第3章：OSI層次：數(shù)據(jù)鏈路層-2015簡化版

OSILayer2:

DataLinkLayerLayer2:DataLinkLayerOverviewoftheDataLinkLayerEthernetandCSMA/CDLLCandMACSub-layersMediaAccessControlinMACSub-layerWirelessLANandCSMA/CALayer2DevicesDataLinkLayerProblem:Howtotransferdatacorrectlyonainstablelink?TheDATALINKLAYERprovides:Access

tothenetworkingmediaPhysical

transmissionacrossthemediaLayer2protocols(procedures)define:TheformatofdataexchangedonalinkTheactionofthetwonodesonthelinkInDataLinkLayer,‘procedure’=‘protocol’NodeNodeframeframeNode：HostandrouterinthenetworkLink：ThechannelconnectstheadjacentnodesLANSandtheDataLinkLayerMaintasks:Error

notificationNetwork

topologyFlow

controlDifferencesbetweenLayer1andLayer2:Layer1cannotcommunicatewiththeupper-levellayers;Layer2doesthatwithLogicalLinkControl(LLC).Layer1cannotdecidewhichhost

willtransmitorreceivebinarydatafromagroup;Layer2doesthatwithMediaAccessControl(MAC)Layer1cannotnameoridentifycomputers;Layer2usesanaddressing(ornaming)process.Layer1canonlydescribestreamsofbits;Layer2usesframingtoorganizeorgroupthebits.Threeservicesprovidedtothenetworklayer(byLLC)Connectionlessservicewithnoacknowledgement,usedon:Reliablelinks(upperlayerstoensurethedatacorrectness)Real-timetasksMostofLANsConnectionlessservicewithacknowledgements:unreliablelink,suchasthewirelessnetworkConnection

servicewithacknowledgementsServicesprovidedbyLayer2MediaAccessControlinCommonLANsEthernet

-logicalbustopology(informationflowisonalinearbus)andphysicalstarorextendedstar(wiredasastar)TokenRing-logicalringtopology(informationflowisinaring)andaphysicalstartopology(wiredasastar)FDDI-logicalringtopology(informationflowisinaring)andphysicaldual-ringtopology(wiredasadual-ring)AccessMethodsforMedia-AccessControlTwobroadcategories:Deterministic—takingturnsTokenRingandFDDINon-deterministic(probabilistic)—firstcome,firstservedEthernet/802.3DeterministicMACProtocolsAspecialdatatokencirculatesthering.Whenahostreceivesthetoken,itcantransmitdatainsteadofthetoken.Thisiscalledseizingthetoken.Whenthetransmittedframecomesbackaroundtothetransmitter,thestationtransmitsanewtoken;theframeisremovedorstrippedfromthering.Non-DeterministicMACProtocolsThisMACprotocoliscalledCarrierSenseMultipleAccesswithCollisionDetection(CSMA/CD)Tousethisshared-mediumtechnology,Ethernetallowsthenetworkingdevicestoarbitratefortherighttotransmit.LANTransmissionMethodsLANdatatransmissionsfallinto3classifications:unicast--asinglepacketissentfromthesourcetoasingledestinationonanetworkmulticast--consistsofasingledatapacketthatissenttoaspecificsubsetofnodesonthenetwork.broadcast--consistsofasingledatapacketthatistransmittedtoallnodesonthenetwork.Layer2:DataLinkLayerOverviewoftheDataLinkLayerEthernetandCSMA/CDLLCandMACSub-layersMediaAccessControlinMACSub-layerWirelessLANandCSMA/CALayer2DevicesLANStandardsDefinethephysicalmediaandtheconnectorsusedtoconnectdevicestomediaDefinethewaydevicescommunicateattheDATALINKLAYERTheDATALINKLAYERdefineshowdataistransportedoveraphysicalmedia.TheDATALINKLAYERalsodefineshowtoencapsulateprotocol-specifictrafficinsuchawaythattrafficgoingtodifferentupper-layerprotocolscanusethesamechannelasitgoesupthestack.LANStandardsDatalinklayerisbrokenintotwopartsbyIEEE:MediaAccessControl

(MAC)(transitionsdowntomedia)

LogicalLinkControl

(LLC)(transitionsuptothenetworklayer)

TheIEEEstandardappears,atfirstglance,toviolatetheOSImodelintwoways.

