計(jì)算機(jī)網(wǎng)絡(luò)英文版課件：slide 04 The Media Access Control Sublayer

TheMediaAccessControlSublayerContentsTheChannelAllocationProblemHowtogetstousethechannelinanybroadcastnetworkBroadcastchannels,referredtoasmultiaccesschannelsorrandomaccesschannelsMultipleAccessProtocolsTheprotocolsusedtodeterminewhogoesnextonamultiaccesschannelbelongtoasublayerofthedatalinklayercalledtheMAC(MediumAccessControl)sublayer.Technically,theMACsublayeristhebottompartofthedatalinklayerEthernetDataLinkLayerSwitchingTheChannelAllocationProblemStaticChannelAllocationinLANsandMANsFDM,TDMDynamicChannelAllocationinLANsandMANsStationModelSingleChannelAssumptionCollisionAssumption.Frametransmissiontime:ContinuousTime,SlottedTime.CarrierSense,NoCarrierSense.MultipleAccessProtocolsALOHA:PureALOHAletuserstransmitwhenevertheyhavedatatobesent,duetothefeedbackpropertyofbroadcasting,asendercanalwaysfindoutwhetheritsframewasdestroyedbylisteningtothechannelIftheframewasdestroyed,thesenderjustwaitsarandomamountoftimeandsendsitagainSlottedALOHA:dividetimeintodiscreteintervals,eachintervalcorrespondingtooneframePureALOHAInpureALOHA,framesaretransmittedatcompletelyarbitrarytimes.PureALOHA(2)Vulnerableperiodfortheshadedframe.MultipleAccessProtocolsCarrierSenseMultipleAccessProtocols(CSMA)Protocolsinwhichstationslistenforacarrier(i.e.,atransmission)andactaccordinglyarecalledcarriersenseprotocols.1-persistentCSMA:

Ifthechannelisbusy,thestationwaitsuntilitbecomesidle.Theprotocoliscalled1-persistentbecausethestationtransmitswithaprobabilityof1whenitfindsthechannelidle.Ifthefirststation'ssignalhasnotyetreachedthesecondone,thelatterwillsenseanidlechannelandwillalsobeginsending,resultinginacollision.nonpersistentCSMA:

ifthechannelisalreadyinuse,thestationdoesnotcontinuallysenseitforthepurposeofseizingitimmediatelyupondetectingtheendoftheprevioustransmission.Instead,itwaitsarandomperiodoftimeandthenrepeatsthealgorithm.p-persistentCSMA:Whenastationbecomesreadytosend,itsensesthechannel.Ifitisidle,ittransmitswithaprobabilityp.Withaprobabilityq=1-p,itdefersuntilthenextslot.PersistentandNonpersistentCSMAComparisonofthechannelutilizationversusloadforvariousrandomaccessprotocols.MultipleAccessProtocolsCSMAwithCollisionDetection(CSMA/CD)toaborttheirtransmissionsassoonastheydetectacollision.quicklyterminatingdamagedframessavestimeandbandwidth.iswidelyusedonLANsintheMACsublayer.CSMAwithCollisionDetectionCSMA/CDcanbeinoneofthreestates:contention,transmission,oridle.EthernetEthernetCablingManchesterEncodingTheEthernetMACSublayerProtocolTheBinaryExponentialBackoffAlgorithmSwitchedEthernetIEEE802.2:LogicalLinkControlRetrospectiveonEthernetIEEE802StandardsThe802workinggroups.Theimportantonesaremarkedwith*.Theonesmarkedwitharehibernating.Theonemarkedwith?gaveup.EthernetArchitectureoftheoriginalEthernet.EthernetCablingThemostcommonkindsofEthernetcabling.EthernetCabling(2)ThreekindsofEthernetcabling.(a)10Base5,(b)10Base2,(c)10Base-T.EthernetCabling(3)Cabletopologies.(a)Linear,(b)Spine,(c)Tree,(d)Segmented.Arepeaterisaphysicallayerdevice.Itreceives,amplifies(regenerates),andretransmitssignalsinbothdirections.EthernetCoding(a)Binaryencoding,(b)Manchesterencoding,

