FTTHnetworkswillnotonlyenablethesupportofawiderangeofnewandemergingser - PDF document

1 FTTHnetworkswillnotonlyenablethesupporto
FTTHnetworkswillnotonlyenablethesupportofawiderangeofnewandemergingservicesandapplicationsbutalsounleashtheireconomicpotentialandsocietalbenetsbyopeninguptherst/lastmilebandwidthbottleneckbetweenbandwidth-hungryendusersandhigh-speedbackbonenet-works[7].Inthispaper,weassumethatopticalberpavesallthewaytoandpenetratesintothehomeofresidentialandbusinesscustomers.Arguingthatduetoitsuniquepropertiesopticalberislikelytoentirelyreplacecopperwiresintheneartomidterm,wewillelaborateonthenalfrontierofopticalnetworks,namely,theconvergencewiththeirwirelesscounterparts.Opticalandwirelesstechnologiescanbethoughtofasquitecomplementaryandwillexpectedlycoexistoverthenextdecades.Futurebroadbandaccessnetworkswillbebimodal,capitalizingontherespectivestrengthsofbothtechnologiesandsmartlymergingtheminordertorealizefuture-proofber-wireless(FiWi)networksthatstrengthenourinformationsocietywhileavoidingitsdigitaldivide.Bycom-biningthecapacityofopticalbernetworkswiththeubiquityandmobilityofwirelessnetworks,FiWinetworksformapowerfulplatformforthesupportandcreationofemergingaswellasfutureunforeseenapplicationsandservices,e.g.,telepresence.FiWinetworksholdgreatpromisetochangethewayweliveandworkbyreplacingcommutingwithteleworking.Thisnotonlyprovidesmoretimeforprofessionalandpersonalactivitiesforcorporateandourownpersonalbenet,butalsohelpsreducefuelconsumptionandprotecttheenvironment,issuesthatarebecomingincreasinglyimportantinourlives.Theremainderofthepaperisstructuredasfollows.InSectionII,wesetthestagebybrieyreviewingradio-over-ber(RoF)networks,apreviouslystudiedapproachtointe-grateopticalbernetworksandwirelessnetworks,andexplaintheirdifferencetoso-calledradio-and-ber(R&F)networks.SectionIIIelaboratesonenablingtechnologiesandthestateoftheartofFiWinetworks.InSectionIV,weintroduceourproposalforfutureFiWinetworks.SectionVconcludesthepaper.II.ROFVS.R&FNETWORKSRoFnetworkshavebeenstudiedformanyyearsasanapproachtointegrateopticalberandwirelessnetworks.InRoFnetworks,radiofrequencies(RFs)arecarriedoveropticalberlinksbetweenacentralstationandmultiplelow-costremoteantennaunits(RAUs)insupportofavarietyofwirelessapplications.Forinstance,adistributedantennasystemcon-nectedtothebasestationofamicrocellularradiosystemviaopticalberswasproposedin[8].Toefcientlysupporttime-varyingtrafcbetweenthecentralstationanditsattachedbasestations,acentralizeddynamicchannelassignmentmethodisappliedatthecentralstationoftheproposedberopticmicrocellularradiosystem.Toavoidhavingtoequipeachradioportinaberopticmicrocellularradionetworkwithalaseranditsassociatedcircuittocontrolthelaserparameterssuchastemperature,outputpower,andlinearity,acost-effectiveradioportarchitecturedeployingremotemodulation Fig.1.Radio-over-SMFnetworkdownlinkusingEAMsfordifferentradioclientsignals[10].canbeused[9].Apartfromrealizinglow-costmicrocellularradionetworks,opticalberscanalsobeusedtosupportawidevarietyofotherradiosignals.RoFnetworksareattractivesincetheyprovidetran

2 sparencyagainstmodulationtechniquesandar
sparencyagainstmodulationtechniquesandareabletosupportvariousdigitalformatsandwirelessstandardsinacost-effectivemanner.Itwasexperimentallydemonstratedin[10]thatRoFnetworksarewellsuitedtosimultaneouslytransmitwidebandcodedivisionmultipleaccess(WCDMA),IEEE802.11a/gwirelesslocalareanetwork(WLAN),per-sonalhandyphonesystem(PHS),andglobalsystemformobilecommunications(GSM)signals.Fig.1illustratesthemethodinvestigatedin[10]fortwodifferentradioclientsignalstransmittedbythecentralstationonasingle-modeber(SMF)downlinktoabasestationandonwardtoamobileuserorvehicle.Atthecentralstation,bothradioclientsignalsarerstupconvertedtoahigherfrequencybyusingafrequencyconverter.ThenthetwoRFsignalsgointotwodifferentelec-troabsorptionmodulators(EAMs)andmodulatetheopticalcarrierwavelengthemittedbytwoseparatelaserdiodes.AnopticalcombinercombinesthetwoopticalsignalsontotheSMFdownlink.Atthebasestation,aphotodiodeconvertstheincomingopticalsignaltotheelectricaldomainandradiatestheampliedsignalthroughanantennatoamobileuserorvehiclewhichusestwoseparatefrequencyconverterstoretrievethetwodifferentradioclientsignals.WhileSMFsaretypicallyfoundinoutdooropticalnet-works,manybuildingshavepreinstalledmultimodeber(MMF)cables.Cost-effectivemultimodeber(MMF)-basednetworkscanberealizedbydeployinglow-costverticalcavitysurfaceemittinglasers(VCSELs).In[11],differentkindsofMMFinconjunctionwithcommercialoff-the-shelf(COTS)componentswereexperimentallytestedtodemonstratethefea-sibilityofindoorradio-over-MMFnetworksforthein-buildingcoverageofsecond-generation(GSM)andthird-generationcellularradionetworks[universalmobiletelecommunicationssystem(UMTS)]aswellasIEEE802.11a/b/gWLANanddig-italenhancedcordlesstelecommunicationpacketradioservice Fig.2.SimultaneousmodulationandtransmissionofFTTHbasebandsignalandRoFRFsignalusinganexternalintegratedmodulatorconsistingofthreeMach-Zehndermodulators(MZMs)[12].(DECTPRS).Torealizefuturemultiserviceaccessnetworks,itisim-portanttointegrateRoFsystemswithexistingopticalac-cessnetworks.In[12],anovelapproachforsimultaneousmodulationandtransmissionofbothRoFRFandFTTHbasebandsignalsusingasingleexternalintegratedmodulatorwasexperimentallydemonstrated,asshowninFig.2.TheexternalintegratedmodulatorconsistsofthreedifferentMach-Zehndermodulators(MZMs)1,2,and3.MZM1andMZM2areembeddedinthetwoarmsofMZM3.TheRoFRFandFTTHbasebandsignalsindependentlymodulatetheopticalcarriergeneratedbyacommonlaserdiodebyusingMZM1andMZM2,respectively.Subsequently,theopticalwirelessRFandwired-linebasebandsignalsarecombinedatMZM3.AfterpropagationoveranSMFdownlink,anopticallter(e.g.,bergrating)isusedtoseparatethetwosignalsandforwardthemtothewirelessandFTTHapplication,respectively.Itwasexperimentallydemonstratedthata1.25Gb/sbasebandsignalanda20-GHz622Mb/sRFsignalcanbesimultaneouslymodulatedandtransmittedover50kmstandardSMFwithacceptableperformancepenalties.Theaforementionedresearchprojectssuccessfullydemon-stratedthefeasibilityandmaturityoflow-costmultiserviceRoFnetworks.Theirfocuswasontheinv

3 estigationofRoFtransmissioncharacteristi
estigationofRoFtransmissioncharacteristicsandmodulationtechniques,con-sideringprimarilyphysicallayerrelatedperformancemetrics,e.g.,powerpenalty,errorvectormagnitude(EVM),andbiterrorrate(BER)measurements.ItwasshownthatRoFnet-workscanhaveanopticalberrangeofupto50km.However,insertinganopticaldistributionsysteminwirelessnetworksmayhaveamajorimpactontheperformanceofmediumaccesscontrol(MAC)protocols[13].Theadditionalpropa-gationdelaymayexceedcertaintimeoutsofwirelessMACprotocols,resultinginadeterioratednetworkperformance.Moreprecisely,MACprotocolsbasedoncentralizedpollingandscheduling,e.g.,IEEE802.16WiMAX,arelessaffectedbyincreasedpropagationdelaysduetotheirabilitytotakelongerwalktimesbetweencentralstationandwirelesssub-scriberstationsintoaccountbymeansofinterleavedpollingandschedulingofupstreamtransmissionsoriginatingfromdifferentsubscriberstations.However,indistributedMACprotocols,e.g.,thewidelydeployeddistributedcoordinationfunction(DCF)inIEEE802.11a/b/gWLANs,theadditionalpropagationdelaybetweenwirelessstationsandaccesspointposesseverechallenges.Toseethis,notethatinWLANsasourcestationstartsatimeraftereachframetransmissionandwaitsfortheacknowledgment(ACK)fromthedestinationstation.BydefaulttheACKtimeoutvalueissetto9sand20sin802.11a/gand802.11bWLANnetworks,respectively.IfthesourcestationdoesnotreceivetheACKbeforetheACKtimeoutitwillresendtheframeforacertainnumberofretransmissionattempts.Clearly,onesolutiontocompensatefortheadditionalberpropagationdelayistoincreasetheACKtimeout.Note,however,thatinDCFtheACKtimeoutmustnotexceedtheDCFinterframespace(DIFS),whichpreventsotherstationsfromaccessingthewirelessmediumandthusavoidingcollisionwiththeACKframe(inIEEE802.11WLANspecicationsDIFSissetto50s).DuetotheACKtimeout,opticalbercandeployedinWLAN-basedRoFnetworksonlyuptoamaximumlength.Forinstance,itwasshownin[14]thatinastandard802.11bWLANnetworktheberlengthmustbelessthan1948mtoensuretheproperoperationofDCF.Inaddition