计算机网络专业英语 习题 T15.docx
Lesson15CSMA/CDandIEEE802
IEEE802
IEEE802referstoafamilyofIEEEstandardsdealingwithlocalareanetworksand
metropolitanareanetworks.Morespecifically,theIEEE802standardsarerestrictedtonetworks
carryingvariable-sizepackets.Bycontrast,incellrelaynetworksdataistransmittedinshort,
uniformlysizedunitscalledcells.Isochronousnetworks,wheredataistransmittedasasteady
streamofoctets,orgroupsofoctets,atregulartimeintervals,arealsooutofthescopeofthis
standard.Thenumber802wassimplythenextfreenumberIEEEcouldassign,though“802”is
sometimesassociatedwiththedatethefirstmeetingwasheld—February1980.
TheservicesandprotocolsspecifiedinIEEE802maptothelowertwolayers(DataLinkand
Physical)oftheseven-layerOSInetworkingreferencemodel.Infact,IEEE802splitstheOSI
DataLinkLayerintotwosub-layersnamedLogicalLinkControl(LLC)andMediaAccess
Control(MAC),sothatthelayerscanbelistedlikethis:
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DataLinkLayer
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LLCSublayer
MACSublayer
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PhysicalLayer
TheIEEE802familyofstandardsismaintainedbytheIEEE802LAN/MANStandards
Committee(LMSC).ThemostwidelyusedstandardsarefortheEthernetfamily,TokenRing,
WirelessLAN,BridgingandVirtualBridgedLANs.AnindividualWorkingGroupprovidesthe
focusforeacharea.
IEEE802.3isaworkinggroupandacollectionofIEEEstandardsproducedbytheworking
groupdefiningthephysicallayeranddatalinklayer’smediaaccesscontrol(MAC)ofwired
Ethernet.Thisisgenerallyalocalareanetworktechnologywithsomewideareanetwork
applications.Physicalconnectionsaremadebetweennodesand/orinfrastructuredevices(hubs,
switches,routers)byvarioustypesofcopperorfibercable.802.3isatechnologythatsupportsthe
IEEE802.1networkarchitecture.802.3alsodefineLANaccessmethodusingCSMA/CD.
IntroducedbyIBMin1984,itwasthenstandardizedwithprotocolIEEE802.5andwas
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