First,itdefinesitsownlayer(LLC),includingitsinterfaces,etc.

Second,itappearsthattheMAClayerstandards,802.3and802.5,crossovertheLayer2/Layer1interface.However,802.3and802.5definethenaming,framing,andMediaAccessControlrulesaroundwhichspecifictechnologieswerebuilt.LANStandardsMACsublayer(802.3)DefineshowtotransmitframesonthephysicalwireHandlesphysicaladdressing

Define

networktopologyDefine

linediscipline.LLC

sublayer(802.2)logicallyidentifiesdifferentprotocoltypesandthenencapsulatesthem.UseSAPidentifiertoperformthelogicalidentificationThetypeofLLCframedependsonwhatidentifiertheupperlayerprotocolexpects.LANStandardsMediaAccessControlSublayerPreambleDest.add.Sourceadd.LengthDataFCS#ofbytes8662Variable4EthernetIIuses“Type”hereanddoesnotuse802.20000.0CXX.XXXXIEEEassignedVendorassignedMACaddressMACLayer802.3MACSub-layer:FieldsinaFrameBeginwithanalternatingpatternof1sand0scalledapreamble.(10101011)Thepreambletellsreceivingstationsthataframeiscoming.PreambleDest.add.Sourceadd.LengthDataFCS#ofbytes8662Variable4EthernetIIuses“Type”hereanddoesnotuse802.20000.0CXX.XXXXIEEEassignedVendorassignedMACSub-layer:FieldsinaFrameDestinationandsourcephysicaladdressfieldssourceaddress:alwaysaunicastaddressdestinationaddress:unicast,multicast,orbroadcast.PreambleDest.add.Sourceadd.LengthDataFCS#ofbytes8662Variable4EthernetIIuses“Type”hereanddoesnotuse802.20000.0CXX.XXXXIEEEassignedVendorassignedMACSub-layer:FieldsinaFramelength

field

indicatesthenumberofbytesofdatathatfollowthisfieldandprecedetheframechecksequencefield.Thedatafieldcontainstheinformationyouwanttosend.PreambleDest.add.Sourceadd.LengthDataFCS#ofbytes8662Variable4EthernetIIuses“Type”hereanddoesnotuse802.20000.0CXX.XXXXIEEEassignedVendorassignedMACSub-layer:FieldsinaFrameFCSfield(fourbytes)containsacyclicredundancycheckvalueThesendingdevicecreatestheCRCThereceivingdevicerecalculatestheCRCtocheckfordamagethatmighthaveoccurredtotheframeintransit.PreambleDest.add.Sourceadd.LengthDataFCS#ofbytes8662Variable4EthernetIIuses“Type”hereanddoesnotuse802.20000.0CXX.XXXXIEEEassignedVendorassignedLogicalLinkControlSublayerTheLogicalLinkControl(LLC)sublayermanagescommunicationbetweendevicesoverasinglelinkLLCisdefinedintheIEEE802.2specificationandsupportsbothconnectionlessandconnect-orientedservices.LLCsublayerallowspartoftheDATALINKLAYERtofunctionindependentlyfromexistingtechnologies.AsingleLLCsub-layercanbecompatiblewithdifferentMACsub-layers.LLCSub-layer:Encapsulation(a)PositionofLLC (b)EncapsulationLLCMACDataLinkLayerPhysicalLayerNetworkLayerLLCPacketMACLLCPacketMACMediaPacketLLCSub-layer:EncapsulationTheLLCtakesthenetworkprotocoldata(packet),andaddsmorecontrolinformationtohelpdeliverthepackettoitsdestination.Itaddstwoaddressingcomponentsofthe802.2specificationtoidentifytheupperlayerprotocolateachend:TheDestinationServiceAccessPoint(DSAP)TheSourceServiceAccessPoint(SSAP)ThisrepackageddatathentravelstotheMACforfurtherencapsulationofthedata.Layer2:DataLinkLayerOverviewoftheDataLinkLayerEthernetandCSMA/CDLLCandMACSub-layersMediaAccessControlinMACSub-layerWirelessLANandCSMA/CALayer2DevicesHexadecimalNumbersasMACAddressesMACaddressesare48bitsandarealwaysexpressedas12hexadecimaldigits.Thefirst6hexadecimaldigits(fromlefttoright),whichtheIEEEadministers,identifythemanufacturerorvendorandcomprisetheOrganizationalUniqueIdentifier(OUI).Theremaining6hexdigitscomprisetheinterfaceserialnumber,administeredbyspecificvendor.0000.0c12.3456or00-00-0c-12-34-56Ethernet802.3BroadcastBroadcastThedestinationMAC:all1s(FFFF.FFFF.FFFF)BroadcastingcanseriouslyaffecttheperformanceofstationsbyinterruptingthemunnecessarilySobroadcastsshouldbeusedonlywhen:TheMACaddressofthedestinationisunknownThedestinationisallhostsFramingFramingistheLayer2encapsulationprocess.AframeisaLayer2protocoldataunit(PDU).Aframehassectionscalledfields,andeachfieldiscomposedofbytes.datafieldframechecksequencefieldframestopfieldframestartfield addressfieldlength/type/controlfieldEthernetMediaAccessControl——CSMA/CD:HistoryEthernet/IEEE802.3MACmechanism:CSMA/CD.CSMA/CDstemsfromALOHAIn1970s,NormanAbramsondesignedALOHAPureALOHA:Hostsalwayscansendthedataiftheywant:Ifthesignalscollided,backoffappropriateamountoftimeandresendSlottedALOHA:Hostsalwayscansendthedataiftheywant,buttheymustwaitforanewslot:Ifthesignalscollided,backoffappropriateamountoftimeandresendEthernetMediaAccessControl——CSMA/CD:HistoryQuestion:Whydon’twelistentothechannelfirsttomakesurethatthechannelisfree?ThatisCSMA(CarrierSenseMultipleAccess)1-persistCSMA:Hostsalwayscansendthedata(havetheprobabilityof1)iftheyfindthechannelisfree;ifthechannelisbusy,theywilllistentothechannelandwait:Ifthesignals

collided,backoffappropriateamountoftimeandresendEthernetMediaAccessControl——CSMA/CD:Historynon-persistCSMA:Hostsalwayscansendthedata(havetheprobabilityof1)iftheyfindthechannelisfree.ifthechannelisbusy,theywillwaitforawhiletolistento

thechannelagain:Ifthesignalscollided,backoffappropriateamountoftimeandresendp-persistCSMA(workingonslottedchannel):Ifthehostsfindthechannelisfree,theywillsendthedatainthisslotataprobabilityofp,orproponethesendingoperationtothenextslotataprobabilityof1-p;ifthechannelisbusy,theywillwaituntilthenextslot:Ifthesignalscollided,backoffappropriateamountoftimeandresendEthernetMediaAccessControl——CSMA/CDAnotherimprovementofALOHA:Listentothechannelwhiletransmittingdata.Ifacollisiondetected,alltransmissionswillbecancelledatonceCSMA/CD(CarrierSenseMultipleAccess)UseCSMAmechanismtojudgeifthehostshouldsendthedataInthetransmittingprocess,listentothechannelatthesametimeWhenacollisiondetected,broadcastthejamsignalBackoffalgorithmsdeterminewhenthecollidingstationscanretransmit.EthernetOperation1234Ethernetisbroadcast

