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电子信息工程

班级：

.

学生姓名：

导师姓名：

.职称：

.

起止时间：

2013年3月4日——2013年6月14日

ZigBee:

WirelessTechnologyforLow-PowerSensorNetworks

GaryLegg

5/6/200412:

00AMEDT

Technologistshaveneverhadtroublecomingupwithpotentialapplicationsforwirelesssensors.Inahomesecuritysystem,forexample,wirelesssensorswouldbemucheasiertoinstallthansensorsthatneedwiring.Thesameistrueinindustrialenvironments,wherewiringtypicallyaccountsfor80%ofthecostofsensorinstallations.Andthenthereareapplicationsforsensorswherewiringisn'

tpracticalorevenpossible.

Theproblem,though,isthatmostwirelesssensorsusetoomuchpower,whichmeansthattheirbatterieseitherhavetobeverylargeorgetchangedfartoooften.Addtothatsomeskepticismaboutthereliabilityofsensordatathat'

ssentthroughtheair,andwirelesssensorssimplyhaven'

tlookedveryappealing.

Alow-powerwirelesstechnologycalledZigBeeisrewritingthewirelesssensorequation,however.AsecurenetworktechnologythatridesontopoftherecentlyratifiedIEEE802.15.4radiostandard（Figure1）,ZigBeepromisestoputwirelesssensorsineverythingfromfactoryautomationsystemstohomesecuritysystemstoconsumerelectronics.Inconjunctionwith802.15.4,ZigBeeoffersbatterylifeofuptoseveralyearsforcommonsmallbatteries.ZigBeedevicesarealsoexpectedtobecheap,eventuallysellingforlessthan$3pernodebysomeestimates.Withpricesthatlow,theyshouldbeanaturalfiteveninhouseholdproductslikewirelesslightswitches,wirelessthermostats,andsmokedetectors.

Figure1:

ZigBeeaddsnetwork,security,andapplication-serviceslayerstothePHYandMAClayersoftheIEEE811.15.4radio

AlthoughnoformalspecificationforZigBeeyetexists（approvalbytheZigBeeAlliance,atradegroup,shouldcomelatethisyear）,theoutlookforZigBeeappearsbright.TechnologyresearchfirmIn-Stat/MDR,inwhatitcallsa"

cautiousaggressive"

forecast,predictsthatsalesof802.15.4nodesandchipsetswillincreasefromessentiallyzerotodayto165millionunitsby2010.NotalloftheseunitswillbecoupledwithZigBee,butmostprobablywillbe.ResearchfirmONWorldpredictsshipmentsof465millionwirelesssensorRFmodulesby2010,with77%ofthembeingZigBee-related.

Inasense,ZigBee'

sbrightfutureislargelyduetoitslowdatarates—20kbpsto250kbps,dependingonthefrequencybandused（Figure2）—comparedtoanominal1MbpsforBluetoothand54MbpsforWi-Fi'

s802.11gtechnology.ButZigBeewon'

tbesendingemailandlargedocuments,asWi-Fidoes,ordocumentsandaudio,asBluetoothdoes.Forsendingsensorreadings,whicharetypicallyafewtensofbytes,highbandwidthisn'

tnecessary,andZigBee'

slowbandwidthhelpsitfulfillitsgoalsoflowpower,lowcost,androbustness.

Figure2:

ZigBee'

sdataratesrangefrom20kbpsto250kbps,dependingonthefrequencyused

BecauseofZigBeeapplications'

lowbandwidthrequirements,aZigBeenodecansleepmostofthetime,thussavingbatterypower,andthenwakeup,senddataquickly,andgobacktosleep.And,becauseZigBeecantransitionfromsleepmodetoactivemodein15msecorless,evenasleepingnodecanachievesuitablylowlatency.SomeoneflippingaZigBee-enabledwirelesslightswitch,forexample,wouldnotbeawareofawake-updelaybeforethelightturnson.Incontrast,wake-updelaysforBluetootharetypicallyaroundthreeseconds.

AbigpartofZigBee'

spowersavingscomefromtheradiotechnologyof802.15.4,whichitselfwasdesignedforlowpower.802.15.4usesDSSS（direct-sequencespreadspectrum）technology,forexample,becausethealternativeFHSS（frequency-hoppingspreadspectrum）wouldhaveusedtoomuchpowerjustinkeepingitsfrequencyhopssynchronized.

ZigBeenodes,using802.15.4,cancommunicateinanyofseveraldifferentways,however,andsomewaysusemorepowerthanothers.Consequently,ZigBeeuserscan'

tnecessarilyimplementasensornetworkanywaytheychooseandstillexpectthemultiple-yearbatterylifethatisZigBee'

shallmark.Infact,sometechnologistswhoareplanningverylargenetworksofverysmallwirelesssensorssaythatevenZigBeeistoopowerhungryfortheiruses.

AZigBeenetworknodecanconsumeextrapower,forexample,ifittriestokeepitstransmissionsfromoverlappingwithothernodes'

transmissionsorwithtransmissionsfromotherradiosources.The802.15.4radiousedbyZigBeeimplementsCSMA/CA（carriersensemultipleaccesscollisionavoidance）technology,andaZigBeenodethatusesCSMA/CAisessentiallytakingalisten-before-talkapproachtoseeifanyradiotrafficisalreadyunderway.But,asnotedbyVenkatBahl,marketingvicepresidentforsensorcompanyEmberCorp.andvicechairmanoftheZigBeeAlliance,that'

snotapreferredapproach."