(c)DifferentialManchesterencoding.EthernetMACSublayerProtocolFrameformats.(a)DIXEthernet,(b)IEEE802.3.IEEE802.2/802.3encapsulation(RFC1042)andEthernetencapsulation(RFC894)6destinationaddr.6sourceaddr.2typedata46-1500CRC4type08002IPdatagram46-1500type08062ARPrequest/reply28PAD18type80352RARPrequest/reply28PAD1846-1500Ethernetencapsulation(RFC894):6destinationaddr.6sourceaddr.2lengthDSAPAA1SSAPAA1cntl1orgcode003type2data38-1492CRC4type08002IPdatagram38-1492type08062ARPrequest/reply28PAD10type80352RARPrequest/reply28PAD10IEEE802.2/802.3encapsulation(RFC1042):802.3MAC802.2LLC802.2SNAPThespecialEthernetdestinationaddressofallonebitsmeansthebroadcast

address.EthernetdestinationaddrEthernetsourceaddrframetypehardtypeprottypeopsenderIPaddrtargetIPaddrSenderEthernetaddrtargetEthernetaddrEthernetheader28bytesARPrequest/reply662221126464protsizehardsizeFigure4.3FormatofARPrequestorreplypacketwhenusedonanEthernet=0x0806=1:forEthernet=0x0800:forIPaddress=1:ARPrequest;=2:ARPreply(2);=3:RARPrequest;=4:RARPreplyEthernetMACSublayerProtocol(2)TheBinaryExponentialBackoffAlgorithmAfteracollision,timeisdividedintodiscreteslotswhoselengthisequaltotheworst-caseround-trippropagationtimeontheether(2t).Afterthefirstcollision,eachstationwaitseither0or1slottimesbeforetryingagain.Ingeneral,aftericollisions,arandomnumberbetween0and2i-1ischosen,andthatnumberofslotsisskipped.However,aftertencollisionshavebeenreached,therandomizationintervalisfrozenatamaximumof1023slots.After16collisions,thecontrollerthrowsinthetowelandreportsfailurebacktothecomputer.Furtherrecoveryisuptohigherlayers.SwitchedEthernetAsimpleexampleofswitchedEthernet.IEEE802.2:LogicalLinkControl(a)PositionofLLC.(b)Protocolformats.LLCprovidesthreeserviceoptions:1）unreliabledatagramservice,2）acknowledgeddatagramservice,and3）reliableconnection-orientedservice.RetrospectiveonEthernetLongevitySimpleandflexible,cheap,easytomaintainEthernetinterworkseasilywithTCP/IPSpeedsofEthernethavegoneupbyseveralordersofmagnitudeandhubsandswitcheshavebeenintroduced,butthesechangeshavenotrequiredchangingthesoftware.DataLinkLayerSwitchingBridgesfrom802.xto802.yLocalInternetworkingSpanningTreeBridgesRemoteBridgesRepeaters,Hubs,Bridges,Switches,Routers,GatewaysVirtualLANsDataLinkLayerSwitchingMultipleLANsconnectedbyabackbonetohandleatotalloadhigherthanthecapacityofasingleLAN.Bridgesfrom802.xto802.yOperationofaLANbridgefrom802.11to802.3.Bridgesfrom802.xto802.y(2)TheIEEE802frameformats.Thedrawingisnottoscale.LocalInternetworkingAconfigurationwithfourLANsandtwobridges.SpanningTreeBridgesTwoparalleltransparentbridges.SpanningTreeBridges(2)(a)InterconnectedLANs.(b)AspanningtreecoveringtheLANs.Thedottedlinesarenotpartofthespanningtree.RemoteBridgesRemotebridgescanbeusedtointerconnectdistantLANs.Repeaters,Hubs,Bridges,Switches,RoutersandGateways(a)Whichdeviceisinwhichlayer.(b)Frames,packets,andheaders.Repeaters,Hubs,Bridges,Switches,RoutersandGateways(2)(a)Ahub.(b