计算机网络英文版课件：5-7 以太网

以太网EthernetHistoryDevelopedbyBobMetcalfeandothersatXeroxPARCinmid-1970sRootsinAlohapacket-radionetworkStandardizedbyXerox,DEC,andIntelin1978LANstandardsdefineMACandphysicallayerconnectivityIEEE802.3(CSMA/CD-Ethernet)standard–originally2MbpsIEEE802.3ustandardfor100MbpsEthernetIEEE802.3zstandardfor1,000MbpsEthernetCSMA/CD:Ethernet’sMediaAccessControl(MAC)policyCS=carriersenseSendonlyifmediumisidleMA=multipleaccessCD=collisiondetectionStopsendingimmediatelyifcollisionisdetectedEthernet“dominant”wiredLANtechnology:cheap$20forNICfirstwidelyusedLANtechnologysimpler,cheaperthantokenLANsandATMkeptupwithspeedrace:10Mbps–10GbpsMetcalfe’sEthernetsketchEthernetFrameStructureSendingadapterencapsulatesIPdatagram(orothernetworklayerprotocolpacket)inEthernetframePreamble:

7byteswithpattern10101010followedbyonebytewithpattern10101011usedtosynchronizereceiver,senderclockratesEthernetFrameStructure(more)Addresses:6bytesifadapterreceivesframewithmatchingdestinationaddress,orwithbroadcastaddress(egARPpacket),itpassesdatainframetonetworklayerprotocolotherwise,adapterdiscardsframeType:indicateshigherlayerprotocol(mostlyIPbutotherspossible,e.g.,NovellIPX,AppleTalk)CRC:checkedatreceiver,iferrorisdetected,frameisdroppedEthernetusesCSMA/CDNoslotsadapterdoesn’ttransmitifitsensesthatsomeotheradapteristransmitting,thatis,carriersensetransmittingadapterabortswhenitsensesthatanotheradapteristransmitting,thatis,collisiondetectionBeforeattemptingaretransmission,adapterwaitsarandomtime,thatis,randomaccessEthernetCSMA/CDalgorithm1.NICreceivesdatagramfromnetworklayer,createsframe2.IfNICsenseschannelidle,startsframetransmissionIfNICsenseschannelbusy,waitsuntilchannelidle,thentransmits3.IfNICtransmitsentireframewithoutdetectinganothertransmission,NICisdonewithframe!4.IfNICdetectsanothertransmissionwhiletransmitting,abortsandsendsjamsignal5.Afteraborting,NICentersexponentialbackoff:aftermthcollision,NICchoosesKatrandomfrom

{0,1,2,…,2m-1}.NICwaitsK·512bittimes,returnstoStep2ExponentialBackoffIfacollisionisdetected,delayandtryagainDelaytimeisselectedusingbinaryexponentialbackoff1sttime:chooseKfrom{0,1}thendelay=K*51.2us2ndtime:chooseKfrom{0,1,2,3}thendelay=K*51.2usnthtime:delay=Kx51.2us,forK=0..2n

–1Notemaxvaluefork=1023giveupafterseveraltries(usually16)ReporttransmiterrortohostIfdelaywerenotrandom,thenthereisachancethatsourceswouldretransmitinlockstepWhynotjustchoosefromsmallsetforKThisworksfineforasmallnumberofhostsLargenumberofnodeswouldresultinmorecollisionsStateDiagramforCSMA/CDCSMA/CDefficiencyTprop=maxpropdelaybetween2nodesinLANttrans=timetotransmitmax-sizeframeefficiencygoesto1astpropgoesto0asttransgoestoinfinitybetterperformancethanALOHA:andsimple,cheap,decentralized!EthernetPerformanceEfficiencyofEthernetat10Mbpswith512-bitslottimes.802.3EthernetStandards:Link&PhysicalLayersmanydifferentEthernetstandardscommonMACprotocolandframeformatdifferentspeeds:2Mbps,10Mbps,100Mbps,1Gbps,10Gbpsdifferentphysicallayermedia:fiber,cableapplicationtransportnetworklinkphysicalMACprotocolandframeformat100BASE-TX100BASE-T4100BASE-FX100BASE-T2100BASE-SX100BASE-BXfiberphysicallayercopper(twisterpair)physicallayerEthernetCablingThemostcommonkindsofEthernetcabling.EthernetProblemsEthernet’speakutilizationisprettylow(likeAloha)PeakthroughputworstwithMorehostsMorecollisionsneededtoi