data link layer - electrical engineering and computer sciencesugih/courses/eecs489/... · 2016. 2....

Computer Networks

Lecture22:MediaAccessControl

DataLinkLayer

MACtopics:•  framingandMACaddressassignment• LANforwarding•  IPtoMACaddressresolution•  IPtoMAC:AddressResolutionProtocol(ARP)

• MACtoIP:DynamicHostConfigurationProtocol(DHCP)

• mediaaccesscontrol

application

transport

network

LLC MAC

physical

MultipleAccessProblemBroadcastchannelofrateRbps,sharedmedium•  iftwohostssendatthesametime,collisionresultsinnopacketbeingreceived(interference)

•  ifnohostissending,channelgoesidle• wewanttohaveonlyonehostsendingatatime

MediaAccessControl:• determineswhichhostgetstosendnext• whattodowhenthere’scollision?

Duplexmode:• halfduplex:onlyoneendcansendatatime•  fullduplex:bothendscansendsimultaneously

IdealMultipleAccessProtocol• whenonehostwantstosend,itcansendatrateR • whenMhostswanttosend,eachcansendatanaveragerateofR/M • fullydecentralized:

• nospecialhosttocoordinatetransmissions• nosynchronizationofclocks,slots• distributedalgorithmthatdetermineshowhostssharechannel,i.e.,determinewhenhostcantransmit

• noout-of-bandchannelforcoordination•  coordinationaboutchannelsharingmustusethechannelitself!

CategorizationofMACProtocols1. Randomaccess:• SlottedALOHA• ALOHA• CarrierSenseMultipleAccesswithCollisionDetection(CSMA/CD)• CSMA/CAvoidance

2. Tokenpassing3. Channelpartitioning:TDMA,FDMA,CDMA

Standards:•  802.3(CSMA/CD),802.3ab(GigEovertwistedpair),802.3zand802.3ah(fiber)•  802.4(tokenbus)•  802.5(tokenring)•  802.11[bagn](WiFi),802.11a[dc]

RandomAccessMACProtocolCharacteristics:•  sendertransmitsatfullchannelrateR bps•  nopriorcoordinationamongnodes• bitsarepropagatedalongtheentirenetwork• destinationrecognizesthattheframeisforitself• destinationgrabsframe• whileonehostistransmitting,allothersmustwait

Randomaccessmeans:•  reliesoncollisiontocontrolaccess• howtodetectcollisions?• howtorecoverfromcollisions?

Ethernet:CSMA/CD

CarrierSense(CS):1.  checkforpresenceofelectricalsignal

(carrier)onwirebeforetransmission2.  presenceofcarriermeanssomeone

elseissending,wait3.  starttransmissionifnocarrierdetected

Problem:collision

CSMACollisions•  collisionsoccursbecauseduetopropagationdelay,twohostsmaynothearofeachother’stransmissionwhentheystarttransmitting(Batt0,Datt1)

• whencollisionoccurs(att2),entireframetransmissiontime(t3-t0or,equivalently,t4-t1)iswasted

•  notetheroledistanceandpropagationdelayplayindeterminingcollisionprobability

•  acollisionisdetectedifpowerreceivedislargerthanpowertransmitted

spatiallayoutofhosts

collision

t2

t3 t4

CollisionDetection(CD)

4. sendermustcontinuetodetectcollisionaftertransmission•  oncollision,framesmustberetransmitted

•  problem:morecollision

4. ifadaptordetectscollisionwhiletransmitting,abortsandsendsjammingsignal

5. afteraborting,adaptorentersexponentialback-offperiod

collisiondetecttoaborttime

JammingSignalandExponentialBack-offJammingsignal:makesureallothertransmittersareawareofcollision;48bitsExponentialback-off:senderspickauniformlydistributedrandomdelaybetween[0, 20d]beforeretransmission.Whyrandom?

