计算机网络专业英语 课件 Lesson 15 CSMACD and IEEE802.pptx
L15-1
Lesson15CSMA/CDandIEEE802Text1CarrierSenseMultipleAccesswithCollisionDetection
KeyPoints要点冲突域向后/前兼容IEEEStd802.3来回时间Isolate、insulate理解课文
Carriersensemultipleaccesswithcollisiondetection(CSMA/CD)isamediaaccesscontrolmethodusedmostnotably尤其inlocalareanetworkusingearlyEthernettechnology.Itusesacarriersensingschemeinwhichatransmittingdatastationdetectsothersignalswhiletransmittingaframe,andstopstransmittingthatframe,transmitsajamsignal,andthenwaitsforarandomtimeintervalbeforetryingtoresendtheframe.CSMA/CDisamodificationofpurecarriersensemultipleaccess(CSMA).CSMA/CDisusedtoimproveCSMAperformancebyterminatingtransmissionassoonasacollisionisdetected,thusshorteningthetimerequiredbeforearetrycanbeattempted.shorten和liken都是形容词加en构成的动词。enhance加强，improve改进，increase增加。
Thejamsignalorjammingsignalisasignalthatcarriesa32-bitbinarypattern模式sentbyadatastationtoinformtheotherstationsofthecollisionandthattheymustnottransmit.Themaximumjam-timeiscalculatedasfollows:ThemaximumalloweddiameterofanEthernetinstallationislimitedto232bits.Thismakesaround-trip-timeof464bits.Astheslot间隙timeinEthernetis512bits,thedifferencebetweenslottimeandround-trip-timeis48bits(6bytes),whichisthemaximum“jam-time”.Thisinturn反过来means:Astationnotingacollisionhasoccurredissendinga4byteslongpatterncomposedof161-0bitscombinations.Note:Thesizeofthisjamsignalisclearlybeyondtheminimumallowedframe-sizeof64bytes.Thepurposeofthisistoensurethatanyothernodewhichmaycurrentlybereceivingaframewillreceivethejamsignalinplaceofthecorrect32-bitMACCRC(CyclicRedundancyCheck),whichcausestheotherreceiverstodiscard抛弃theframeduetoaCRCerror.
In10megabitsharedmediumEthernet,ifacollisionerroroccursafterthefirst512bitsofdataaret