,itwasshownthatthereisatrade-offbetweenberlengthandnetworkthroughput.Asmoreberisdeployedthenetworkthroughputdecreasesgradually.TheaforementionedlimitationsofWLAN-basedRoFnet-workscanbeavoidedinso-calledradio-and-ber(R&F)networks[15].WhileRoFnetworksuseopticalberasananalogtransmissionmediumbetweenacentralcontrolstationandoneormoreRAUswiththecentralstationbeinginchargeofcontrollingaccesstobothopticalandwirelessmedia,inR&FnetworksaccesstotheopticalandwirelessmediaiscontrolledseparatelyfromeachotherbyusingingeneraltwodifferentMACprotocolsintheopticalandwirelessmedia,withprotocoltranslationtakingplaceattheirinterface.Asaconsequence,wirelessMACframesdonothavetotravelalongtheopticalbertobeprocessedatthecentralcontrolstation,butsimplytraversetheirassociatedaccesspointandremainintheWLAN.InWLAN-basedR&Fnetworks,accesscontrolisdonelocallyinsidetheWLANwithoutinvolvinganycentralcontrolstation,thusavoidingthenegativeimpactofberpropagationdelayonthenetworkthroughput.R&FnetworksarewellsuitedtobuildWLAN-basedFiWinetworks

4 ofextendedcoveragewithoutimposingstringe
ofextendedcoveragewithoutimposingstringentlimitsonthesizeoftheopticalbackhaul,asopposedtoRoFnetworksthatlimitthelengthofdeployedberstoacoupleofkilometers.RecallthatthisholdsonlyfordistributedMACprotocolssuchasDCF,butnotforMACprotocolsthatdeploycentralizedpollingandscheduling,e.g.,WiMAX.III.FIWINETWORKSBothRoFandR&FtechnologiescanbefoundinFiWinetworks.Inthissection,wediscussenablingtechnologiesingreaterdetailandelaborateonthestateoftheartofFiWinetworks. A.EnablingTechnologies1)RoFTechnologies:SeveralRoFtechnologieshavebeenemergingfortherealizationoflow-costFiWinetworks.Inthefollowing,webrieysummarizesomeofthekeyenablingRoFtechnologies.Forfurtherdetailsandatechnicallymoreprofounddiscussion,werefertheinterestedreaderto[16].a)OpticalRFGeneration:Toavoidtheelectronicbot-tleneck,thegenerationofRFsignalsisbestdoneoptically.ThefollowingnovelopticalRFgenerationtechniqueswereexperimentallystudiedanddemonstratedin[16]:FWMinHNL-DSF:Four-wavemixing(FWM)inahighlynonlineardispersion-shiftedber(HNL-DSF)canbeusedtorealizesimultaneousall-opticalup-conversionofmultiplewavelengthchannelsbyusingopticalcarriersuppression(OCS)techniques.FWMistransparenttothebitrateandmodulationformatwhichmaybedifferentoneachwavelength.DuetotheultrafastresponseofHNL-DSF,TerahertzopticalRFgenerationispossible.XPMinHNL-DSF:Cross-phasemodulation(XPM)inanonlinearopticalloopmirror(NOLM)inconjunctionwithstraightpassinHNL-DSFenablestheall-opticalup-conversionofmultiplewavelengthchannelswithoutanyinterference-andsaturation-effectlimitation.XAMinEAM:All-opticalwavelengthup-conversionbymeansofcross-absorptionmodulation(XAM)inanelectroabsorptionmodulator(EAM)hasseveraladvan-tagessuchaslowpowerconsumption,compactsize,polarizationinsensitivity,andeasyintegrationwithotherdevices.ExternalIM:Externalintensitymodulation(IM)isan-otherapproachforopticalRFgeneration,deployingoneofthreefollowingmodulationschemes:double-sideband(DSB),single-sideband(SSB),andOCS.ExternalPM:InsteadofexternalIM,externalphasemodulation(PM)canbeusedforopticalRFgeneration.Accordingto[16],externalintensityandphasemodulationschemesarethemostpracticalsolutionsforall-opticalRFgenerationduetotheirlowcost,simplicity,andlong-distancetransmissionperformance.b)RemoteModulation:Aninterestingapproachtobuildlow-costFiWinetworksistheuseofasinglelightsourceatthecentralofce(CO)togenerateadownlinkwavelengththatisreusedatRAUsforupstreamtransmissionbymeansofremotemodulation,therebyavoidingtheneedforanadditionallightsourceateachRAU.Thefollowingremodulationschemeswereexperimentallystudiedin[16]:DPSKforDownstream/OOKforUpstream:PMisdeployedtogenerateadifferentialphase-shift-keyed(DPSK)opticaldownstreamsignal.TheDPSKisup-convertedthroughOCSmodulation.AnopticalsplitterisusedateachRAUtodividethearrivingopticalsignalintotwoparts.OnepartisdemodulatedbyaMach-Zehnderinterferometerandissubsequentlydetectedbyaphotodetector.Theotherpartison-off-keyed(OOK)remodulatedwithupstreamdatausingaMach-ZehndermodulatorandissenttotheCO.

