Features
Thetransmissionmediumisthephysicalpathbetweenthesenderandthereceiverinthecommunicationnetwork.Thetransmissionmediausedincomputernetworkscanbedividedintotwocategories:wiredandwireless.Twistedpair,coaxialcableandopticalfiberarethethreecommonlyusedtransmissionmedia.Satellitecommunications,wirelesscommunications,infraredcommunications,lasercommunications,andmicrowavecommunicationsincludeinformationcarriersthatareallwirelesstransmissionmedia.Thecharacteristicsofthetransmissionmediumhaveagreatinfluenceonthequalityofnetworkdatacommunication.Thesecharacteristicsare:1)Physicalcharacteristics:describethecharacteristicsofthetransmissionmedium.2)Transmissioncharacteristics:includingwhethertouseanalogsignaltransmissionordigitalsignaltransmission,modulationtechnology,transmissionvolumeandtransmissionfrequencyrange.3)Connectivity:point-to-pointormultipointconnection.4)Geographicalrange:ThemaximumdistancebetweenpointsontheInternet,whichcanbeusedinbuildings,betweenbuildingsorextendedtotheentirecity.5)Anti-interference:theabilitytopreventnoiseandinterferencefromaffectingdatatransmission.6)Relativeprice:Basedonthepriceofcomponents,installationandmaintenance.
Impact
Thespaceorentitythroughwhichinformationistransmitted.Itisanintegralpartofthetelecommunicationsystem,anditsfunctionistotransmittheinformationsentbythesendingdevicetothereceivingdevice.Thecharacteristicsofinformationwhenitistransmittedinthemediumhaveanimportantinfluenceonthecomposition,characteristicsandcommunicationqualityofthecommunicationsystem.Themediumhastwomaineffectsonthepropagationofelectromagneticwaves.Oneisthatthediffusionduringpropagationandtheabsorptionorreflectionofthemediummaketheenergyweaker,whichismanifestedastransmissionloss.Thesizeofthelossandthechangeovertimeareimportantcharacteristicsofthemedium.Thesecondisdistortionwhenitspreadsinthemedium.Forexample,thedistortioncausedbydifferentfrequencycomponentsinthesignalatdifferenttransmissionspeedsinthemedium(dispersioneffect)limitsthetransmissionfrequencyrangeofthemedium;orthereceivedsignalisdistortedorevenunstableduetotheexistenceofmultipatheffectsinspace;orduetothemediumThereareotherelectromagneticwavespropagatinginit,includingman-madeandnaturalones,whichcauseinterferencetothedesiredsignal.Thesewillcausethereceivedsignaltobedifferentfromtheoriginalsignal,whichisunrecognizableinseverecases.
Classification
Accordingtothewayelectromagneticwavespropagate,themediacanbedividedintotwocategories:oneisthephysicalcircuitsthatguideelectromagneticwavesbetweenthetransceiverequipment,suchaswiresandcoaxialcables.Tubes,waveguidesoropticalfibers,etc.,electromagneticwavespropagatealongtheline,andtheenergyisconcentratednearthewireorconfinedinthetubeoftheguidedwave.Thisiscalledthewiredmethod,andthechannelformedbyitiscalledthewiredinformation;theotheristhetransmissionandreceptionequipmentThereisnophysicalconnectionbetweenthetwo,andtheuseofelectromagneticwavestopropagateinspacetotransmitinformationiscalledawirelessmethod,andthechannelformedbyitiscalledawirelesschannel.
