Wireless multihop mesh networks无线多跳mesh网络Word文件下载.docx

Wireless multihop mesh networks无线多跳mesh网络Word文件下载.docx

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Alongwiththeexpandingofthewirelessnet’sscale,therequiringofwiredconnectionforeachaccess-pointinWLANmakesitmeetalotofchallengesandinconvenienceintheenvironmentoflackofcableinfrastructure.ThetraditionalWLANshowsit’sinsufficientgradually.

ThedisadvantagesoftraditionalWLAN:

a.Thepoorreliability:

InthetraditionalWLAN,severalusersgetaccesstothewirelessnetthroughoneaccess-pointdirectly—itiscall“single-hop”networks.Solongasonestoppagecanbreakdownthewholenetworks.

Figure1.TraditionalWLAN

b.Thesmallcoverage:

Thenormaltechnologyof“point-to-point”or“point-to-multipoint”usestheshortnetworks,withthelimitationsofcoverage,justlikenetworksof802.11andBluetooth.

c.Thepoorscalability:

Asthejoiningandsharingdataflowofaccess-pointsofextranodes,thebandwidthofwholenetworkswillcomedowngradually.

d.Thebadqualityofcommunication:

Becauseofnoiseinlinkandmistakesincommunication,thebandwidthwilldecreaseasthedistance’sincreasing.Mostkindofwirelessnetworkshave“blindpoints”initseffectivedistance,inwhichitshowsthepoorsignalquality.

Ⅱ.Theconceptofwirelessmeshnetwork（WMN）

Withtheevolutionofcommunicationtechniqueandcateringtotheneedofthemarket,anewwirelessnetworkemerges,whichiscalled“wirelessmeshnetwork”,shortfor“WMN”.Wirelessmeshnetworkisaflexiblecommunicationsystemtoimprovetheefficiencyandqualityinsingletransmission.WMNshaveemergedasacost-effectivealternativeforlarge-areacommunications.WMNshavealsobecomeanimportanttechnologyforconstructingwirelessnetworksbeyondthe3Ghigh-speedcellphonenetworks.Ameshnetworkisanadhocnetworkthatemploysoneoftwoconnectionarrangements:

fullmeshtopologyorpartialmeshtopology.

WMNsdonotrequirewiredinfrastructureforlong-haulcommunications,thusofferingthemajorbenefitoflow-costandflexibledeploymentandmaintenance.WMNs,however,sufferfromthesamesecurityvulnerabilitiesasconventionalwirelessnetworks.Becausedataaretransmittedintheformofradiowavesintheairwithoutphysicalboundaries,packeteavesdroppingandpacketinterceptingcanbecarriedouteasilybyattackerswiththehelpofrelativelycheapequipment.Moreover,wirelesschannelsmaybedisturbedbynoiseandinterference,whichaffectthequalityofcommunications.WMNsaremulti-hopnetworks,whichareespeciallyvulnerabletointerferenceproblems:

Adjacenthopsonthesamepathandneighboringpathscouldinterferencewitheachother.WMNsaredistributednetworks,whichhaveadditionalissuesonrouting,authentication,andkeymanagement.Therefore,securityisamajorconcerninWMNs.

Ⅲ.AdhocnetworksandWirelessmeshnetworks

Figure2.Adhocnetwork

a.Adhocnetworks

Adhocnetworksarewirelessnetworksthatcanbedeployedquickly.AmobileadhocnetworkisapotentialsolutionfordeployingadataandvoicenetworkforFRsinscenarioswithlimitedornoinfrastructure.AllnodesinthenetworkactasequalpeersasFigure2.AnadhocnetworktypicallyusesasuitableroutingprotocolsuchasAODVorDSRtoallowmulti-hopcommunicationbetweenthenodes.OneormorenodesmayactasgatewaysandconnecttheadhocnetworkwiththeInternetorsomeothernetwork.Thegatewaysmayuseawiredorevenawirelessback-haultoexchangedatawithanexternalnetwork.Self-containedadhocnetworksmaynotevenhaveaback-haul.Adhocnetworksgenerallyhaveonlysingle-radionodesandcanbeconsideredtobesingle-tiermeshnetworks.

b.Wirelessmeshnetworks

Inthefullmeshtopology,eachnodeisconnecteddirectlytoeachothernode.Inthepartialmeshtopology,nodesareconnectedtoonlysome,butnotalltheothernodes.Wirelessmeshnetworksthuscombineameshtopologywithadhocwirelessnetworkcharacteristics.Meshnetworkscanbebuiltonamixoffixedandmobilenodesinterconnectedviawirelesslinkstoformamulti-hopadhocnetwork.

