外文翻译adhoc网络的一个简要概述挑战与研究方向教学内容.docx
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外文翻译adhoc网络的一个简要概述挑战与研究方向教学内容.docx
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外文翻译adhoc网络的一个简要概述挑战与研究方向教学内容
附录
一、英文原文
ABriefOverviewofadhocNetworks:
ChallengesandDirections
Oneofthemostvibrantandactive“new”fieldstodayisthatofadhocnetworks.Significantresearchinthisareahasbeenongoingfornearly30years,alsounderthenamespacketradioormulti-hopnetworks.
adhocnetworkisa(possiblymobile)collectionofcommunicationsdevices(nodes)thatwishtocommunicate,buthavenofixedinfrastructureavailable,andhavenopre-determinedorganizationofavailablelinks.Individualnodesareresponsiblefordynamicallydiscoveringwhichothernodestheycandirectlycommunicatewith.
Adhocnetworkingisamulti-layerproblem.Thephysicallayermustadapttorapidchangesinlinkcharacteristics.Themultipleaccesscontrol(MAC)layerneedstominimizecollisions,allowfairaccess,andsemi-reliablytransportdataoverthesharedwirelesslinksinthepresenceofrapidchangesandhiddenorexposedterminals.Thenetworklayerneedstodetermineanddistributeinformationusedtocalculatepathsinawaythatmaintainsefficiencywhenlinkschangeoftenandbandwidthisatapremium.It’salsoneedstointegratesmoothlywithtraditional,nonadhoc-awareinternetworksandperformfunctionssuchasauto-configurationinthischangingenvironment.Thetransportlayermustbeabletohandledelayandpacketlossstatisticsthatareverydifferentthanwirednetworks.Finally,applicationsneedtobedesignedtohandlefrequentdisconnectionandreconnectionwithpeerapplicationsaswellaswidelyvaryingdelayandpacketlosscharacteristics.
Adhocnetworksaresuitedforuseinsituationswhereinfrastructureiseithernotavailable,nottrusted,orshouldnotbereliedonintimesofemergency.Afewexamplesinclude:
militarysoldersinthefield;sensorsscatteredthroughoutacityforbiologicaldetection;aninfrastructurelessnetworkofnotebookcomputersinaconferenceorcampussetting;theforestryorlumberindustry;rareanimaltracking;spaceexploration;underseaoperations;andtemporaryofficessuchascampaignheadquarters.
History
Thehistoryofadhocnetworkscanbetracedbackto1972andtheDoD-sponsoredPacketRadioNetwork(PRNET),whichevolvedintotheSurvivableAdaptiveRadioNetworks(SURAN)programintheearly1980s[l].Thegoaloftheseprogramswastoprovidepacketswitchednetworkingtomobilebattlefieldelementsinaninfrastructureless,hostileenvironment(soldiers,tanks,aircraft,etc.,formingthenodesinthenetwork).
Intheearly1990saspateofnewdevelopmentssignaledanewphaseinadhocnetworking.Notebookcomputersbecamepopular,asdidopen-sourcesoftware,andviablecommunicationsequipmentbasedonRFandinfrared.Theideaofaninfrstructurelesscollectionofmobilehostswasproposedintwoconferencepapers[2,3],andtheIEEE802.11subcommitteeadoptedtheterm“adhocnetworks.”Theconceptofcommercial(non-military)adhocnetworkinghadarrived.Othernovelnon-militarypossibilitiesweresuggested(asmentionedintheintroduction),andinterestgrew.
Ataroundthesametime,theDoDcontinuedfromwhereitleftoff,fundingprogramssuchastheGlobalMobileInformationSystems(GloMo),andtheNear-termDigitalRadio(NTDR).ThegoalofGloMowastoprovideoffice-environmentEthernet-typemultimediaconnectivityanytime,anywhere,inhandhelddevices.ChannelaccessapproacheswerenowintheCSMA/CAandTDMAmolds,andseveralnovelroutingandtopologycontrolschemesweredeveloped.TheNTDRusedclusteringandlinkstaterouting,andself-organizedintoatwo-tieradhocnetwork.NowusedbytheUSArmy,NTDRistheonly“real”(non-prototypical)adhocnetworkinusetoday.
Spurredbythegrowinginterestinadhocnetworking,anumberofstandardsactivitiesandcommercialstandardsevolvedinthemidtolate’90s.WithintheIETF,theMobileAdhocNetworking(MANET)workinggroupwashorn,andsoughttostandardizeroutingprotocolsforadhocnetworks.ThedevelopmentofroutingwithintheMANETworkinggroupandthelargercommunityforkedintoreactive(routesondemand)andproactive(routesready-to-use)routingprotocols[4].The802.11subcommitteestandardizedamediumaccessprotocolthatwasbasedoncollisionavoidanceandtoleratedhiddenterminals,makingitusable,ifnotoptimal,forbuildingmobileadhocnetworkprototypesoutofnotebooksand802.11PCMCIAcards.HIPERLANandBluetoothweresomeotherstandardsthataddressedandbenefitedadhocnetworking.