5 OCSforDownstream/ReuseforUpstream:A
OCSforDownstream/ReuseforUpstream:AttheCO,anopticalcarrierissplitpriortoopticalRFgenerationbymeansofOCSandisthencombinedwiththeRFsignalandsentdownstream.EachRAUutilizesaberBragggrating(FBG)toreecttheopticalcarrierwhilelettingtheRFsignalpasstoaphotodetector.ThereectedopticalcarrierisremodulatedwithupstreamdataandisthensentbacktotheCO.PMforDownstream/DirectlyModulatedSOAforUp-stream:Similartotheaforementionedscheme,anopticalcarrieriscombinedwithanRFsignal,generatedbymeansofPM,andsentdownstreamwhereanFBGisusedattheRAUtoreecttheopticalcarrierandpasstheRFsignal.Thereectedopticalcarrierisampliedanddirectlymodulatedwithupstreamdatausingasemi-conductoropticalamplier(SOA).Theuseofacolorless(i.e.,wavelength-independent)SOAasanamplierandmodulatorforupstreamtransmissionprovidesapromisinglow-costRoFsolutionthatiseasytomaintain[16].2)R&FTechnologies:R&F-basedFiWiaccessnetworksmaydeployanumberofenablingopticalandwirelesstech-nologies.a)OpticalTechnologies:ApartfromPONs,thefollow-ingopticaltechnologiesareexpectedtoplayanincreasinglyimportantroleinthedesignofaexibleandcost-effectiveopticalbackhaulforFiWinetworks[17].TunableLasers:Directlymodulatedexternalcavitylasers,multisectiondistributedfeedback(DFB)/distributedBraggreector(DBR)lasers,andtunableVCSELscanbeusedastunablelaserswhichrenderthenetworkexibleandrecongurableandhelpminimizeproductioncostandreducebackupstock.TunableReceivers:Atunablereceivercanberealizedbyusingatunableopticallterandabroadbandphotodiode.Othermoreinvolvedimplementationsexist(see[17]).ColorlessONUs:ReectiveSOAs(RSOAs)canbeusedtobuildcolorlessONUsthatremotelymodulateopticalsignalsgeneratedbycentralizedlightsources.Burst-ModeLaserDrivers:Burst-modetransmittersarerequiredforONUs.Theyhavetobeequippedwithlaserdriversthatprovidefastburston/offspeed,sufcientpowersuppressionduringidleperiod,andstable,accuratepoweremissionduringbursttransmission.Burst-ModeReceivers:Burst-modereceiversarerequiredatthecentralopticallineterminal(OLT)ofaPONandmustexhibitahighsensitivity,widedynamicrange,andfasttimeresponsetoarrivingbursts.Amongothers,de-signchallengesforburst-modereceiversincludedynamicsensitivityrecovery,fastlevelrecovery,andfastclockrecovery.b)WirelessTechnologies:Aplethoraabroadbandwire-lessaccesstechnologiesexist[18].Currently,thetwomostimportantonesfortheimplementationofthewirelesspartofFiWinetworksareWiFiandWiMAX. Fig.4.OpticalinterconnectedbidirectionalberringsintegratedwithWiFi-basedwirelessaccesspoints[20].wavelengthchannel.Thatis,eachRAUisallocatedaseparatededicatedwavelengthchannelfortransmissionandreceptiontoandfromthecentralstation.Atthecentralstation,aWDMlasergeneratesthedesiredwavelengthsinordertoreachthecorrespondingRAUs.ThegeneratedwavelengthsareopticallyswitchedandpassedtoanarrayofRFmodulators,oneforeachRAU.ThemodulatedwavelengthsaremultiplexedontotheopticalberringandreceivedbyeachaddressedRAUonitsassignedwavelength.AnRAUretrievestheRFsignalan

6 dtransmitsittotheantennasofapassingtrain
dtransmitsittotheantennasofapassingtrain.Intheupstreamdirection,theRAUsreceiveallRFsignalsandsendsthemtothecentralstationforprocessing.ByprocessingthereceivedRFsignals,thecentralstationisabletokeeptrackofthetrainlocationandidentifyingtheRAUclosesttothemovingtrain.Inconventionalcellularradionetworks,ahand-overwouldtakeplacewheneverthetraincrossesthecellboundarybe-tweentwoneighboringRAUs.Toavoidhand-overs,theap-pliedconceptofmovingcellsletsacellpatternmovetogetherwiththepassingtrainsuchthatthetraincancommunicateonthesameRFfrequenciesduringthewholeconnectionwithoutrequiringhand-overs.ThecentralstationimplementsthemovingcellsbysubsequentlysendingtheRFfrequenciesusedbythetraintothenextRAUfollowinginthedirectionthetrainismoving.BasedonthereceivedupstreamRFsignals,thecentralstationisabletotrackthelocationofthetrainandassigndownstreamRFsignalstothecorrespondingRAUclosesttothetrainsuchthatthetrainandmovingcellsmovealonginasynchronousfashion.Fig.4showsatwo-levelbidirectionalpath-protectedringR&FarchitecturefordenseWDM/subcarriermultiplexing(SCM)broadbandFiWinetworks[20].Inthisarchitecture,theCOinterconnectsremotenodes(RN)viaadual-berring.EachRNcascadeswirelessaccesspoints(WAPs)throughconcentrationnodes(CNs),whereeachWAPoffersservicestomobileclientnodes(MCNs).Forprotection,theCOis Fig.5.Opticalhybridstar-ringnetworkintegratedwithWiFi-basedwirelessaccesspoints[21].equippedwithtwosetsofdevices(normalandstandby).EachRNconsistsofaprotectionunitandabidirectionalwavelengthadd-dropmultiplexerbasedonamultilayerdielectricinter-ferencelter.EachCNcontainsaprotectionunit.TheWAPcomprisesanopticaltransceiver,aprotectionunit,up/downRFconverters,andasleeveantenna.EachWAPprovideschannelbandwidthofatleast5MHzandcoversupto16MCNsbymeansoffrequencydivisionmultiplexing(FDM).Undernormaloperatingconditions,theCOtransmitsdownstreamsignalsinthecounter-clockwisedirectionviaRNsandCNstotheWAPs.IfabercutoccursbetweentwoRNsorbetweentwoCNs,theirassociatedcontrollersdetectthefailurebymonitoringthereceivedopticalsignalandthenswitchtotheclockwiseprotectionring.IfafailurehappensataWAP,theretransmittedsignalsareprotectionswitchedthroughotheropticalpathsbythrowinganopticalswitchinsidetheaffectedWAP.Thisarchitectureprovideshighreliability,exibility,capacity,andself-healingproperties.Fig.5depictsanR&F-basedhybridFiWinetworktopologythatcombinesopticalstarandringnetworks[21].EachberringaccommodatesseveralWiFi-basedWAPsandiscon-nectedtotheCOandtwoneighboringberringsviaopticalswitches.TheopticalswitcheshavefullwavelengthconversioncapabilityandinterconnecttheWAPsandCObymeansofsharedpoint-to-pointlightpaths.Thenetworkisperiodicallymonitoredduringprespeciedintervals.Attheendofeachinterval,thelightpathsmaybedynamicallyreconguredinresponsetovaryingtrafcdemands.Whentrafcincreasesandtheutilizationoftheestablishedlightpathsislow,theloadontheexistinglightpathsisincreasedbymeansofloadbalancing.Otherwise,iftheestablishedlightpathsareheavilyloaded,newlightpathsneedtobesetup,provide

7 denoughcapacityisavailableontheberli
denoughcapacityisavailableontheberlinks.Intheeventofoneormorelinkfailures,theaffectedlightpathsaredynamicallyreconguredusingtheredundantberpathsofthearchitecture. Fig.6.OpticalunidirectionalWDMringinterconnectingmultiplePONsintegratedwithaWiFi-basedwirelessmeshnetwork[22].TheFiWinetworkproposedin[22]consistsofanop-ticalWDMbackhaulringwithmultiplesingle-channelormultichannelPONsattachedtoit,asshowninFig.6.AnOADMisusedtoconnecttheOLTofeachPONtotheWDMring.WirelessgatewaysareusedtorealizeanR&FnetworkthatbridgesthePONstoaWiFi-basedWMN.Inthedownstreamdirection,datapacketsareroutedfromtheCOtothewirelessgatewaysthroughtheopticalbackhaulandarethenforwardedtotheMCNsbywirelessmeshrouters.Intheupstreamdirection,wirelessmeshroutersforwarddatapacketstooneofthewirelessgateways,wheretheyarethentransmittedtotheCOononeofthewavelengthchannelsoftheopticalbackhaulWDMring,aseachPONoperatesonaseparatedynamicallyallocatedwavelengthchannel.SincetheopticalbackhaulandWMNusedifferenttechnologies,aninterfaceisdenedbetweeneachONUandthecorre-spondingwirelessgatewayinordertomonitortheWMNandperformroutecomputationtakingthestateofwirelesslinksandaveragetrafcratesintoaccount.WhenthetrafcdemandssurpasstheavailablePONcapacity,someofthetimedivisionmultiplexing(TDM)PONsmaybeupgradedtoWDMPONs.IfsomePONsareheavilyloadedandothershavelesstrafc,someheavy-loadedONUsmaybeassignedtoalightly-loadedPONbytuningtheiropticaltransceiverstothewavelengthassignedtothelightly-loadedPON.Thisarchitectureprovidescost-effectiveness,bandwidthefciency,widecoverage,highexibility,andscalability.Inaddition,therecongurableTDM/WDMopticalbackhaulhelpsreducenetworkcongestionandaveragepacketlatencybymeansofloadbalancing.Moreover,thedynamicallocationofradioresourcesenablescost-effectiveandsimplehand-overs.IV.SUPERMANAswehaveseenintheprevioussection,mostpreviouslyreportedFiWinetworksusedWiFitechnologiesforthewire-lesspart.OnlyafewreportedFiWinetworksconsideredthedeploymentofWiMAXtechnologies.Onenotableex-ampleistheintegrationofsingle-channelTDMEPONandWiMAXnetworks.SeveralTDMEPON-WiMAXintegrationapproacheswereoutlinedanddiscussedin[23],rangingfromindependenttouniedconnection-orientedarchitectures.TheintegrationofTDMEPONandWiMAXaccessnetworksseemstobeinterestingduetothesimilarityofthetwotechnologies.BothEPONandWiMAXnetworkstypicallyhaveapoint-to-multipointtopologywithacentralcontrolstation(OLTinEPON,BSinWiMAX)performingdynamicbandwidthallocationbymeansofcentralizedpollingandscheduling.Thesesimilaritiesgiverisetointerestingconver-genceproblemswhoseoptimizationisexpectedtoleadtoanimprovedFiWinetworkperformance.InourproposedFiWinetwork,wetakeadifferentapproach.GiventhesimilaritiesofEPONandWiMAX,wearguethatthetwotechnologiesaremorelikelytotargetthesamenetworksegmentratherthanbeingcascadedtocoverdifferentnetworksegments.Inotherwords,weexpectthatnetworkoperatorswillmakeachoicebetweenEPONandWiMAXdependingonanumberoffactors,e.g.,right-of-way.Furthermore,recallfromSectionIthatEPONnetworkswillbr

8 ingbercloseorallthewaytoendusers.Its
ingbercloseorallthewaytoendusers.Itseemssomewhatimpracticaltodeployametropolitan-reachwirelesstechnologysuchasWiMAXforrealizingwirelessdroplinesofrathershortlengthtoorinsideofcesandhomes.Instead,usingnext-generationlow-costWiFitechnologiesinconjunctionwithWDM-enhancedEPONaccessnetworkswhileintegratingWiMAXwithopticalmetropolitanareanetwork(MAN)technologiesappearstobeamorepromisingapproach,givingrisetoanovelFiWinetworkarchitecturewhichwecallSuperMAN.Fig.7depictsthenetworkarchitectureofSuperMAN.Itbuildsonourall-opticallyintegratedEthernet-basedaccess-metronetwork,describedatlengthin[24],extendedbyoptical-wirelessinterfaceswithnext-generationWiFiandWiMAXnetworks.Morespecically,theopticalpartofSuper-MANconsistsofanIEEE802.17ResilientPacketRing(RPR)metronetworkthatinterconnectsmultipleWDMEPONaccessnetworksattachedtoasubsetofRPRnodes.RPRisanopticaldual-berbidirectionalringnetworkthataimsatcombiningEthernet'sstatisticalmultiplexinggain,lowequipmentcost,andsimplicitywithSONET/SDH'scarrier-classfunctionalitiesofhighavailability,reliability,andprotableTDM(voice)support.InRPR,destinationstrippingisdeployedtoimprovespatialreuseofbandwidthandthusincreasethecapacityofthenetwork.EachoftheattachedWDMEPONshasatreetopologywiththeOLTattheroottreebeingcollocatedwithoneofthePCOs.NoparticularWDMarchitectureisimposedontheONUs,thusallowingthedecisiontobedic-tatedbyeconomics,state-of-the-arttransceivermanufacturingtechnology,trafcdemands,andserviceproviderpreferences.TherecommendedWDMextensionstotheIEEE802.3ahMultiPointControlProtocol(MPCP),describedingreaterdetailin[25],guaranteebackwardcompatibilitywithlegacyTDMEPONsandenabletheOLTtoscheduletransmissions