Figure3.Wirelessmulti-hopmeshnetworkoftwotiers

Inasufficientlydensewirelessmeshnetwork,thereisrarelyasinglepointoffailureandthenetworkisabletorecoverfromthefailureofanindividualnodebyroutingaroundittoothernodesinadirectormulti-hopmanner.Ameshnetworkallowsnodestocommunicatewithothernodeswithoutbeingroutedthroughacentralswitchpoint,thuseliminatingcentralizedfailureandprovidingself-healingandself-organizationbehavior.AsshowninFigure3,severalmeshroutersactasAPstothemobileclients.Themeshprovidesawirelessinfrastructuretomobileclientsusinganadhocwirelessback-bonetoroutepacketstotheirrespectivedestinations.Oneormoremeshrouterscanactasgatewaysfortheback-haultraffictoexternalnetworks,andmayuseawiredorwirelessconnection.Anadhocwirelessnetworkprotocol,suchasAdhocOn-demandDistanceVectorrouting,OptimizedLinkStateRouting,orDynamicSourceRouting,istypicallyusedforroutingpacketsinthenetworkbetweenmeshrouters.Thereexistover70po-tentialprotocolsthatcouldbeusedforroutinginameshnetwork.Pathdiversityandredundancyarethekeyfeaturesofasuccessfulmeshnetwork.

Two-tiermeshnetworks,suchasthatshowninFigure3,avoidthecompletelyself-organizednatureofadhocnetworks.Suchnetworkshavetwoormoreradios.Typicallyoneradioisusedinthe“accesstier”asanAPtoprovidewirelessservicetomobileclientsintheareaconnectedtoit（justlikeinaWi-Fiaccesspoint）.Thesecondradioisusedinthe“meshtier”bythemeshnodestoformawirelessbackbone,usingasuitablemeshroutingprotocol.Inthiscontext,theterm“meshroutingprotocol”issynonymouswiththeterm“adhocroutingprotocol”.Thus,eachmeshnodeinatwo-tierwirelessmeshnetworkprovidesthefunctionalityofbothanaccesspointaswellasrouter.

Theback-haulofthemeshnodewirelesslyrelaysthetrafficfrommeshroutertomeshrouteruntilitreachesagateway.ThegatewaynodethenconnectstotheInternetortoanotherprivatenetwork（throughawiredconnection）.Anotheroptionistouseanalternativetechnologysuchassatelliteorcellularnetworklinks,towirelesslyconnectthemeshnetworkwithanexternalnetwork.

Weusethistwo-tiermodelinDart-Mesh,ourmeshimplementationtestbed.OnebenefitofthisarchitectureisthatforenduserstheexperienceofjoiningsuchanetworkisidenticaltotheprocessforaclientjoiningaWi-Fiinfrastructure-basedWLAN.ThisprocessiswellsupportedbyallcompatiblewirelessclientdevicesthatsupportthepublishedstandardsoftheWi-Fidevicemanufacturersconsortium.Theclientsdonotneedtorunanyspecialroutingsoftwareorconfiguretheradiotorunin“adhocmode.”Indeed,the“adhocmode”isnotfullysupportedbymanydevicemanufacturers,devicedriversandoperatingsystems,andconfiguringawirelesscardtousethismodeisnotaneasyprocess.

c.RelationshipbetweenadhocnetworksandWirelessmeshnetworks

Ad-hocnetworksandWMNsareoftenusedinterchangeably,buttherearesubtledifferencesbetweenthemonanumberofaspects.

First,end-userdevicesinad-hocnetworksalsoperformroutingandconfigurationfunctionalitiesforothernodes.InWMNsthesefunctionalitiesareperformedbymeshrouters.Intermsoffunctionalities,meshclientdevicesarethesameasthoseinIEEE902.11WLANs,providinglowerenergyconsumptionandhigh-endapplicationcapabilitiestomobileusersandenergyconstrainedusers.Moreover,becauseend-usersdonotsupportroutingfunctionalitiesthecostofWMNclientdevicesmaybereduced.

Second,WMNsadoptthemultiple-radiointerfacetechnology,whichdiffersfromad-hocnetworks.Themulti-radiointerfacetechnologymakesitpossibletoseparaterouting（controlpackets）fromdatatransmission（datapackets）.Thissignificantlyimprovesthecapacityofthenetwork.Ontheotherhand,routinganddatatransmissioninad-hocnetworksareperformedonthesameradiointerface,whichwouldreducetheperformance.

Third,becauseend-userdevicesinad-hocnetworksprovideroutingfunctionalities,thenetworktopologyandconnectivitydependonthemovementofusers.Thisimposesadditionalchallengestoroutingprotocols,networkconfigurations,andnetworkdeployment.Differentfromah-hocnetworks,routingisperformedbymeshroutersthatareeitherstationaryorslightlymobile.MeshroutersconstitutetheinfrastructureforWMNs.Whileprovidingcontinuousconnectivitythroughoutthenetworks,WMNscontinuetosupportmobileend-userswithoutcompromisingtheperformance.

Thedifferencesbetweenad-hocnetworksandWMNsaresubtle,makingitdifficulttodrawacleanlinebetweenthetwotypesofnetworks.Itmaybehelpfultoviewad-hocnetworksasasubsetofWMNsorviceversa.

Ⅳ.ChallengesandproblemstobesolvedintheresearchofWirelessmeshnetworks

SeveralchallengesneedtobeovercometorealizeawirelessmeshnetworksuitableforFirstResponders.Theoverarchingchallengeistoengineerandbuildawirelessmeshnetworkthatprovidesallthedesiredcharacteristicsinacosteffectivemanner.Weenumeratesomeofthedesirablecharacteristicsofamobilemeshnetworkandtheircorrespondingchallenges:

a.Robustness:

Themeshnetworkshouldbefunctionalevenifafewindividualnodesfail.Forexample,inahazardousscenario