Asysmmetricalcableismadeofseveralpairsofdoubleconductorscalledcorewiresinaprotectivesheath.Inordertoreducetheinterferencebetweenthepairsofwires,eachpairofwiresismadeintoatwistedshape,whichiscalledatwistedwire.Twistedpairwiresaretwocopperwireswithadiameterof0.32mm~0.8mm,whicharetwistedafterinsulationandotherprocesses.Theyaredividedintounshieldedtwistedpair(UTP)andshieldedtwistedpair(ShieldedTwistedPair,STP)twokinds.Multiplepairsoftwistedpairsintoacablearesymmetricalcables.Thecorewireofthesymmetricalcableisthinnerthantheopenwire,withadiameterof0.4mm~1.4mm,soitslossislargerthanthatoftheopenwire,buttheperformanceismorestable.Symmetricalcablesarewidelyusedforuseraccesscircuitsinwiredtelephonenetworks.Theamplitude-frequencycharacteristicsofsymmetricalcablesareoflow-passtype,andthecrosstalkincreasesasthefrequencyrises,sothedegreeofmultiplexingisnothigh.
Definition
Symmetricalcable:consistsoftwowireswiththesamequality,wirediameterandinsulationresistancetoground,andTheinsulatedwiresformapairoftransmissionloops,andacommunicationcableformedbytwistingmultiplepairsofsuchwires.
Chinesename:SymmetricalCable
Foreignname:SymmetricalCable
Alternativename:None
ApplicationSubject:InformationCommunication
Features:Sameresistancetoground,twoidenticalwires,transmissionloop,twisting
Twoidenticalwiresaretwistedtogetherforcommunication.Itseemsthatthishasbecomeasymmetricalcable,butthingsarenotthatsimple.Figure1showsapictureofasymmetricalcableforofficeuse.Wewillintroducethestructureofsymmetricalcableagain:
Figure1Symmetricalcableforlocaluse
Insulationlayer
Theconductorsinsymmetricalcablesgenerallyusecopperwires,withawirediameterof0.25mmto1.4mm.Thediameterofthesymmetricalcableusedinthecityisusually0.4mm～0.9mm;thediameterofthesymmetricalcableusedforthelongdistanceisusually1.2mm.Inafewcases,symmetricalcableswithaluminumcoresalsoappear.
Asfortheinsulationmaterialsofsymmetricalcables,theyaredividedintothefollowingthreecategories:
1)Paper:Paper-insulatedcableshavethreeforms:paperbag,pulpandrope.Themethodofropeinsulationistowindtheropeinaspiralonthewire,andthenwrapitwith1to2layersofpapertape.
2)Polyvinylchloride:Thedielectricconstantanddielectriclossofpolyvinylchloridearerelativelylarge,soitislimitedtoshort-distancesymmetricalcables.However,thecableofthismaterialhasflame-retardantpropertiesandcanbeusedforindoorwiring.
3)Polyolefinandpolystyrene:Thesetwomaterialshavegoodhigh-frequencyperformance,andarecommonlyusedincablesthattransmitsignalsatthemegahertzlevelorbelowthetertiarygroup.Polystyreneisusuallymadebyrope.Andpolyolefinsmostlyusesolid,foamorfoamleather.
Inordertofacilitateidentification,differentinsulatinglayersusuallychoosedifferentcolors.
Strandingmethod
Inthe"communicationcable"Theconceptofstar-twistingwasmentionedintheintroductionofterminology,andhereisabriefintroductiontothedifferenttwistingmethods.
1)2or4insulatedwiresaretwistedintoa2-wiregroupora4-wiregroupwithanappropriatetwist.Fora4-wiregroup,whenviewedfromthecrosssection,whenthepositionsofthe4wiresaresquare,wecallitastar-quad4-wiregroup,inwhichthe2wiresonthediagonalformaloop.
2)The4wiresthataretwistedintotwo2-wiregroupsfirst,andthentwistedtoeachother,wecallitadouble-pairtwisted4-wiregroup.
3)Layer-strandedtype:twistseveralwiregroupsintoacablecoreinlayers.
4)Unittype:firsttwistseveralwiregroupsintoindependentunits,andthentwistseveralunitsintocablecores.
Ontheoutsideofthelinegroup,differentcolortiemarksareoftenusedtodistinguishdifferentlinegroups.
Sheath
Inordertoensurethesafetyofsymmetricalcables,Wraptapeontheoutsideofthecablecore,andsometimesaddashieldinglayerandasheath.
1)Paper-insulatedandpolystyrenerope-tapeinsulatedcablesrequiretheuseofasealedsheath.Intheearlydays,lead-sealedsheathswereused,andlater,aluminum-sealedsheathswerealsoused.Insomecases,analuminum-polyethylenebondedsheathisalsoused,whichisasemi-hermeticsheath.
2)Forindoorcables,sheathsmadeofpolyvinylchlorideorlowhalogen(orhalogen-free)materialsareused.
Ontheoutsideofthesheath,accordingtodifferentuseenvironments,differentoutersheathsneedtobeaddedtoensurethesafetyofthecommunicationcable.
Reduceinterference
Inthedesignandmanufactureofsymmetricalcables,scientificandtechnologicalworkershavetakenvariousmeasures,butthewireloopTherewillstillbeacertaindegreeofmutualinterference.Inordertoreducetheimpactofnear-endcrosstalkbetweenloopsontransmissionlinesindifferentdirections,scientistshavethoughtofothermethods:
1)Dualcablesystem:TwoloopsthatarenotinthesamecablehavethesameThefrequencyfortwo-waytransmission.
2)Singlecablesystem:Onlyoneloopisusedfortwo-waytransmissionatdifferentfrequencies.
3)Innershield:Sometimesaspecialshieldisusedtoseparatethecablecoreintotwopartsinacabletoreplacetwocables.Wecallthistypeofcableaninnershieldedcable.