外文翻译 - 无线技术，低功耗传感器网络

0外文资料1ZigBee:WirelessTechnologyforLow-PowerSensorNetworksTechnologistshaveneverhadtroublecomingupwithpotentialapplicationsforwirelesssensors.Inahomesecuritysystem,forexample,wirelesssensorswouldbemucheasiertoinstallthansensorsthatneedwiring.Thesameistrueinindustrialenvironments,wherewiringtypicallyaccountsfor80%ofthecostofsensorinstallations.Andthenthereareapplicationsforsensorswherewiringisntpracticalorevenpossible.Theproblem,though,isthatmostwirelesssensorsusetoomuchpower,whichmeansthattheirbatterieseitherhavetobeverylargeorgetchangedfartoooften.Addtothatsomeskepticismaboutthereliabilityofsensordatathatssentthroughtheair,andwirelesssensorssimplyhaventlookedveryappealing.Alow-powerwirelesstechnologycalledZigBeeisrewritingthewirelesssensorequation,however.AsecurenetworktechnologythatridesontopoftherecentlyratifiedIEEE802.15.4radiostandard(Figure1),ZigBeepromisestoputwirelesssensorsineverythingfromfactoryautomationsystemstohomesecuritysystemstoconsumerelectronics.Inconjunctionwith802.15.4,ZigBeeoffersbatterylifeofuptoseveralyearsforcommonsmallbatteries.ZigBeedevicesarealsoexpectedtobecheap,eventuallysellingforlessthan$3pernodebysomeestimates.Withpricesthatlow,theyshouldbeanaturalfiteveninhouseholdproductslikewirelesslightswitches,wirelessthermostats,andsmokedetectors.Figure1:ZigBeeaddsnetwork,security,andapplication-serviceslayerstothePHYandMAClayersoftheIEEE811.15.4radioAlthoughnoformalspecificationforZigBeeyetexists(approvalbytheZigBeeAlliance,atradegroup,shouldcomelatethisyear),theoutlookforZigBeeappearsbright.TechnologyresearchfirmIn-Stat/MDR,inwhatitcallsacautiousaggressiveforecast,predictsthat2salesof802.15.4nodesandchipsetswillincreasefromessentiallyzerotodayto165millionunitsby2010.NotalloftheseunitswillbecoupledwithZigBee,butmostprobablywillbe.ResearchfirmONWorldpredictsshipmentsof465millionwirelesssensorRFmodulesby2010,with77%ofthembeingZigBee-related.Inasense,ZigBeesbrightfutureislargelyduetoitslowdatarates20kbpsto250kbps,dependingonthefrequencybandused(Figure2)comparedtoanominal1MbpsforBluetoothand54MbpsforWi-Fis802.11gtechnology.ButZigBeewontbesendingemailandlargedocuments,asWi-Fidoes,ordocumentsandaudio,asBluetoothdoes.Forsendingsensorreadings,whicharetypicallyafewtensofbytes,highbandwidthisntnecessary,andZigBeeslowbandwidthhelpsitfulfillitsgoalsoflowpower,lowcost,androbustness.Figure2:ZigBeesdataratesrangefrom20kbpsto250kbps,dependingonthefrequencyusedBecauseofZigBeeapplicationslowbandwidthrequirements,aZigBeenodecansleepmostofthetime,thussavingbatterypower,andthenwakeup,senddataquickly,andgobacktosleep.And,becauseZigBeecantransitionfromsleepmodetoactivemodein15msecorless,evenasleepingnodecanachievesuitablylowlatency.SomeoneflippingaZigBee-enabledwirelesslightswitch,forexample,wouldnotbeawareofawake-updelaybeforethelightturnson.Incontrast,wake-updelaysforBluetootharetypicallyaroundthreeseconds.AbigpartofZigBeespowersavingscomefromtheradiotechnologyof802.15.4,whichitselfwasdesignedforlowpower.802.15.4usesDSSS(direct-sequencespreadspectrum)technology,forexample,becausethealternativeFHSS(frequency-hoppingspreadspectrum)3wouldhaveusedtoomuchpowerjustinkeepingitsfrequencyhopssynchronized.ZigBeenodes,using802.15.4,cancommunicateinanyofseveraldifferentways,however,andsomewaysusemorepowerthanothers.Consequently,ZigBeeuserscantnecessarilyimplementasensornetworkanywaytheychooseandstillexpectthemultiple-yearbatterylifethatisZigBeeshallmark.Infact,sometechnologistswhoareplanningverylargenetworksofverysmallwirelesssensorssaythatevenZigBeeistoopowerhungryfortheiruses.AZigBeenetworknodecanconsumeextrapower,forexample,ifittriestokeepitstransmissionsfromoverlappingwithothernodestransmissionsorwithtransmissionsfromotherradiosources.The802.15.4radiousedbyZigBeeimplementsCSMA/CA(carriersensemultipleaccesscollisionavoidance)technology,andaZigBeenodethatusesCSMA/CAisessentiallytakingalisten-before-talkapproachtoseeifanyradiotrafficisalreadyunderway.But,asnotedbyVenkatBahl,marketingvicepresidentforsensorcompanyEmberCorp.andvicechairmanoftheZigBeeAlliance,thatsnotapreferredapproach.Havingtolistenburnspower,saysBahl,andwedontliketodothat.AnotherZigBeeand802.15.4communicationsoptionisthebeaconmode,inwhichnormallysleepingnetworkslavenodeswakeupperiodicallytoreceiveasynchronizingbeaconfromthenetworkscontrolnode.Butlisteningforabeaconwastespower,too,particularlybecausetiminguncertaintiesforcenodestoturnonearlytoavoidmissingabeacon.In-Your-FaceCommunicationTosaveasmuchpoweraspossible,ZigBeeemploysatalk-when-readycommunicationstrategy,simplysendingdatawhenithasdatareadytosendandthenwaitingforanautomaticacknowledgement.AccordingtoBobHeile,whoischairmanofboththeZigBeeAllianceandIEEE802.15,talk-when-readyisanin-your-facescheme,butonethatsverypowerefficient.Wedidanextensiveanalysisthatledtothebestpower-savingstrategyinvariouskindsofenvironmentsfromquiettonoisy,Heilesays.Wediscoveredthat,handsdown,wewerebetteroffjustsendingthepacketandacknowledgingit.Ifyoudontgetanack,itjustmeansyougotclobbered,sosenditagain.Youwinduphavingmuchbetter4powermanagementthanifyoulistenanddetermineifitsquietbeforeyoutalk.Fortunately,thisin-your-facestrategyleadstoverylittleRFinterference.ThatslargelybecauseZigBeenodeshaveverylowdutycycles,transmittingonlyoccasionallyandsendingonlysmallamountsofdata.OtherZigBeenodes,aswellasWi-FiandBluetoothmodules,caneasilydealwithsuchsmall,infrequentbursts.ZigBeestalk-when-readyschemedoesntsuitallpurposes,however.Forexample,inanetworkofthousandsoftinysensorsdroppedintoawarzonetomonitorenemytroopmovements,thepowersavingsprovidedstillmightnotbeenough.Witheachnetworknodesendingdataperiodicallyandwithtransmissionsrepeatednumeroustimesthroughothernearbynodesofameshnetworkconfigurationinordertoreachanetworkcontrollerlargenumbersofpacketcollisionsandretransmissionscouldwastepowerandsignificantlyshortensensornodebatterylife.Ifthesensorbatteriesareverysmallandpower-limited,thatsespeciallyproblematic.AlthoughcontentionforairwaveaccessisntgenerallyaproblemforZigBee,itcanbe.Sensor-networkcompanyDustNetworks,infact,sayscontentionissuesarekeepingthecompanyfromturningtoZigBeefornow,atleasteventhoughDustremainsamemberoftheZigBeeAlliance.EachZigBeedeviceneedstocontendforairspacewithitsneighbors,saysDustdirectorofproductmanagementRobertShear,sotheresinevitablysomecontentionandsomeinefficiency.ToavoidZigBeesaccesscontention,Dustusescontention-freeTDMA(timedivisionmultipleaccess)technology.ZigBee,throughthe802.15.4MAClayer,providesguaranteedtimeslotsinaschemethatsomewhatresemblesTDMA,butonlyaspartofanoptionalsuperframethatsmorecomplexandlesspower-efficientthanTDMA.ZigBeehasstillmorepower-savingtricksupitssleeve,however.Forexample,itreducespowerconsumptioninZigBeecomponentsbyprovidingforpower-savingreduced-functiondevices(RFDs)inadditiontomorecapablefull-functiondevices(FFDs).EachZigBeenetworkneedsatleastoneFFDasacontroller,butmostnetworknodescanbeRFDs(Figure3).RFDscantalkonlywithFFDs,nottootherRFDs,buttheycontainlesscircuitrythanFFDs,and5littleornopower-consumingmemory.Figure3:ZigBeenetworkscancontainasmanyas65,536nodesinavarietyofconfigurationsZigBeeconservesstillmorepowerbyreducingtheneedforassociatedprocessing.Simple8-bitprocessorslikean8051canhandleZigBeechoreseasily,andZigBeeprotocolstacksoccupyverylittlememory.AnFFDstack,forexample,needsabout32kbytes,andanRFDstackneedsonlyabout4kbytes.Thosenumberscomparewithabout250kbytesforthefarmorecomplexBluetoothtechnology.FromZigBeesrelativelysimpleimplementations,costsavingsnaturallyaccrue.RFDs,ofcourse,reduceZigBeecomponentcostsbyomittingmemoryandothercircuitry,andsimple8-bitprocessorsandsmallprotocolstackshelpkeepsystemcostsdown.Often,anapplicationsmainprocessorcaneasilybearthesmalladditionalloadofZigBeeprocessing,makingaseparateprocessorforZigBeefunctionsunnecessary.ButthemainstrategyforkeepingZigBeepriceslowistohavebigmarketsandhighvolumes.TheZigBeeAlliance,bymakingZigBeeanopenstandardandbyvigorouslypromotinginteroperabilityamongZigBeedevices,expectsthatZigBeewillbeverybiginapplicationssuchashomeandbuildingautomation.Theallianceiscurrentlyworkingoninteroperabilityproceduresforthoseparticularapplications,whichitexpectstocompletelaterthisyearalongwithZigBeeSpecification1.0.OnereasonforoptimismaboutZigBeeadoptionforhomeautomationandsecurityisitseaseofuse.ZigBeenetworksareself-forming,makingiteasyevenforconsumerstoset6themup.Intheresidentialspace,theresnoconfigurationinvolved,saystheZigBeeAlliancesHeile.Youtakesomethingoutofthebox,putthebatteriesin,andmaybedosomethingassimpleasbutton-presssecuritybringtwodevicesclosetogether,pushthebuttonsuntilthegreenlightscomeon,andyouredone.ZigBeenetworkscanalsoself-formincommercialandindustrialsettings,butprofessionalinstallerswillhavetoolsthatprovideadditionalcontrol,particularlyforsecurity.ZigBeesecurityisflexible,saysHeile,togivebothconsumerandprofessionaluserswhattheyneed.Youdonthavetohave128-bitpublic-keyencryptionforasmokedetector,hesays,butifIminahigh-riseofficecomplex,thatsexactlythelevelofsecurityImgoingtohaveformyfluorescentlightfixtures.Ifyoureinahigh-risebuildingonFifthAvenue,youdontwantsomeonegoingdownthestreetandturningyourlightsoff.ProprietaryCompetitionCompetitionforZigBeecomesalmostentirelyfromproprietarytechnologies.SensorcompanyDust,asnoted,isstickingwithitsowntechnology,andEmber,althoughpushingstronglyintotheZigBeearena,planstokeepofferingitsproprietaryEmberNetaswell.Inaddition,ZensysisprovidingitsZ-Wavetechnologytocustomers.Sylvania,forexample,isalreadyusingZ-Waveforlightingcontrol,whileZigBeesystemsremainatleastseveralmonthsaway.Byofferinginteroperability,however,ZigBeeaddscapabilitiesthatproprietaryproductscant.Forexample,saysEmbersBahl,interoperabilityallowstheZigBeenodesofalightingsystemtoworkwiththeZigBeenetworkofanHVACsystem,orviceversa.PhilipsLightingisreallyexcitedaboutthis,Bahl,says,becauseitturnsthemfromaballastmanufacturerintotheinfrastructurebackboneofabuilding-automationsystem.Needlesstosay,manyofthemajorsemiconductorcompanies,andespeciallythosethatarebiginembeddedsystems,areeagerlyanticipatingZigBeesentryintomassmarkets.FreescaleSemiconductor(untilrecentlyknownasMotorolasSemiconductorProductsSector)isalreadyprovidingZigBee-readytechnologytoselectcustomers.Othersemiconductorcompanies,includingAMI,Atmel,Microchip,Philips,andRenesas,are7membersoftheZigBeeAlliance.ZigBeewilllikelybeslowtopenetratetheindustrialmarketforwirelesssensors,however.AccordingtomarketresearchfirmONWorld,itwilltakefivetosevenyearstoconvinceindustrialcustomersofthereliability,robustness,andsecurityofwireless-sensorsystems.ONWorlddoespredictsignificantlong-termgrowthofZigBeeinindustry,though.By2010,thecompanyprojects,RFmodulesusedinindustrialmonitoringandcontrolwillreach165millionunits,upfrom1.9millionin2004.About75%ofthose,ONWorldpredicts,willbebasedonZigBeeand802.15.4.Eventually,ZigBeecouldgointoawidevarietyofapplications.Inhouseholdappliances,itcouldhelpmonitorandcontrolenergyconsumption.Inautomotiveapplications,itcouldprovidetire-pressuremonitoringandremotekeylessentry.ZigBeecouldalsobeusedinmedicaldevicesorevenincomputerperipherals,suchaswirelesskeyboardsormice.Concernisincreasing,though,thatZigBeecouldturnintoaone-size-fits-alltechnologythatdoesntfitanyapplicationparticularlywell.Someskeptics,forexample,worrythatanattempttomakeZigBeeall-encompassingcouldmaketheZigBeeprotocolstacktoolargeforZigBeestwingoalsofverylowpowerconsumptionandverylowcost.Ifthathappens,thenZigBeeslow-power,low-data-ratenichenarrowasitiswillhaveproventobetoobroad.Andthen,perhaps,wellneedyetanotherwirelessstandardtogowiththeburgeoningnumberwealreadyhave.8外文资料翻译9ZigBee：无线技术，低功耗传感器网络工程师们在发掘无线传感器的潜在应用方面从未感到任何困难。