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计算机网络专业英语 习题 T15.docx

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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:

?

DataLinkLayer

?

?

LLCSublayer

MACSublayer

?

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

fa

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