资源描述：
《电子与通信专业英语第十一单元扩展阅读原文与翻译》由会员上传分享，免费在线阅读，更多相关内容在教育资源-天天文库。
1、SynchronousdigitalhierarchySynchronousopticalnetworking(SONET)andsynchronousdigitalhierarchy(SDH)arestandardizedmultiplexingprotocolsthattransfermultipledigitalbitstreamsoveropticalfiberusinglasersorlight-emittingdiodes(LEDs).Lowerdataratescanalsobetransferredviaa
2、nelectricalinterface.ThemethodwasdevelopedtoreplacethePlesiochronousDigitalHierarchy(PDH)systemfortransportinglargeramountsoftelephonecallsanddatatrafficoverthesamefiberwithoutsynchronizationproblems.SONETgenericcriteriaaredetailedinTelcordiaTechnologiesGenericReq
3、uirementsdocumentGR-253-CORE.GenericcriteriaapplicabletoSONETandothertransmissionsystems(e.g.,asynchronousfiberopticsystemsordigitalradiosystems)arefoundinTelcordiaGR-499-CORE.SONETandSDH,whichareessentiallythesame,wereoriginallydesignedtotransportcircuitmodecommu
4、nications(e.g.,DS1,DS3)fromavarietyofdifferentsources,buttheywereprimarilydesignedtosupportreal-time,uncompressed,circuit-switchedvoiceencodedinPCMformat.TheprimarydifficultyindoingthepriortoSONET/SDHwasthatthesynchronizationsourcesofthesevariouscircuitswerediffer
5、ent.Thismeantthateachcircuitwasactuallyoperatingataslightlydifferentrateanddifferentphase.SONET/SDHallowedforthesimultaneoustransportofmanydifferentcircuitsofdifferingoriginswithinasingleframingprotocol.SONET/SDHisnotitselfacommunicationsprotocol,butatransportprot
6、ocol.DuetoSONET/SDH’sessentialprotocolneutralityandtransport-orientedfeatures,SONET/SDHwastheobviouschoicefortransportingAsynchronousTransferMode(ATM)frames.ItquicklyevolvedmappingstructuresandconcatenatedpayloadcontainerstotransportATMconnections.Inotherwords,for
7、ATM(andeventuallyotherprotocolssuchasEthernet),theinternalcomplexstructurepreviouslyusedtotransportcircuit-orientedconnectionswasremovedandreplacedwithalargeandconcatenatedframeintowhichATMcells,IPpackets,orEthernetframesareplaced.BothSDHandSONETarewidelyusedtoday
8、:SONETintheUnitedStatesandCanada,andSDHintherestoftheworld.AlthoughtheSONETstandardsweredevelopedbeforeSDH,itisconsideredavariationofSDHbecauseofSDH’sgr