例如，在家庭安全系统方面，无线传感器相对于有线传感器更易安装。而有线传感器装置布线的花费通常占无线传感器安装费用的80%的工业环境方面也是同样适用。而且在有线传感器的不适合甚至是不能使用的领域，无线传感器更具应用性。虽然，无线传感器需要消耗更多能量，也就是说所需电池的数量会随之增加或充电过于频繁。再加上对无线传感器经空气传送数据可靠性的怀疑，所以无线传感器看起来并不是那么吸引人。一个低功率无线技术被称为ZigBee，它是无线传感器技术的革新。但是，一个安全的网络技术，对最近通过的IEEE802.15.4无线标准（图1）的顶部游戏机，ZigBee承诺把无线传感器应用到从工厂自动化系统、家庭安全系统到消费电子产品的一切领域。与802.15.4的合作下，ZigBee使用的电池寿命可比普通小型电池的长几年。ZigBee设备的价格预计也比较便宜，有人估计销售价格最终不到3美元每节点。由于价格低，他们自然适应用在于如无线光线开关，无线自动调温器，烟雾探测器等家用产品。图1ZigBee在IEEE811.15.4规范定义的物理层和MAC层的基础上添加了网络/安全层和应用服务层虽然还没有正式的ZigBee规范存在（由于ZigBee联盟是一个贸易集团，规范应该在年底推出），但ZigBee的前景一片光明。技术研究公司In-Stat/MDR在它所谓的“谨慎进取”的预测中，预测802.15.4节点和芯片的销售量将从现在基本上为零，增加到2010年的165万个。不是所有这些节点都将与ZigBee结合，但大多数可能会。世界研究公司预测，到2010年射频模块无线传感器出货量达4.65亿，其中77是ZigBee相关的。从某种意义上说，ZigBee的光明前途在很大程度上是由于其较低的数据速率20kbps到250kbps，取决于使用的频率（图2），比1Mbps的蓝牙和54Mbps的802.11g的Wi-Fi的技术。但ZigBee不能像Wi-Fi那样发送电子邮件和大型文件，也不能像蓝牙那样发送文件和音频。对于发送传感器的数据，它只有几十字节数，高带宽10是没有必要的，ZigBee的低带宽有助于它实现低功耗，低成本和鲁棒性。图2ZigBee的数据传输速率为20kbps到250kbps，其取决于使用的频率大小由于ZigBee应用的是低带宽要求，ZigBee节点大部分时间可以采用睡眠模式，从而节省电池电源，然后醒来，快速发送数据，回到睡眠模式。而且，由于ZigBee可以用15毫秒或更少的时间从睡眠模式醒来，即使是睡眠节点也可以达到适当的低延迟。有人反对基于ZigBee的无线光开关，例如，将不会在灯亮起前知道唤醒延迟。与此相反，蓝牙唤醒延迟通常大约三秒钟。ZigBee的功耗节省中的很大一部分来自802.15.4无线电技术，它本身是为低功耗设计的。802.15.4采用DSSS（直接序列扩频）技术，例如，因为FHSS（跳频扩频）会消耗很多能量用于保持它的频率跳变同步。使用802.15.4的ZigBee节点，可以用几种不同的方式通信，然而，某些方式比其他的消耗更多的电能。因此，ZigBee的用户不一定能够实现他们想要的任何传感器网络，他们仍然期望ZigBee的特点多年的电池寿命。事实上，一些技术专家打算用小型无线传感器创建大的网络，即使ZigBee的电池满足不了他们的需求。一个ZigBee网络节点可以消耗额外的功率，例如，如果它试图避免与其他节点的传输或与其他无线电的传输产生冲突。那么采用802.15.4标准的ZigBee使用CSMA/CA（载波侦听多址接入冲突避免）技术，ZigBee节点使用CSMA/CA本质上采取的是监听先于通话的方式，看是否有无线电通信已经进行。但是，正如传感器营销公司恩贝尔公司副总裁兼ZigBee联盟的副主席VenkatBahl所指出的，这不是一个首选的方法。“监听消耗更多的电能”，Bahl说，“我们不喜欢这样做。”当面通信ZigBee和802.15.4通讯的另一个选择是同步模式，通常从睡眠模式醒来的网络从节点定期从网络的控制节点接收同步帧。但是，监听同步帧信号浪费电能，同时也因为时间的不确定性，强制节点提前打开，以免错过同步帧信号。为了尽可能节省电力，ZigBee采用一种“就绪时通话”的通信策略，当数据准备好后发送数据，然后等待自动确认。根据ZigBee联盟兼802.15协会主席BobHeile说，“就绪时通话”是一种“当面通信”的计划，但电力效率很高。“我们广泛地分析了在各种从安静到嘈杂的环境下最省电的策略，”Heile说，“我们发现，我们容易实现在发送数据包和承认它。如果你没收到确认信息，表示你发送失败了，所以进行重发。这种方式比监听并确认通话前是否安静的方式具有更好的电源管理性能。”幸运的是，这种当面策略导致的RF干扰非常小。这主要是因为ZigBee节点具有非常低的占空比，只是偶尔传输并且每次发送少量的数据。其他ZigBee节点，以及Wi-Fi和蓝牙模块，可以轻松应付这么小，不频繁的数据量。但是，ZigBee的就绪时通话计划并不适合所有的目的。例如，在战区中部署由成11千上万的微型传感器组成的传感器网络用于监视敌方部队的调动，提供的电能储备仍可能是不够的。每个网络节点周期性地发送，并通过网状网络中的其他邻近节点多次重发以使数据包到达网络控制器，大量的数据包碰撞和重发数据包会浪费电能，并显著缩短传感器节点的电池寿命。如果传感器电池量非常小并且功率有限，这特别成问题。虽然电波信道竞争对于ZigBee不是一个普遍的问题，但它有成为问题的可能。传感器网络公司Dust网络表示，竞争问题使该公司转向ZigBee，至少在现在，即使Dust公司仍是ZigBee联盟的成员。“每个ZigBee设备需要与相关领域争夺信道，”Dust公司产品管理总监RobertShear说，“所以有一些不可避免的竞争和低效率。”为了避免ZigBee信道的争夺，Dust公司使用免竞争的TDMA（时分多址）技术。ZigBee通过802.15.4的MAC层提保证时隙，有点类似于TDMA的时隙，但只是作为一个可选的“超帧”，比TDMA更复杂，更省电。但是ZigBee仍然有更省电的方式。例如，它在除了性能更好的全功能设备(FFDs)之外还提供了更省电的半功能设备(RFDs)。每个ZigBee网络至少需要一个全功能设备FFDs作为一个控制器，但大多数网络节点可以是半功能设备RFDs。RFDs只能和FFDs通信，而不能和其他RFDs通信，它们比FFDs电路简单，很少或没有功率消耗内存。图3ZigBee网络可在多种配置中包含多达65536个节点ZigBee还通过减少相关处理能力来节省电能。简单的8位处理器像8051可以容易地处理ZigBee的任务，并且ZigBee协议栈占用很少的内存。例如，一个全功能设备的协议栈，大概需要32字节，而一个半功能设备的协议栈只需要4字节。耗费250字节内存空间蓝牙技术与之相比显得复杂的多。由于ZigBee实现简单