Physical address

Conceptdescription

MAC(MediaAccessControl,MediaAccessControl)addressisanidentifierforidentifyingLAN(LocalAreaNetwork)nodes.

Itdefineshowthedataframeistransmittedonthemedium.Inlinksthatsharethesamebandwidth,accesstotheconnectionmediumis"firstcome,firstserved".Physicaladdressingisdefinedhere,andlogicaltopology(thepathofasignalthroughthephysicaltopology)isalsodefinedhere.Linecontrol,errornotification(nocorrection),framedeliverysequenceandoptionalflowcontrolarealsoimplementedinthissublayer.

Thedatalinklayerofthelocalareanetworkisdividedintotwosub-layers,thelogicallinklayerLLCandthemediumaccesscontrolMAC.

LogicalLinkControl(LogicalLinkControlorLLCforshort)istheupperpartofthedatalinklayerinthelocalareanetwork.ThelogicallinkcontrolprotocolisdefinedinIEEE802.2.Theuser'sdatalinkserviceprovidesaunifiedinterfaceforthenetworklayerthroughtheLLCsublayer.BelowtheLLCsublayeristheMACsublayer.

MAC(mediumaccesscontrol)belongstoasub-layerunderLLC(LogicalLinkControl).Twomediaaccesscontrolmethodswidelyusedinlocalareanetworksare:

1.Contention-basedmediaaccesscontrol,alsoknownasrandommediaaccesscontrolprotocols,suchasCSMA/CD.

2.Deterministicmediaaccesscontrol,alsoknownasorderlyaccesscontrolprotocol,suchasToken(token)mode.

Applicationintroduction

Thatistosay,inthephysicaltransmissionprocessatthebottomofthenetwork,thehostisidentifiedbythephysicaladdress,whichisgenerallyuniqueintheworld.Forexample,awell-knownEthernetcardhasaphysicaladdresssizeof48bits(bits),thefirst24bitsarethemanufacturernumber,andthelast24bitsarethenetworkcardnumber,suchas:44-45-53-54-00-00,whichisreadablebyamachineThewayisstoredinthehostinterface.TheEthernetAddressManagementInstitute(IEEE)dividestheEthernetaddress,thatis,differentcombinationsof48bits,intoanumberofindependentconsecutiveaddressgroups.ManufacturersofEthernetnetworkcardspurchaseoneofthem,andtheuniqueaddressesareindividuallyassignedduringspecificproduction.GivetheEthernetcard.

Imagemetaphor

TheMACaddressisliketheIDnumberonourIDcard,whichisgloballyunique.

Detailedtechnicalexplanation

Addressesinthenetworkaredividedintophysicaladdressesandlogicaladdresses.ComparedwiththeportnumberoftheIPaddresstransmissionlayerofthenetworklayerandtheusernameoftheapplicationlayer,TheMAClayeraddressofthelocalareanetworkisprocessedbyhardware,whichiscalledphysicaladdressorhardwareaddress.TheportnumberoftheIPaddresstransportlayerandtheusernameoftheapplicationlayerarelogicaladdressesthatareprocessedbysoftware.TheMACaddressisalsocalledthephysicaladdress.Mostlocalareanetworksidentifyanetworkedcomputerorotherdevicebyassigningahardwareaddresstothenetworkcard.

Theso-calledphysicaladdressreferstotheaddresssolidifiedintheEPROMofthenetworkcard.Thisaddressshouldbeguaranteedtobeuniqueintheentirenetwork.TheIEEERegistrationCommitteeassignsthefirstthreebytesofthephysicaladdress,thecompanylogo,toeachmanufacturer.Thelastthreebytesareallocatedbythemanufacturer,andthenumberofnetworkcardsthatamanufacturercanproducewhenobtaininganaddresswiththefirstthreebytesis16,777,216.Anetworkcardcorrespondstoaphysicaladdress.Thatistosay,thefirstthreebytescorrespondingtothephysicaladdresscanknowhismanufacturer.