Havingtolistenburnspower,"

saysBahl,"

andwedon'

tliketodothat."

AnotherZigBeeand802.15.4communicationsoptionisthebeaconmode,inwhichnormallysleepingnetworkslavenodeswakeupperiodicallytoreceiveasynchronizing"

beacon"

fromthenetwork'

scontrolnode.Butlisteningforabeaconwastespower,too,particularlybecausetiminguncertaintiesforcenodestoturnonearlytoavoidmissingabeacon.

In-Your-FaceCommunication

Tosaveasmuchpoweraspossible,ZigBeeemploysatalk-when-readycommunicationstrategy,simplysendingdatawhenithasdatareadytosendandthenwaitingforanautomaticacknowledgement.AccordingtoBobHeile,whoischairmanofboththeZigBeeAllianceandIEEE802.15,talk-when-readyisan"

in-your-face"

scheme,butonethat'

sverypowerefficient."

Wedidanextensiveanalysisthatledtothebestpower-savingstrategyinvariouskindsofenvironmentsfromquiettonoisy,"

Heilesays."

Wediscoveredthat,handsdown,wewerebetteroffjustsendingthepacketandacknowledgingit.Ifyoudon'

tgetanack,itjustmeansyougotclobbered,sosenditagain.Youwinduphavingmuchbetterpowermanagementthanifyoulistenanddetermineifit'

squietbeforeyoutalk."

Fortunately,thisin-your-facestrategyleadstoverylittleRFinterference.That'

slargelybecauseZigBeenodeshaveverylowdutycycles,transmittingonlyoccasionallyandsendingonlysmallamountsofdata.OtherZigBeenodes,aswellasWi-FiandBluetoothmodules,caneasilydealwithsuchsmall,infrequentbursts.

stalk-when-readyschemedoesn'

tsuitallpurposes,however.Forexample,inanetworkofthousandsoftinysensorsdroppedintoawarzonetomonitorenemytroopmovements,thepowersavingsprovidedstillmightnotbeenough.Witheachnetworknodesendingdataperiodically—andwithtransmissionsrepeatednumeroustimesthroughothernearbynodesofameshnetworkconfigurationinordertoreachanetworkcontroller—largenumbersofpacketcollisionsandretransmissionscouldwastepowerandsignificantlyshortensensornodebatterylife.Ifthesensorbatteriesareverysmallandpower-limited,that'

sespeciallyproblematic.

Althoughcontentionforairwaveaccessisn'

tgenerallyaproblemforZigBee,itcanbe.Sensor-networkcompanyDustNetworks,infact,sayscontentionissuesarekeepingthecompanyfromturningtoZigBee—fornow,atleast—eventhoughDustremainsamemberoftheZigBeeAlliance."

EachZigBeedeviceneedstocontendforairspacewithitsneighbors,"

saysDustdirectorofproductmanagementRobertShear,"

sothere'

sinevitablysomecontentionandsomeinefficiency."

ToavoidZigBee'

saccesscontention,Dustusescontention-freeTDMA（timedivisionmultipleaccess）technology.ZigBee,throughthe802.15.4MAClayer,providesguaranteedtimeslotsinaschemethatsomewhatresemblesTDMA,butonlyaspartofanoptional"

superframe"

that'

smorecomplexandlesspower-efficientthanTDMA.

ZigBeehasstillmorepower-savingtricksupitssleeve,however.Forexample,itreducespowerconsumptioninZigBeecomponentsbyprovidingforpower-savingreduced-functiondevices（RFDs）inadditiontomorecapablefull-functiondevices（FFDs）.EachZigBeenetworkneedsatleastoneFFDasacontroller,butmostnetworknodescanbeRFDs（Figure3）.RFDscantalkonlywithFFDs,nottootherRFDs,buttheycontainlesscircuitrythanFFDs,andlittleornopower-consumingmemory.

Figure3:

ZigBeenetworkscancontainasmanyas65,536nodesinavarietyofconfigurations

ZigBeeconservesstillmorepowerbyreducingtheneedforassociatedprocessing.Simple8-bitprocessorslikean8051canhandleZigBeechoreseasily,andZigBeeprotocolstacksoccupyverylittlememory.AnFFDstack,forexample,needsabout32kbytes,andanRFDstackneedsonlyabout4kbytes.Thosenumberscomparewithabout250kbytesforthefarmorecomplexBluetoothtechnology.

FromZigBee'

srelativelysimpleimplementations,costsavingsnaturallyaccrue.RFDs,ofcourse,reduceZigBeecomponentcostsbyomittingmemoryandothercircuitry,andsimple8-bitprocessorsandsmallprotocolstackshelpkeepsystemcostsdown.Often,anapplication'

smainprocessorcaneasilybearthesmalladditionalloadofZigBeeprocessing,makingaseparateprocessorforZigBeefunctionsunnecessary.

ButthemainstrategyforkeepingZigBeepriceslowistohavebigmarketsandhighvolumes.TheZigBeeAlliance,bymakingZigBeeanopenstandardandbyvigorouslypromotinginteroperabilityamongZigBeedevices,expectsthatZigBeewillbeverybiginapplicationssuchashomeandbuildingautomation.Theallianceiscurrentlyworkingoninteroperabilityproceduresforthoseparticularapplications,whichitexpectstocompletelaterthisyearalongwithZigBeeSpecification1.0.

OnereasonforoptimismaboutZigBeeadoptionfor