Ifcollisionoccursagain,pickanotherrandomdelaybetween[0, 21d],[0, 22d],[0, 23d],...hence(binary)exponentialback-off

Retryatmost16times,butcapback-offexponentat10;forminimumframesizeof512bitsat10Mbps,maxwaittimeisabout50msec

TheCSMA/CDalgorithm:1.  listenforcarrier2.  ifnocarrier,sendframe3.  listenforcollisionorjammingsignal4.  ifcollisiondetected,sendjammingsignal5.  ifcollisionorjammingsignaldetected,retransmitafterexponentialback-off

HistoricalNote:Collisiondetectionandretransmissionwithback-offwasfirstusedintheALOHAandslottedALOHAMACalgorithmsfromtheUniversityofHawaii(1970)foraccesstosatellitechannels,butslottedALOHAhasnoCarrierSenseandislessefficient

CSMA/CDSummary

CarrierSense(CS):listenbeforespeaking,anddon’tinterrupt

CollisionDetection(CD):ifsomeoneelsestartstalkingatthesametime,shoutthemdownExponentialback-off:don’tstarttalkingagainrightaway

Howlongmustasenderlistenforcollision?•  letτbethepropagationtimefromoneendofthewiretotheother

•  withinτtimeafterthetransmissionofaframe(t),allnodesonthesegmentwouldhavesensedcarrier

•  worstcasescenarioforcollision:anodeattheotherendofthewirestartstransmittingattimet +τ − ε

•  thenodeclosesttothecollisionsendsoutajammingsignaltoensurecollisionisdetectedbytheothernode

•  ittakesanotherτperiodforthecollisiontogetbacktotheoriginalsender

Hencetheoriginalsendermustlistenfor2τperiod

CollisionDetectionTime

t + τ − ε

t + τ

t + 2τ ComerPeterson & Davie

t

MinimumFrameSizewhatiftransmissionendshere?

Whenasenderdetectscollisionhowdoesitknowthatthecollisionwascausedbyitspacket?

Answer:sendermustholdcarrierforaperiodof2τ,i.e.,itmustbetransmittingforthewhole2τperiod�eachEthernetframemustbeatleast2τ*linkspeedlong

Example:•  10MbpsEthernetallowsamaximumof5segments,each500mlong•  speedoflight3x108m/s,butcoaxpropagationonly2x108m/s•  round-trippropagationdelay(2τ)on2.5kmcoaxis25µsecs•  allowingfor4repeatersmakesend-to-enddelay50µsecs•  50µsecsat10Mbpsis62.5bytes•  802.3standardrequiresstationstoholdcarrierfor64bytesor 51.2µsecs

min frame size

CSMA/CDEfficiency(η)tprop=maxpropagationtimebetween2hostsintheLANttrans=timetotransmitmaximum-sizeframe�largerframesizemoreefficient

η = ttransttrans + 5t prop

= 11+ 5 t prop

ttrans

η→1 as t prop → 0 or as ttrans →∞

Tanenbaum

TokenRingMACProtocol• atokengoesaroundaringnetwork•  tosenddata,anodemustfirstgrabthetoken• aframesentfromasourceispassedfromnodetonodearoundthering• destinationrecognizesownaddressandmakesacopyofframe•  senderremovesframefromring• eachnodecanonlytransmitoneframeatatime;mustreturntokentotheringaftereachframetransmission

Whyletthesender,insteadofthereceiver,removeframefromthering?

Comer

TokenRingMACProtocol

Token:• aspecialbitpattern• usebit-stuffingifdataresemblestoken• onlyonetokenonringatatime(managedbyamonitor)

IBM’stokenringlinkspeedis16MbpsTokenring:• advantage:nocollision• disadvantage:•  requiresamonitor(singlepointoffailure)

•  latency

TokenRingPerformance

Halsall

(64 bytes) (1500 bytes)

CSMA/CDEfficiency(η)packet sizes

packet sizes

OtherMACProtocols

FDDI:• operatesat100Mbps• usesthetokenringMACprotocol•  forrobustness,usestwocounter-rotatingrings•  ifalink/nodegoesdown,thedual-ringcanbereconfiguredtoasingleringnetwork(hencecalledself-healingnetwork)

SLIP/PPP:serialline,point-to-pointprotocol,noneedformediaaccesscontrol,justframingATM/FrameRelay/SONET:forbackbonelinks....

Cormer