Architecture and Performance of A MAC Protocol for

ArchitectureandPerformanceofAMACProtocolforTheHFCSystemShengmingJiang†,DannyH.K.Tsang†,SamuelT.Chanson††HongKongUniversityofScience&TechnologyClearWaterBay,Kowloon,HongKong†{eejiang,eetsang}@ee.ust.hk,††[e-mail]802.14standardcommitteeiscurrentlyworkingonaprojecttofindacost-effectivemeansofprovidingaccesstointegratednetworksforpeopletoenjoymultimediaprogramsandtoworkathome.AnadvancedsystembasedontheCATVsystemcalledHybridFiberCoax(HFC)isbeingstudied.SincesomepropertiesoftheHFCsystemprecludethepossibilityofdirectlyusingexistingmediumaccesscontrol(MAC)protocolsforitsdatalinklayer,aMACschemebasedontimedivisionisdiscussedinthispaper.ThisMACschemecanbeextendedforwirelessnetworks.Keywords:MAC,HFC,ATM,Multimedia,SCCA/TD1.IntroductionAsATMtechnologyprogresses,researchersarelookingforacost-effectivemeansofprovidinghomeaccesstointegratednetworkssothatpeoplecanenjoymultimediaprogramsandworkathome.DuetothelargegeographicalcoverageofexistingCATVsystems,anadvancedsystembasedonCATVcalledHybridFiberCoax(HFC)isbeingstudiedbytheIEEE802.14ProjectCommittee[1].ThissystemwilllikelyuseATMtechnologytoprovidemultipleservicestohomesuchastelephone,TVandinformationretrieval.AsshowninFigure1,asimpleHFCsystemmaybeabstractedintoadual-branch-busbasednetworkconsistingofadistributinghub(calledheadendinthispaper)andthefibernodes(orsimplynodes).Thecoaxialplantwhichconnectsresidentialzonesisextendedfromthefibernodes.Thetwobusescanbeloopedattheheadendtoformasinglebus.SeveralsimpleHFCsystemscanbeconnectedviatheirdistributinghubs.2HUB12n.....abstractedHFCsystem1.....k..........Figure1TopologyoftheHFCsystemIntheHFCsystem,thedownstreambandandupstreambandoperateasymmetricallyatdifferentfrequencies,andanodehasonlyonelinktoeachbus.Inthedownstreamdirection(fromtheheadendtothenodes)broadcastmodeisused,andtheheadendistheonlytransmitter.Thismakescontrolofthedownstreambandsimple.Intheupstreamdirection(fromthenodestotheheadend),thenodessharetheupstreambandfortransmission,andcontentionaccessmodeisused.Toallowthenodestosharetheupstreambandinafairmanner1,anefficientmediumaccesscontrol(MAC)protocolisnecessary.Thisprotocolhastobecost-effectiveandefficientforlongdistancecommunication,supportingawiderangeoftransmissionratesaswellasbothanaloganddigitalbroadbandservices[2].Unfortunately,thesepropertiesandrequirementsmakeitdifficulttouseexistingMACprotocolsdirectlyforthefollowingreasons:1)Thering-basedprotocolssuchasToken-Ring[5],FDDI[5],ATMR[3]andOrwell[4]arenotsuitableforthedual-branch-bustopologyrequiredbytheHFCsystem.Ontheotherhand,bothCSMA/CDandToken-Bus[5]aresuitablefortheHFCtopology.However,CSMA/CDcannotguaranteeboundeddelayandisonlysuitableforrelativelyshortdistances.SincedifferentfrequenciesareusedfortransmittingandreceivingintheHFCsystem,messagesforinter-nodecommunicationneedtoberetransmittedbytheheadend.Retransmissionfromtheheadendwillresultinlongtoken-passingtimeintoken-passing-basedschemeslikeToken-Bus,especiallywhenthebusisverylongandcoversawidearea.AnotherweaknessofToken-Busisthatitrequirestheuserstoremainon-linetobeincludedinthelogicalringwhichneedstobereconfiguredwheneverusersareaddedorremovedfromthenetwork.1By'afairmanner',wemeaneachnodewillhaveequalopportunityofaccessingthebus.32)ItisdifficulttouseMACprotocolsbasedontheslottedmediaapproachlikeDQDB[5]andCRMA[6].Theseprotocolsoftenrequireanodetohavetwolinkstoeachbusfordatainsertion.Moreover,DQDBneedstwoheadendsandCRMArequiresafoldedbuswhilethereisonlyoneheadendandthetopologyisrequiredtobedual-branch-busintheHFCsystem.AlthoughthetwobusesintheHFCsystemcanbeloopedtoformafoldedbuslikeCRMA,theoutboundbus(fromtheheadendtothenodes)isreservedfortransmission,andtheinboundbus(fromthenodestotheheadend)forreceivinginthefolded-busCRMA.ThisisoppositetotheHFCsystemwherethedownstreambandisforreceivingandupstreambandfortransmission.Inaddition,todividebandwidthintoslots,theslotheaderhastobebroadcasttothenodescontinuously.IntheHFCsystem,thiscanbedonebytheheadendandtheslotheaderwillbebroadcasttothenodesviathedownstreamband.However,inthiscase,thedownstreambandcannotbeusedbythenodesfortransmission.WithregardstothespecialpositionoftheheadendintheHFCsystem,aMACschemebasedoncentralizedcontrolisanaturalchoice.OnesuchschemeisthetraditionalpollingsystemsuchasRoll-callPolling[9].However,thesesystemshavebeendesignedforlowormediumspeeddataapplicationsundertheassumptionthatthepropagationdelayisnegligiblecomparedtothetransmissiontime.Thisassumptionisnottrueinhighspeednetworks[7].Inthispaper,wewilldiscussanewMACschemecalledSelf-ControlCycleAccesswithTimeDivision(SCCA/TD).ThephilosophyofSCCA/TDissimilartothatofTimeDivisionMultiplexAccess(TDMA)andPacketReservationMultipleAccess(PRMA[8])intermsoftimedivisionandcycliccontrolforbandwidthallocation.Themaindifferencesareasfollows:1)InTDMAandPRMA,theusershavetocontendforidletimeslotsforbandwidthreservationatthebeginningoftransmission.Thiscompetitionisbasedonusersmonitoringthebroadcastmessagesonuplinkchannelusageinthedownlinkchannel.Thisschemeisunfairsincethephysicalposi