Fig.7.SuperMANarchitectureintegratingnext-generationWiFitechnologieswithWDMEPONandnext-generationWiMAXtechnologieswithRPR.toandreceptionsfromONUsonanysupportedwavelengthchannel.Theopticalaccess-metronetworkletslow-costPONtechnologiesfollowlow-costEthernettechnologiesfromac-cessnetworksintometronetworksbyinterconnectingthePcollocatedOLTs/COswithapassiveopticalstarsubnet-workwhosehubconsistsofanathermalwavelength-routingPParrayedwaveguidegrating(AWG)inparallelwithawavelength-broadcastingPPpassivestarcoupler(PSC).ItisimportanttonotethatineachWDMEPONtwodifferentsetsofwavelengths,OLTandAWG,areused.Therstwavelengthset,OLT,isusedforupstreamanddownstreamtransmissionsbetweenONUsandrespectiveOLTresidinginthesameWDMEPON.Whereasthesecondset,AWG,compriseswavelengthsthatopticallybypassthecollocatedOLT/COandallowONUsresidingindifferentWDMEPONstocommunicateall-opticallywitheachotherinasinglehopacrosstheAWGofthestarsubnetwork,providedtheONUsareequippedwithtransceiversoperatingonthesewavelengths.WenallynotethatsimilartoIEEE802.3ahEPON,theopticalpartofSuperMANisnotrestrictedtoanyspecicdynamicbandwidthallocation(DBA)algorithm.AplethoraofDBAalgorithmsforWDMEPONsexist[26].TheseDBAalgo-rithmsneedtobeadaptedtoSuperMAN.TheaforementionedopticalpartofSuperMANinterfaceswithnext-generationWiFiandW

9 iMAXnetworks.Bothoptical-wirelessinterfa
iMAXnetworks.Bothoptical-wirelessinterfacesaredescribedingreaterdetailinthefollowing.A.RPR/WiMAXInterfaceAsshowninFig.7,someoftheRPRnodesmayinterfacewithWiMAXratherthanEPONaccessnetworks.Fig.8de- Fig.8.Optical-wirelessinterfacebetweenRPRandWiMAXnetworks.pictstheoptical-wirelessinterfacebetweenRPRandWiMAXnetworksingreaterdetail,whereanintegratedratecontroller(IRC)isusedtoconnectanRPRnodetoaWiMAXBS.InRPR,packetsundergooptical-electrical-optical(OEO)conversionateachringnode.AnRPRnodedeploysingeneraltwoseparateelectricaltransitqueues,oneprimarytransitqueue(PTQ)andonesecondarytransitqueue(STQ),forservicedifferentiation.Inaddition,anelectricalstagequeueisusedtostoretrafcreadytobesentbytheRPRstation. TheRPRschedulergivesprioritytoin-transitringtrafcoverstationtrafcsuchthatin-transitpacketsarenotlostduetobufferoverow.Furthermore,RPRdeploysadistributedfairnesscontrolprotocolthatdynamicallythrottlestrafcinordertoachievenetwork-widefairnesswhilemaintainingspatialreuse.TheWiMAXBSdeploysadownlink(DL)schedulerandanuplink(UL)scheduler,wherebythelatteroneprocessesULrequestsfromandsendsULgrantstoitsattachedSSs.Inourongoingwork,weconsiderIEEE802.16eandtheemergingamendmentIEEE802.16m.Therstoneaddsmobilitysup-porttoconventionalIEEE802.16dWiMAXnetworks,whilethelatteroneprovidesincreasesthedatarateto1Gb/s.TheIRCinFig.8playsakeyroleinourongoingworkonintegratingRPRandWiMAXtechnologies.TheIRCcomprisesaBScontroller,trafcclassmappingunit,CPU,andtrafcshaper.ItwillbeusedtoseamlesslyintegratebothtechnologiesandjointlyoptimizetheRPRschedulerandWiMAXDLandULschedulers.B.WDMEPON/Next-GenerationWiFiInterfaceRecallfromSectionIIthatWiFi-basedRoFnetworkscansustainacceptablethroughputperformanceonlyiftheinsertedberdoesnotexceedacertainmaximumlength.DuetothefactthatEPONscanhaveareachofupto20km,theWDMEPONtreenetworkswithWiFiextensionsarerealizedasR&Fnetworks,whereeachWiFi-basednetworkoperatesindependentlyofitsattachedWDMEPONtreenetwork.Inourongoingwork,wefocusontheMACenhancementsofnext-generationIEEE802.11nWLANsandpathselectionalgorithmsforIEEE802.11sWLANmeshnetworks.Next-generationWLANswillofferathroughputofatleast100Mb/smeasuredattheMACserviceaccesspoint(SAP).TheIEEEstandard802.11nisn'texpectedtobeapproveduntilMarch2009,butdevicesbuilttothecurrent802.11ndraftwillrequireonlysoftwareupgradestobecompliantwiththeratiedstandard.ThedraftprovidesbothPHYandMACenhancements.ByusingMIMO-OFDMandchannelbonding,802.11nWLANsofferrawdataratesofabout600Mb/satthephysicallayer.ToachieveanetMACthroughputof100Mb/sandhigher,802.11nWLANsallowwirelessstationsforthetruncationoftransmissionopportunities(TXOPs),reversedirection(i.e.,bidirectionalTXOP),anduseofareducedinterframespace(RIFS)todecreasethedeadtimebetweenframes(aTXOP,speciedinIEEE802.11e,isatimeintervalduringwhichawirelessstationfollowingasinglechannelaccessisallowedtosendmultipledataframes).ThemostimportantMACenhancementofnext-generationWLANsisframeaggregation.In802.11n,thefollowingtwomethodsexistforframeaggregatio

10 n:(i)aggregateMACprotocoldataunit(A-MPDU
n:(i)aggregateMACprotocoldataunit(A-MPDU),and(ii)aggregateMACservicedataunit(A-MSDU).A-MPDUconcatenatesupto64MPDUsubframesintoasinglephysicallayerSDU,providedallconstituentMP-DUsaredestinedtothesamereceiver.A-MSDUconcatenatesmultipleMSDUsubframesintoasingleMPDU,wherebyallconstituentMSDUsnotonlyhavetobedestinedtothesamereceiverbutalsomusthavethesametrafcidentier(TID),i.e.,thesameQoSlevel.A-MPDUandA-MSDUcanbeusedseparatelyorjointlytoincreasetheMACthroughputofnext-generationWLANs.AsshowninFig.7,SuperMANdeploysanext-generation802.11nWLANmeshnetwork.TheemergingamendmentIEEE802.11saimsatspecifyingawirelessdistributionsystem(WDS)amongWLANAPswhichcanbeusedtorealizemunicipalnetworksthatprovidepublicwirelessac-cessthroughoutcities,neighborhoods,andcampuses.IEEE802.11sintroducesanewmeshframeformatandradio-awareroutingframeworkwhichusestheso-calledHybridWirelessMeshProtocol(HWMP)asdefaultroutingprotocol.HWMPworksonlayer2,usesMACaddressesforpathselection,andcontainsbothreactiveandproactiveroutingcomponents.InSuperMAN,proactiveroutingcanbeusedtocongureroutingtreestowardthecollocatedAP/ONU(s)thatactasmeshportalsbridgingtheWLANmeshnetworktotheoptical(wired)WDMEPONaccessnetwork.Forintra-meshcommunicationbetweenwirelessstations,agivenmeshportal(i.e.,AP/ONU)mayapplyreactiveroutingbysettingupadirectroutebetweentheinvolvedwirelessstations,therebyeliminatingtheneedtosendintra-meshtrafcthroughthemeshportal.ItisimportanttonotethattheroutingframeworkofIEEE802.11sisextensible.Thus,otherroutingprotocolsandroutingmetricscanbedeployedinordertooptimizenetworkperformanceaccordingtogiventrafcdemandsandusagescenarios.Inourongoingwork,westudyintegratedhybridpathselectionalgorithmsforSuperMANthattakebothproactiveandreactivecomponentsaswellasdifferentroutingmetriccombinationsintoaccount.ParticularattentionwillbepaidtothedesignandperformanceevaluationofQoS-awareschedul-ingalgorithmsthatensureQoScontinuityacrosstheWDMEPON/next-generationWiFiinterfaceandprovideend-to-endQoSassurancesacrossSuperMAN.V.CONCLUSIONSHybridoptical-wirelessFiWinetworksformapowerfulfuture-proofplatformthatprovidesanumberofadvantages.Introducingopticalberintobroadbandwirelessaccessnet-workshelpsrelieveemergingbandwidthbottlenecksintoday'swirelessbackhaulduetoincreasingtrafcloadsgeneratedbynewapplications,e.g.,iPhone.Bysimultaneouslyprovidingwiredandwirelessservicesoverthesameinfrastructure,FiWinetworksareabletoconsolidate(optical)wiredandwirelessaccessnetworksthatareusuallyrunindependentlyofeachother,thuspotentiallyleadingtomajorcostsavings.Moreinterestingly,andcertainlysomewhatcontroversially,bypavingallthewaytoandpenetratingintohomesandofceswithhigh-capacityberandconnectingwirelesslaptopsandhandheldswithhigh-throughputWiFitechnologiestohigh-speedopticalwirednetworks,SuperMAN,andFiWinetworksingeneral,giveaccesstotheeverincreasingprocessingandstoragecapabilitiesofmemoryandCPUsofwidelyuseddesktops,laptops,andotherwirelesshandhelds,e.g.,Wii.Notethatnowadaysdesktopandlaptopcomputerscommo