Forexample,theaddressfixedinthenetworkcardis002514895423,thenthisnetworkcardisinsertedintohostA,andthephysicaladdressofhostAis002514895423,regardlessofwhetherhostAisconnectedtoLAN1orLAN2,Nomatterwherethecomputerismoved,thephysicaladdressofhostAis002514895423.Itisconstantandwillnotbethesameasanycomputerintheworld.WhenthehostAsendsaframe,thenetworkcarddirectlywritesthisaddressasthesourceaddressintotheframewhenthenetworkcardexecutesthesendingprogram.WhenhostAreceivesaframe,itdirectlycomparesthisaddresswiththedestinationaddressofthereceivedframetodeterminewhethertoreceiveit.Thephysicaladdressisgenerallydenotedas00-25-14-89-54-23(theaddressofhostAis002514895423).

CSMA/CDworkingprinciple

InCSMA,duetothechannelpropagationdelay,evenifthestationsonbothsidesofthecommunicationdonotdetectthecarriersignal,itisstillpossibletosenddataConflictswilloccurbecausetheymaysenddataatthesametimewhentheydetectthatthemediumisfree,causingconflictstooccur.AlthoughCSMAcandetectconflicts,itdoesnothavetheconflictdetectionandpreventionfunctionsoftheprophet,resultinginfrequentconflicts.

AnimprovementofCSMAistomakethesendingstationcontinuetolistentothemediumduringtransmissiontodetectwhetherthereisaconflict.Ifbothstationsdetectthatthechannelisfreeatacertaintimeandstarttotransmitdataatthesametime,theywillalmostimmediatelydetectthatthereisaconflict.Ifaconflictoccurs,electromagneticwavesthatexceedtheamplitudeofthecarriersignalsentbythesendingsitecanbedetectedonthechannel,andtheexistenceoftheconflictcanbejudgedfromthis.Onceaconflictisdetected,thesendingstationimmediatelystopssendingandsendsaseriesofblockingsignalstothebustonotifytheoppositestationcommunicatingonthebustoquicklyterminatethecorruptedframe,whichcansavetimeandbandwidth.ThissolutionistheCSMA/CD(CarrierSenseMultipleAccesswithCollisionDetection)introducedinthissection,andithasbeenwidelyusedinlocalareanetworks.

Theso-calledcarriersense(CarrierSense)meansthateachworkstationonthenetworkmustconfirmwhetherthereisdatatransmissiononthebusbeforesendingdata.Ifthereisdatatransmission(calledthebusasbusy),nodatawillbesent;ifthereisnodatatransmission(calledthebusasempty),theprepareddatawillbesentimmediately.

Theso-calledmultipleaccess(MultipleAccess)meansthatallworkstationsonthenetworkusethesamebusforsendingandreceivingdata,andthedatasentisbroadcast.

Theso-calledcollision(Collision)meansthatiftherearetwoormoreworkstationsontheInternetsendingdataatthesametime,thesignalwillbemixedonthebus,sothatnoworkstationcandistinguishtherealdata.what.Thissituationiscalledadataconflict,alsocalledacollision.

Inordertoreducetheimpactofconflicts,theworkstationmustconstantlycheckthedatasentbyitselfduringtheprocessofsendingdatatoseeifthereisanyconflictwiththedataofotherworkstationsduringthetransmissionprocess.Thisisaconflict.CollisionDetected.

PrincipleofCSMA/CDconflictdetection

CSMA/CDisamediacontentionhandlingprotocoluniformlyadoptedinstandardEthernet,fastEthernetandGigabitEthernet(butin10GigabitInEthernet,becauseitusesfull-duplexcommunication,thisprotocolisnolongerused).Thereasonwhyitiscalled"CarrierSense"("Carrier"istheelectromagneticwavecarryingthesignal)insteadof"MediaSense"isbecauseifthereisacarrieronthemedium,itprovesthatthemediumisbusy.(Becausethesignalordataisnotdirectlytransmitted,butcarriedoutthroughelectromagneticcarrier);ifthereisnocarrier,themediumisinanidlestate.Thatis,throughthedetectionofthecarrier,thestateofthemediumcanbeknown,buttheidlestateofthemediumitselfcannotbeobtainedbydirectlylisteningtothemedium.