Wiredchannels
Includeopenwires,cables,waveguides,opticalcables,etc.Theyhavestabletransmission,lowinterference,largecapacityandstrongconfidentiality.However,itisnecessarytolaycablesandbuildsound-increasingstationsalongthelinetocompensateforlinelosses,whichrequiresalargeinvestmentandalongconstructiontime.Therefore,expandingsystemcapacity,improvinglineutilization,andreducingthecostofeachvoicechannelareimportantissuesforwiredcommunications.Therearethefollowingtypesofwiredlines.OverheadopenwireAcommunicationlinecomposedofoneormorepairsofbarecopperwires.Usuallythreetwo-waycarriervoicechannelsareprovidedbelow30kHz,and12two-waycarriervoicechannelsareprovidedbetween30and150kHz.Whenthefrequencyishigher,thelossandelectromagneticradiationincreasefaster,andthefrequencyexceeding150kHzisonlyusedinafewcases.Thecrosstalkbetweenlong-distanceparallelwirepairsisreducedbywire-paircrossovertechnology.Therearenotmanywirepairsthatcanbeerectedonthepole,sotheopenwiresystemhasasmallcapacityandiseasilyaffectedbyclimaticconditions.symmetricalcablethecablecoreiscomposedofoneormorepairsofmutuallyinsulatedmetalwires,andtheoutersheathismadeofmetalorothercompositematerials.Sincethecorewireissymmetricaltotheground,itiscalledasymmetricalcable.Thecrosstalkbetweenthewirepairsisweakenedbytwistingandbalancing.Symmetricalcablesaremostlyburiedundergroundandshieldedbyanoutersheath,sotheelectromagneticradiationissmall,andtheanti-interferenceandconfidentialityperformanceisbetterthanthatofopenwires.Amongthem,thereareaudiocablessuitableforaudiocommunicationandhigh-frequencycablessuitableforcarriercommunication.Apairofcorewiresinahigh-frequencycablecancommunicatewith60or120unidirectionalcarrierphones,andthehighestmultiplexingfrequencyis252or552kHz,respectively.Thelimitationonthehighestfrequencyismainlycrosstalk.Whencrosstalkisnotconsidered,thereusefrequencycanbeincreased.CoaxialcableThecablecoreiscomposedofseveralcoaxialtubes,andthecoaxialtubeisformedbyconcentricallysleevedtwoconductorsinsulatedfromeachother.Duetotheshieldingeffectoftheouterconductorathighfrequencies,theexternalelectromagneticinterferenceandthemutualcrosstalkbetweenthecoaxialtubeshavebeenreducedtoanegligibledegree.Coaxialcablecanensurehighcommunicationqualityinawidefrequencyband,andissuitableforuseasatransmissionmediumforlarge-capacitysystems.Losschangesapproximatelyaccordingtothelaw,sothehigherthereusefrequencyf,thegreatertheloss,andtheshorterthepitchdistance.Thetableliststhesystemcapacityandaveragepitchdistanceofthetwospecificationsofcoaxialcables.Largercoaxialcables(suchas3.7/13.5mminsize)arenotusedmuch.Therearealsomicrocoaxialcables(0.7/2.9mm)speciallyusedfordigitaltransmissionsystems.Cableslaidontheseabedoftenadoptasingle-tubecoaxialstructure,withsteelwirearmorontheoutside,andsteelstrandscanbeinstalledintheinnerconductortoincreaseitstensilestrengthaccordingtoneeds.WaveguideAmetaltubethatcantransmitelectromagneticwaveswithextremelyhighfrequencies.Inthelate1950s,acircularwaveguidesuitableforlong-distancetransmissionmediawasdeveloped.Whenthefrequencyis40-110GHz,thelossislessthan5dB/km.Therefore,itcanbeusedtoconstructamillimeterwavewaveguidecommunicationsystemwithasuper-largecapacity(tensofthousandstohundredsofthousandsofvoicechannels).However,themanufacturingandlayingrequirementsofwaveguidesareextremelyhigh,whichlimitsthepracticalityanddevelopmentofthistransmissionsystem.Opticalfibersandopticalcablescancarryahugeamountofinformationbymonochromaticlight.Therefore,sincetheinventionoflaserin1960,peoplehavealwaysattachedgreatimportancetothestudyofmediasuitableforlasertransmission.Amongthem,themosteffectiveistheuseofhigh-purityquartzglassfiberGuidelightwaves.Inasingle-modefiber,thelightwaveistransmittedinasinglemode,thedispersiondistortionissmall,andthetransmissionratecanexceed10Gbit/s).Therefore,single-modefiberisaverypromisingtransmissionmediumforlarge-capacityorultra-large-capacitysystems,butbecauseitscorediameterisonlyafewmicrons,theprecisionrequiredforcouplingandconnectionisextremelyhigh(seeopticalfibercommunication)."Wirelesschannel"radiocommunicationhasbecomeanimportantmeansofcommunication.Sincespectrumresourcesarelimited,openingupnewbandstoavoidmutualinterferenceisanimportantissueinthedevelopmentofradiocommunications.Intermsofthewavelengthsused,thelongoneshavebeenexpandedtoultra-longwavesandextremelylongwaves;theshortoneshavebeenreducedtomillimeterwaves,sub-millimeterwaves,andevenlightwaves.Intermsofspaceused,ithasalsodevelopedintounderground,underwaterandevenouterspace.Thepropagationmodeofelectromagneticwavesthroughthemediumcanbedividedintothreetypes:earthsurfacepropagation,ionosphericreflectionpropagationandline-of-sightpropagation.