network—thatis,eachstationcanseealltheframes,regardlessofwhethertheyarethedestinationWhetherastationisthedestinationisjudgedbyMACaddressDestinationstationsendsdataupOSIlayers.OthernodesdiscardframeEthernetOperationListenthentransmitBroadcastjamsignalCollisionoccursDevicesbackoffappropriateamountoftimeandthenretransmit1234Layer2:DataLinkLayerOverviewoftheDataLinkLayerEthernetandCSMA/CDLLCandMACSub-layersMediaAccessControlinMACSub-layerWirelessLANandCSMA/CALayer2DevicesWirelessLANWirelessLANCommunicationsbasedoncellsThesignalssentbyastationcanonlybereceivedbythestationsnearbyShort-distancetransmissionWirelessLANStandardIEEE802.11IEEE802.11bIEEE802.11aIEEE802.11gIEEE802.11nWirelessLANStandardIEEE802.11Akeytechnology:DirectSequenceSpreadSpectrum(DSSS)DSSSappliestowirelessdevicesoperatingwithina1to2Mbpsrange.DSSSmayoperateatupto11Mbpsbutwillnotbeconsideredcompliantabove2MbpsIEEE802.11bAlsocalledWi-Fi?Itincreasedtransmissioncapabilitiesto11MbpsAll802.11bsystemsarebackwardcompliantinthattheyalsosupport802.11for1and2MbpsdataratesforDSSSonlyAchieveshigherdatathroughputratebyusingadifferentcodingtechniquefrom802.11Operatewithin2.4GHzWirelessLANStandardIEEE802.11aCoversWLANdevicesoperatinginthe5GHztransmissionband.Usingthe5GHz802.11aiscapableofsupplyingdatathroughputof54Mbpsandwithproprietarytechnologyknownas"ratedoubling"hasachieved108Mbps.Inpractice,amorestandardratingis20-26Mbps.WirelessLANStandardIEEE802.11gprovidesthesamethroughoutas802.11a(54Mbps)butwithbackwardscompatibilityfor802.11busingOthogonalFrequencyDivisionMultiplexing(OFDM)technology.IEEE802.11n:nextgenerationWLANprovidedoublebandwidththan802.11g,thatis,108Mbps,andtheoreticallyupto500-600MbpsWirelessLANTopologyInfrastructuremode

andad-hocmodeFig.InfrastructureModeWirelessLAN:InfrastructureModeABasicServiceSet(BSS)includesaBaseStation(BS)andseveralwirelesshostsAllhostscancommunicatewitheachotherdirectlyinlocalBSSAccessPoint(AP)actsasaBaseStation(BS)forinfrastructuremodeAPishardwiredtothecabledLANtoprovideInternetaccessandconnectivitytothewirednetworkWhenanAPisinstalled,aServiceSetIdentifier(SSID)andachannelareassignedTherangeofthecellwillbefrom91.44to152.4meters(300to500feet)ABSScanconnecttoanotherBSSviaaDistributionSystem(DS),andconstructsanExtendedServiceSet(ESS)AccessingProcedureWhenaclientisactivatedwithintheWLANitwillstart"listening"foracompatibledevicewithwhichto"associate“Thisisreferredtoas"scanning"ActivescanningPassivescanningActivescanningCauseaproberequesttobesentfromthewirelessnodeseekingtojointhenetwork.TheproberequestwillcontaintheServiceSetIdentifier(SSID)ofthenetworkitwishestojoinWhenanAPwiththesameSSIDisfound,theAPwillissueaproberesponseTheauthenticationandassociationstepsarecompleted.PassivescanningListenforbeaconmanagementframes(beacons),whicharetransmittedbytheAP(infrastructuremode)orpeernodes(adhoc)WhenanodereceivesabeaconthatcontainstheSSIDofthenetworkitistryingtojoin,anattemptismadetojointhenetwork.PassivescanningisacontinuousprocessandnodesmayassociateordisassociatewithAPsassignalstrengthchanges.FramesinWLANWLANsdonotuseastandard802.3frame.TherearethreetypesofframesControlFramesManagementframesDataframes(Onlydataframesaresimilarto802.3frames)Thepayloadofwirelessdataframesand802.3framesis1500bytesHowever,anEtherframemaynotexceed1518byteswhereasawirelessframecouldbeaslargeas2346bytes.UsuallytheWLANframesizewillbelimitedto1518bytesasitismostcommonlyconnectedtoawiredEthernetnetwork.WhyWeNeedCSMA/CA?CollisionscanoccurinWLAN,butthestationscanonlyknowthetransmissionnearby,soCSMA/CDisnotagoodchoice.HiddenStationProblemWhenAistransmittingdatatoB,Ccan’tdetectthetransmissionbetweenAandB,soperhapsCwilldecidetotransmitdatatoBandresultinacollisionatB.ExposedStationProblemWhenBistransmittingdatatoA,Ccandetectthetransmission,soCwillnottransmitdatatoD.Butthatisamistake.ABCDBACDMultipleAccessingMechanismEthernetSignalsistransmittedtoallstationsonthecable.