[Explanation]Infact,whatislisteninghereshouldbethe"channel",notthe"medium"itself,becauseatransmissionmediummaycontainmultiplechannelsfordifferenttransmissionlinks.

Asmentionedearlier,theimprovementofCSMA/CDoverCSMAisthatithasaconflictdetectionfunction.Thenthequestionarises.HowdoesCSMA/CDdetectconflicts?

TheworkingprincipleofCSMA/CDcanbesummarizedinthefollowingsentences:

Listenfirst,thenspeak,listenandspeak.

Incaseofconflict,stoptalkingimmediately.

Waitforthemoment,andthentalk.

Here,"listening"meansmonitoringandtesting;"speaking"meanssendingdata.Thespecificdetectionprincipleisdescribedasfollows:

(1)Whenastationwantstosenddata,itchecksthenetworktoseeifotherstationsaretransmitting,thatis,whetherthelisteningchannelisfree.

(2)Ifthechannelisbusy,waituntilthechannelisfree;ifthechannelisfree,thestationisreadytosendthedata.

(3)Whilesendingdata,thesitecontinuestolistentothenetwork,makingsurethatnoothersitesaretransmittingdataatthesametimebeforecontinuingtotransmitdata.Becauseitispossiblethattwoormorestationssimultaneouslydetectthatthenetworkisidleandthenstarttransmittingdataatalmostthesametime.Iftwoormorestationssenddataatthesametime,conflictswilloccur.Ifthereisnoconflict,continuesendinguntilalldataissent.

(4)Ifthereisaconflict,stopsendingdataimmediately,butsendaJAM(blocking)signalthatstrengthenstheconflict,sothatallworkstationsonthenetworkknowthatthereisaconflictonthenetwork,andthenwaitforaAtapredeterminedrandomtime,andwhenthebusisidle,theunsentdatawillberesent.

TheadvantagesoftheCSMA/CDcontrolmethodare:

Theprincipleisrelativelysimple,technicallyeasytoimplement,allworkstationsinthenetworkareonanequalfooting,andthereisnoneedtoconcentrateControl,doesnotprovideprioritycontrol.However,whenthenetworkloadincreases,thesendingtimeincreases,andthesendingefficiencydropssharply.

Theworkingprincipleoftokenaccesscontrol

TokenaccesscontrolmethodscanbedividedintotokenringaccesscontrolandTherearetwotypesoftokenbusaccesscontrol.Tokenbusaccesscontrolhasbeenusedlessfrequently.

Application

Whetherinthetraditionalwiredlocalareanetwork(LAN)orinthepopularwirelesslocalareanetwork(WLAN),theMACprotocoliswidelyused.Inthetraditionallocalareanetwork,thephysicallayerofvarioustransmissionmediacorrespondstothecorrespondingMAClayer.ThecommonlyusednetworkadoptstheIEEE802.3MAClayerstandardandadoptstheCSMA/CDaccesscontrolmethod;whileinthewirelesslocalareanetwork,theMACaddressThecorrespondingstandardisIEEE802.11,anditsworkingmodeadoptsDCF(distributedcontrol)andPCF(centralcontrol).

TokenRingMediaAccessControl

IEEE802.5TokenRingMediaAccessControlusesatokentocirculatealongthering,anditshouldbeensuredthatthetokenisuniqueinthering.Theworkingprincipleoftokenring:

Onlinesitesrequesttosendframesandmustwaitforanemptytoken.

Whenanemptytokenisobtained,itischangedtoabusytoken,followedbyadataframe;otherstationsintheringcannotsenddata.