Wirelesschannel
Electromagneticwavespropagatingalongthesurfaceoftheeartharealsocalledgroundwaves.Ontheonehand,itspropagationabilitydependsonthediffractionabilityofradiowavesalongthesurfaceoftheearth,includingtheabilitytobypassgroundobstacles;ontheotherhand,itdependsontheabsorptionlossofradiowavesontheground.Longwave,ultralongwaveandextremelylongwavehavestrongdiffractionabilityandlowabsorptionloss,sotheabilitytopropagatealongthegroundisthestrongest,reachingthousandsoreventensofthousandsofkilometers.However,thefrequencyrangeisnarrow,thechannelcapacityissmall,theatmosphericinterferenceislarge,andtheequipmentandantennaarehuge.Thehigherthefrequency,theworsetheabilitytopropagatealongtheground.Forexample,short-wavegroundwavescanonlytraveltensofkilometers.Asformicrowaves,groundwavesaregenerallynotused(seegroundwavepropagation).Whentheelectricwavepropagatesalongtheground,partoftheenergyistransmittedundergroundorunderwater,whichcanbeusedtorealizepartoftheundergroundorunderwatercommunication.Thelongerthewavelength,thestrongerthepenetrationability,whichcanreachseveralmetersortensofmeters(seeundergroundcommunication,underwatercommunication).Electromagneticwavesreflectandpropagatethroughtheionospherealsocalledskywaves.Theionospherehasareflectioneffectonlong,mediumandshortwaves.Amongthem,shortwavesaremostsuitableforthepropagationofskywaves.Smallerequipmentandantennascanbeusedtoachievelong-distanceandevenglobalcommunications.However,theshortwavefrequencybandisnarrowandthecapacityisnotlarge.Ultrashortwavesandmicrowavescanpenetratetheionosphereandcannotbepropagatedbyionosphericreflection.Whenusingsky-wavecommunication,duetothechangesintheionosphere(suchasdifferentdaysandnights,differentseasons,changesinsunspots,etc.),theionospherehasagreatinfluenceonthepropagationofelectricwaves,sothestabilityofcommunicationispoor(seeIonosphericelectricwavepropagation)).Theunevenairmassinthetropospherehasascatteringeffectonultrashortwavesandmicrowaves,andcanbeusedtorealizescatteringcommunicationforhundredsofkilometers.However,duetothelimitationofthedispersioneffect,theeffectivetransmissionbandwidthisrelativelynarrow,whichcanonlymeettheneedsofsmallandmedium-capacitycommunications(tensorhundredsofvoicechannels).Electromagneticwaveline-of-sightpropagationFromthesendingendtothereceivingendinastraightlinethroughthespacemedium.Ultrashortwavesandmicrowavesaremostlyspreadinthisway.Ifboththereceivingandtransmittingendsareontheground,thepropagationdistanceislimitedtothedirectviewingrange,abouttensofkilometers.Torealizelong-distancecommunication,severalrelaystationsmustbeestablished,andthesignalsaretransmittedtotheremoteendinturn,soitiscalledrelaycommunication(seemicrowaverelaycommunication).Althoughthismethodhasalargeinvestmentandpoormobility,itsfrequencyrangeismuchwiderthanthatofmediumandshortwaves,anditscapacityismuchlarger,anditstransmissionisrelativelystable,soitdevelopsrapidly.However,inthemillimeterwaveandsub-millimeterwavebands,theinfluenceofatmospherictemperatureandwatervaporcontentchangeshasbeensignificant.Rainandsnowwillalsocauseseriousattenuation,whichhasacertainimpactonthestabilityofcommunication.Ifoneendofthecommunicationisonthegroundandtheotherendisinouterspace,toformground-aircommunication(seespacecommunication),youmustchooseafrequencyrangethatisnotreflectedbytheionosphereandisnotseriouslyabsorbedbyrain,fog,etc.,thatis,theso-calledground-aircommunication"Window".