Thesendingstationdetectsthecollisions.Atatime,onlyaneffectiveframecanbetransmittedonthechannel.WLANSignalsistransmittedtostationsneartothesendingstationonthecableTheMAC

protocolmusttryitbesttoensureonlyasendingstationneartothereceivingstationThereceivingstationdetectsthecollisions.Atatime,multipleeffectiveframescanbetransmittedonthechannel.CSMA/CACSMA/CA（CarrierSenseMultipleAccesswithCollisionAvoidance）發(fā)送站點在發(fā)送數(shù)據(jù)前，以控制短幀刺激接收站點發(fā)送應(yīng)答短幀，使接收站點周圍的站點監(jiān)聽到該幀，從而在一定時間內(nèi)避免數(shù)據(jù)發(fā)送基本過程A向B發(fā)送RTS（RequestToSend）幀，A周圍的站點在一定時間內(nèi)不發(fā)送數(shù)據(jù)，以保證CTS幀返回給A；B向A回答CTS（ClearToSend）幀，B周圍的站點在一定時間內(nèi)不發(fā)送數(shù)據(jù)，以保證A發(fā)送完數(shù)據(jù)；A開始發(fā)送若控制幀RTS或CTS發(fā)生沖突，采用二進(jìn)制指數(shù)后退算法等待隨機時間，再重新開始。TheActualThroughputWhenasourcenodesendsaframe,thereceivingnodereturnsapositiveacknowledgment(ACK).Thiscancauseconsumptionof50%oftheavailablebandwidth.Thisreducestheactualdatathroughputtoamaximumof5.0to5.5Mbpsonan802.11bwirelessLANratedat11Mbps.PerformanceofthenetworkwillalsobeaffectedbysignalstrengthAsthesignalbecomesweaker,AdaptiveRateSelection(ARS)maybeinvokedThetransmittingunitwilldropthedataratefrom11Mbpsto5.5Mbps,from5.5Mbpsto2Mbpsor2Mbpsto1Mbps.Layer2:DataLinkLayerOverviewoftheDataLinkLayerEthernetandCSMA/CDLLCandMACSub-layersMediaAccessControlinMACSub-layerWirelessLANandCSMA/CALayer2DevicesLayer2Devices—NICsNICsperformimportantLayer2datalinklayerfunctions:LogicalLinkControl-communicateswithupperlayersinthecomputerMediaAccessControl-providesstructuredaccesstosharedaccessmedianaming-providesauniqueMACaddressidentifierframing-partoftheencapsulationprocess,packagingthebitsfortransportsignaling-createssignalsandinterfacewiththemediabyusingbuilt-intransceiversLayer2Devices—BridgesBridgesdividetrafficintosegmentsandfilterstrafficbasedontheMACaddress,notbasedonprotocols.Bridgescanimprovenetworkperformancebyreducinglargecollisiondomains.Bridgesworkbestwheretrafficislowfromonesegmentofanetworktoothersegments.Whentrafficbetweennetworksegmentsbecomesheavy,bridgescanbecomeabottleneckandslowdowncommunication.

