiberscanbedividedintomulti-modefibers(MMF:Multi-ModeFiber)andsingle-modefibers(SMF:SingleModeFiber).Opticalsignalshavemultiplepropagationmodes,butthereisonlyonepropagationmodeinasingle-modefiber.Thesignallightsourceoftheopticalfibercanhavealight-emittingdiode(LED:Light-EmittedDioxide)andalaser.Theopticalwavelengthusedinpracticalapplicationsismainlyinthetwolow-losswavelengthwindowsof1.31and1.55.Forexample,the1000Base-LXphysicalinterfaceintheEthernetnetworkusesanopticalsignalwithawavelengthof1.31.850nmwavelengthsignallightsourceshavealsoappearedincomputerlocalareanetworks,suchasthe1000Base-SXphysicalinterfaceintheEthernetnetwork.ThespectralpurityoftheLEDlightsourceislow,andtheopticalsignalsofdifferentwavelengthstravelatdifferentspeedsintheopticalfiber.Therefore,asthedistanceincreases,theopticalsignalpropagateschromaticdispersion,whichcausessignaldistortion,andlimitsthedistanceofopticalfibertransmission.Therefore,forlong-distancetransmission,Thesignalneedstoberelayedeverysomedistance.Thedispersionofsingle-modefiberismuchsmallerthanthatofmulti-modefiber(thereismodaldispersioninmulti-modefiber),sothetransmissiondistancewithoutrelayislonger,andthedispersioncausedbylasersourcetransmissionwithhighspectralpurityissmaller..

Principle

Wiredchannelsusewiresasthetransmissionmedium.Signalsaretransmittedalongthewires.Thesignalenergyisconcentratednearthewires.Therefore,thetransmissionefficiencyishigh,butthedeploymentisnotflexibleenough.Thetransmissionmediausedinthistypeofchannelincludesoverheadopenwires,telephonelines,twistedpairs,symmetricalcables,andcoaxialcablesthattransmitelectricalsignalswithwires,aswellasopticalfibersthattransmitmodulatedopticalpulsesignals.

Features

Thetransmissionmediumisawire(twistedpairoropticalfiber,etc.).Thesignalistransmittedalongthewire,andtheenergyisrelativelyconcentratednearthewire,soithasahightransmissionefficiency.Highsignal-to-noiseratio,narrowfrequencybandresources,echoandnonlineardistortion.