OpenProblems
Despitethelonghistoryofadhocnetworking,therearestillquiteanumberofproblemsthatareopen.Sinceadhocnetworksdonotassumetheavailabilityofafixedinfrastructure,itfollowsthatindividualnodesmayhavetorelyonportable,limitedpowersources.Theideaofenergy-efficiencythereforebecomesanimportantprobleminadhocnetworks.Surprisingly,therehasbeenlittlepublishedworkintheareaofenergy-efficiencyofadhocnetworksuntilfairlyrecently.Mostexistingsolutionsforsavingenergyinadhocnetworksrevolvearoundthereductionofpowerusedbytheradiotransceiver.AttheMAClevelandabove,thisisoftendonebyselectivelysendingthereceiverintoasleepmode,orbyusingatransmitterwithvariableoutputpower(andproportionateinputpowerdraw)andselectingroutesthatrequiremanyshorthops,insteadofafewlongerhops[8].
Theabilityoffixed,wirelessnetworkstosatisfyqualityofservice(QoS)requirementsisanotheropenproblem.AdhocnetworksfurthercomplicatetheknownQoSchallengesinwirelinenetworkswithRFchannelcharacteristicsthatoftenchangeunpredictably,alongwiththedifficultyofsharingthechannelmediumwithmanyneighbors,eachwithitsownsetofpotentiallychangingQoSrequirements.Reflectingthemulti-layernatureofadhocnetworks,therearenumerousattemptstoimprovetheQoSproblemsfromtheservicecontracts[9]totheMAClayer.ApromisingmethodforsatisfyingQoSrequirementsisamoreunifiedapproachofcross-layerorvertical-layerintegration.Theideaistoviolatemanyofthetraditionallayeringstylestoallowdifferentpartsofthestacktoadapttotheenvironmentinawaythattakesintoaccounttheadaptationandavailableinformationatotherlayers.
Asimilarmulti-layerissueisthatofsecurityinadhocnetworks[10].Sincenodesusethesharedradiomediuminapotentiallyinsecureenvironment,theyaresusceptibletodenialofservice(DoS)attacksthatarehardertotrackdownthaninwirednetworks.Also,sincealargeportionofthenetworknodeswillbedynamicallyreorganizingandforwardingpacketsonbehalfofothers,adhocnetworksareparticularlysusceptibletotheinjectionofbogusnetworkcontroltraffic.Finally,adhocnetworkscanhevictimsofspecializedkindsofsecurityattackssuchasDoSattacksthatcauseanodetouseitstransceiversomuchthatitdepletesitsbattery.
Robotsandsensorsalsoprovidenewhardwarecapabilitiesripefornewmethodsofenhancingadhocefficiency.Robots,forexample,haveatightintegrationbetweentheprocessesofmovement,decision-making,andnetworkingthatallowthemtomodifytheiractionswhiletakingintoaccounttheeffectsonmanydifferentsystemaspects[12].Similarly,sensorsareoftendeployedinawaythatmakestheirrolesandcapabilitiesredundant,suggestingnewwaysofcombiningapplicationknowledgeofdeliveredinformationwiththeroutinglayer[13].
Finally,aproblemthatoverarchesalltheseothersisthelackofwelldefinedandwidelyacceptedmodelsforRFpathattenuation,mobility,andtraffic.Thesetightlyinterrelatedmodelsareneededforquantifyingandcomparingadhocsystemperformancetoacommonbaseline.Thephysicalprocessesofrefraction,reflection,andscatteringofRFradiationismoderatelywellunderstoodbutdifficulttoquantifyindetailwhenincludingalargenumberofcomplexobjectssuchasfoliage,cars,orbuildings.Incontrast,thepatternofmovementofthenodesandtheflowoftrafficcancertainlybeeasilydescribedindetail,butthedependencyonthetargetapplication,thelackofexistingsystemsavailableforstudy,andthelikelyinteractionsbetweenconnectivity,movement,anduserapplications,causesthesemodelstobeill-defined.
TheFuture
Imaginethefollowingscenarios:
awirelessmeshofrooftop-mountedadhocrouters;anadhocnetworkofcarsforinstanttrafficandotherinformation;sensorsandrobotsformingamultimedianetworkthatallowsremotevisualizationandcontrol;multipleairbornerouters(fromtinyrobotstoblimps)automaticallyprovidingconnectivityandcapacitywhereneeded(e.g.,atafootballgame);anadhocnetworkofspacecraftaroundandintransitbetweentheEarthandMars.
Thesemayseemlikesciencefiction,butareinfactideaspursuedseriouslybytheadhocresearchcommunity.Whileonlytimecantellwhichoftheseimaginedscenarioswillbecomereal,theaboveoffersaglimpseintoboththetechnologicalpotentialandtheevolvingstateoftheart.Wediscussinthissectiontheforcesatplaythatarelikelytoshapethefutureofadhocnetworking,anddiscussthedirectionsinwhichitmayevolve.
Toappreciatetheroleadhocnetworksarelikelytoplayinthefuture,considerthis:
bandwidth-hungryapplicationsandthelawsofphysicsdrivewirelessarchitecturesawayfromcellulartowardadhoc.Thisisbecausemorecapacityimpliestheneedforahighercommunicationsbandwidthandbetterspatialspectralreuse.Higherbandwidthisfoundathigherfrequencies,wherethepropagationisdismal.Further,mobiledeviceshavetobepower-thrifty.Propagation,spectralreuse,andenergyissuessupportashiftawayfromasinglelongwirelesslink(asincellular)toameshofshort
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