Thestationontheringreceives,shiftsdata,andperformsdetection.Iftheaddressisthesameasthisstation,thedatawillbereceivedatthesametime.Afterthereceptioniscompleted,thecorrespondingflagwillbeset.

Aftertheframecirculatesontheringforaweek,itreturnstothesendingstation.Afterthesendingstationdetectsthecorrespondingmark,theframeisremoved.

Changethebusytokentoanemptytokenandcontinuetotransmitforsubsequentstationstosendframes.Thelengthoftheloopiscalculatedinbits:

Becausethepropagationspeedofelectromagneticwavesislimited,theremaybemultipledatabitsinthetransmissionmediumatthesametime.

Eachrepeaterontheringintroducesatleast1bitdelay

Thenumberofbitsreservedonthering:

Propagationdelay(us/km)×mediumlength×datarate+repeaterdelay

Example:mediumlengthL=1km,datarateC=4Mbit/s,numberofstationsN=50.

Solution:Propagationdelaytp=L/v

v=2×105km/stp=5usNumberofbitsreservedonthering=5×1×4+50=70bit

TokenRingMACframestructure

IEEE802.5MACframeformattokenanddataframemanagement:

Howtopreventdataframesfromendlesslyontheringcycle?

Setupthemonitor

Leaveamarkontheframestructure

Howtomonitorthetokenerror?

Notoken

Multipletokens

Busytokenendlessloop

Centralizeddetection:

Setupacentralizedmonitoringstation(timeoutcounter)/markloss

Detectbusymarkidentification/endlessloopontheframestructure

Distributeddetection:

EachstationSettimer:whenthestationhasdatatosendandthetimewaitingforthemarkexceedsthelimit/themarkislost

TokenControlProtocol

Topology:Workingprinciple:TokenBusinthephysicalsystemEstablishalogicalringonline.

Onthelogicalring,thetokenisanecessaryconditionforthestationtosenddata.

Thetokenistransmittedtothenextstationinthelogicalringindescendingorderofaddresses.

Fromaphysicalpointofview,thetokenframecontainingDAisbroadcasttotheBUS,andallstationsjudgewhethertoreceiveitaccordingtoDA=theaddressofthestation.

Features:Noconflict,thelengthoftheinformationframeofthetokenringcanbedeterminedaccordingtoneeds.

SequentiallyreceivingFairness(fairness),thetimethatthestationwaitsfortheTokenisknownwithcertainty.

(Needtolimitthemaximumnumberofframessentbyeachstation).

Informationbitsneedtobefilledduetodetectionconflicts(notallowedtobelessthan46bytes).

MediaAccessControlTechnology

Thecentraltopicofthemediaaccesssublayerishowtoallocateasinglebroadcastchannelamongcompetingusers.     

1.Staticallocation:Aslongasoneusergetsthechannel,itwillnotconflictwithotherusers.(Userdatatrafficisburstyandintermittent) .

2.Dynamicallocation:itiscalledmulti-accessormulti-pointaccess,whichreferstomultipleuserssharingaline,andthechannelisnotfixedlyallocatedtotheuserwhentheusercommunicates.SuchasystemisalsocalledCompetitivesystem.Dynamicallocationmethodscanbefurtherdividedinto:randomaccess,typicalALOHAprotocol,CSMAprotocol ;controlledaccess,typicaltokennetworkcompetitionsystemandcentralizedcontrolofmulti-pointlinepolling. 

3.Fivekeyassumptionsinchanneldynamicallocation. 

Stationmodel:Stationsareindependent,framesaregeneratedataconstantrate,andeachstationhasonlyoneprogram.

Single-channelhypothesis(core)

Conflicthypothesis:twoframesaretransmittedatthesametime,therewillbecollisions,allstationscandetect,theconflictingframesneedtoberetransmitted 

Sendtime:1.Continuoustime;2.Timeslot. 

Carrierdetection:1.Withcarriersensing;2.Withoutcarriersensing.