Layer2Devices—BridgesTransparentBridge“透明”是指局域網(wǎng)上的站點并不知道所發(fā)送的幀將經(jīng)過哪幾個網(wǎng)橋，因為網(wǎng)橋?qū)Ω髡緛碚f是看不見的透明網(wǎng)橋是一種即插即用設(shè)備，其標(biāo)準(zhǔn)是IEEE802.1D目前以太網(wǎng)中使用得最多的網(wǎng)橋SourceroutingBridge源路由(sourceroute)網(wǎng)橋在發(fā)送幀時將詳細(xì)的路由信息放在幀的首部中,從而使每個經(jīng)過的網(wǎng)橋都了解幀的路徑在令牌環(huán)網(wǎng)絡(luò)中被廣泛使用Layer2Devices—Bridges站表接口管理軟件網(wǎng)橋協(xié)議實體緩存接口1接口2①②③網(wǎng)段B網(wǎng)段A1112①③⑤2②④⑥2站地址接口網(wǎng)橋網(wǎng)橋④⑤⑥接口1接口212以太網(wǎng)網(wǎng)橋的工作原理（透明網(wǎng)橋）TransparentBridge若從A發(fā)出的幀從接口x進(jìn)入了某網(wǎng)橋，那么從這個接口出發(fā)沿相反方向一定可把一個幀傳送到A。網(wǎng)橋每收到一個幀，就記下其源地址和進(jìn)入網(wǎng)橋的接口，作為轉(zhuǎn)發(fā)表中的一個項目。在收到一個新的幀時，將轉(zhuǎn)發(fā)表中已收集到的地址記錄取出，與此幀的目的地址匹配，找到對應(yīng)的接口，并向該接口轉(zhuǎn)發(fā)。在網(wǎng)橋的轉(zhuǎn)發(fā)表中寫入的信息除了地址和接口外，還有幀進(jìn)入該網(wǎng)橋的時間，其原因是：拓?fù)淇赡芙?jīng)常變化站點也可能會更換適配器（這就改變了站點的地址）工作站并非總是處于工作狀態(tài)把每個幀到達(dá)網(wǎng)橋的時間登記下來，就可以在轉(zhuǎn)發(fā)表中只保留網(wǎng)絡(luò)拓?fù)涞淖钚聽顟B(tài)信息，使得網(wǎng)橋中的轉(zhuǎn)發(fā)表能反映當(dāng)前網(wǎng)絡(luò)的最新拓?fù)銽ransparent

BridgeProblem:Whenadeviceonanetworkwantstosenddata,butdoesnotknowthedestinationaddress.Sendoutabroadcasttoalldevicesonanetwork.Sinceeverydeviceonthenetworkhastopayattentiontosuchbroadcasts,bridgesalwaysforwardthem.Toomanybroadcastscanresultinabroadcaststorm,anditcancause:networktime-outstrafficslowdownslessthanacceptableperformance.SourceRouteBridge

透明網(wǎng)橋容易安裝，但網(wǎng)絡(luò)資源的利用不充分源路由(sourceroute)網(wǎng)橋在發(fā)送幀時將詳細(xì)的路由信息放在幀的首部中源站以廣播方式向欲通信的目的站發(fā)送一個發(fā)現(xiàn)幀，每個發(fā)現(xiàn)幀都記錄所經(jīng)過的路由發(fā)現(xiàn)幀到達(dá)目的站時就沿各自的路由返回源站源站在得知這些路由后，從所有可能的路由中選擇出一個最佳路由凡從該源站向該目的站發(fā)送的幀的首部，都必須攜帶源站所確定的這一路由信息Layer2Devices—SwitchesPerformtwobasicoperations:switchingdataframes:aframeisreceivedonaninputmediumandthentransmittedtoanoutputmediummaintenanceofswitchingoperations:Switchesbuildandmaintainswitchingtablesandsearchforloops.Routersbuildandmaintainbothroutingtablesandservicetables.Layer2Devices—SwitchesSwitchingisatechnologythatalleviatescongestioninEthernetLANsbyreducingtrafficandincreasingbandwidth.Switchescreatededicatednetworksegments,orpoint-to-pointconnections,andconnectingthesesegmentsinavirtualnetworkwithintheswitch.Thisiscalledavirtualcircuitbecauseitexistsonlywhentwonodesneedtocommunicate

andisestablishedwithintheswitchYouca