基于双绞线的千兆以太网1000Base-T与1000BaseTX

基于双绞线的千兆以太网1000Base-T与1000Base-TX1000Base-T1000Base-T技术能在五类线(通过TSB-95认证)上提供1000Mbps的传输带宽，而五类线是在LAN体系中最广泛采用的物理媒体。IEEE的标准化委员会在1999年6月正式批准1000Base-T成为一种以太网标准。它是一种使用5类UTP作为网络传输介质的千兆以太网技术，最长有效距离与1000Base-TX一样可以达到100米。用户可以采用这种技术在原有的快速以太网系统中实现从100Mbps到1000Mbps的平滑升级。与1000Base-LX、1000Base-SX和1000Base-CX网络介质不同，1000Base-T不支持8B/10B编码/译码方案，需要采用专门的更加先进的编码/译码机制。如图所示，1000Base-T是千兆以太网4个物理层之一，收发机制为千兆以太网的两个标准之一：IEEE802.3z或称1000Base-X和IEEE802.3ab或称1000Base-T。在1000Base-T之前的1000Base-X技术采用三种物理介质：单模光纤、多模光纤和25米的短距离屏蔽铜缆。由于目前的大多数布线系统是采用的5类双绞线，1000Base-T标准可支持在符合ANSI/TIA/EIA-568A（1995）标准的5类双绞线上运行千兆以太网。1000Base-T采用4对5类双绞线完成1000Mbps的数据传送，每一对双绞线传送250Mbps的数据流。1000Base-T要点：4对线全都使用（全双工）全双工运行网络设备需要串扰/回声消除技术超5类及更高的布线系统都可以支持4级编码（PAM-5）每个信号电平代表2比特每秒发送125M符号与100Base-Tx符号速率相同降低噪声的干扰每对线支持250Mbps的数据速率（每个方向）1000Base-Tx1000Base-TX也是基于四对双绞线，但却是以两对线发送，两对线接收(类似于100Base-TX的一对线发送一对线接收)。由于每对线缆本身不进行双向的传输，线缆之间的串扰就大大降低，同时其编码方式也相对简单。这种技术对网络的接口要求比较低，不需要非常复杂的电路设计，降低了网络接口的成本。但由于使用线缆的效率降低了(两对线收，两对线发)，要达到1000mbps的传输速率，要求带宽就超过100mhz，也就是说在五类和超五类的系统中不能支持该类型的网络。一定需要六类系统的支持。2对线接收，2对线发送网络设备无需回声消除技术只有6类或更高的布线系统才能支持CablingInstallation&Maintenance的相关文章ThecaseforCategory6asaGigabitEthernetinfrastructureThe1000Base-TXstandardwillpavethewayforlowercostgigabit-speedsystems.Weoftenhearthemantraofthecableindustry-futureproofyourcablingsystem.Butwith1000Base-T(GigabitEthernet)designedtoworkoverexistingCategory5and5esystems,isthereasolideconomicjustificationtodeployCategory6cabling?Inshort,yes.Category6isnotonlythebestchoicetoassuresufficientheadroomforreal-worldimplementationofGigabitEthernet,italsoprovidesthenecessaryfoundationforloweringthetotalsystemcostofGigabitEthernet.TounderstandwhyCategory6cablingwillsoonbecomeacriticalpathintheprogressionofEthernet,let'sfirstexaminethepathourindustryhastaken.Dr.RobertMetcalfinventedEthernetwhileworkingforXeroxin1976.Thetechnologywasrefined,andasecondgeneration-calledEthernetII,andoftenreferredtoasDIXafteritscorporatesponsorsDigital,IntelandXerox-waswidelyused.WhileEthernet'shumblebeginningswereonly2.94Mbits/sec,itsoonevolvedintoa10Mbits/secnetworkprotocolthatquicklysurpassedtokenringarchitectures.EngineersmayargueaboutthetechnicaladvantagesofEthernet,butitstrueinitialadvantagewasinitseconomics.TheacceptanceofEthernetbychipmanufacturersledtolowercostnetworkinterfacecards(NICs),whichinturnfueledenduseracceptance.Morethan22yearsago,EthernetbecamethedefactoLANprotocoleverywhere.TheflexibilityofEthernetdesignallowedforthedevelopmentofFastEthernet(100Mbits/sec)in1995,withcompleteretentionoftheoriginalframestructureof10Base-T.ClickheretoenlargeimageThe1000Base-Tconfigurationaddsasignificantamountofcomplexityandcostsbecauseofthebi-directionaltransmission(two-waytransmissiononasinglepair).Thisdesignrequirestheuseofhybridstoseparatethetransmissionpathfromthereceivepath.Italsorequireshigh-powereddigitalsignalprocessors(DSPs)tocanceltheechoesgeneratedbythenearandfar-endhybrids.Incontrast,thedesignof1000Base-TXdoesnotrequirehybrids,nordoesitnecessitateechocancellation.TheGigabitEthernetNICmarketisshapinguptobeanother10/100market,andmostlikelywilltaketheformofa10/100/1000productofferingwithauto-sensingandauto-configurationforanyofthespeedssupportedbytheLAN.Thekeyquestionis:Whenwillthe10/100/1000NICpricefalltoa50%premiumover10/100NICs?Whenthathappens,wewouldarguethattherateofmarketadoptionofthetri-modeNICwillacceleratequickly.Currently,1000Mbits/sectransceiverchippricesfor1000Base-Tareinthe$17range.Thiscostisasignificantpremiumtothecurrent10/100Mbits/sectransceivercost,andthegapisnotexpectedtoshrinksignificantlyanytimesoon.100Base-Tvs.1000Base-TXAllthishistoryandthecurrentdynamicsbringusto1000Base-TX.Lastyear,theTelecommunicationsIndustryAssociation/ElectronicIndustriesAlliance(TIA/EIA)approvedTIA/EIA-854FullDuplexEthernetSpecificationfor1000Mbits/secOperationOverBalancedTwisted-PairCabling,withlittlefanfare.WhilethisnewspecificationdoesnothingtoimprovethethroughputcapacitiesofGigabitEthernet,itsdesignallowsforsignificantreductionsinthecomplexityandquantityofchipsneededforGigabitEthernetNICs.Thetechnicaldifferencesof1000Base-TX,whencomparedto1000Base-T,willtranslatetolowercostNICsforendusers.Thepriceof1000Base-TXchipsisexpectedtofallbelowthatof1000Base-Tchipslatethisyear,withapricegapthatwillcontinuetowidenthereafter.ThepriceofNICsandEthernetswitches,beingdirectlycorrelatedtothechipcost,willcorrespondinglyfavor1000Base-TXtechnology.Consequently,themarketplaceisexpectedtoembrace1000Base-TXNICs(basedonTIA/EIA-854)over1000Base-TNICs(basedonIEEE802.3ab),andtherapidacceptanceof10/100/1000Mbits/secNICsover10/100Mbits/secNICsmaystartasearlyas2003.ClickheretoenlargeimageSinceGigabitEthernetrequirestheuseofallfourpairsinthecable,delayskewcanbeaconcernifallfourpairswithinthecabledonotmeettheproperspecification.Byusingcablesthathavedifferentinsulatingmaterialsoneachpair,thedelayskewmaybesignificantenoughthata250Mbits/sectransmissionprotocolwillnotworkproperly.Thereareseveralreasonswhy1000Base-Tequipmentwillbemadeobsoletebyanimprovedtechnologythatislesse