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1ZigbeeWirelessSensorNetworkinEnvironmentalMonitoringApplicationsI.ZIGBEETECHNOLOGYZigbeeisawirelessstandardbasedonIEEE802.15.4thatwasdevelopedtoaddresstheuniqueneedsofmostwirelesssensingandcontrolapplications.Technologyislowcost,lowpower,alowdatarate,highlyreliable,highlysecurewirelessnetworkingprotocoltargetedtowardsautomationandremotecontrolapplications.It’sdepictstwokeyperformancecharacteristics–wirelessradiorangeanddatatransmissionrateofthewirelessspectrum.ComparingtootherwirelessnetworkingprotocolssuchasBluetooth,Wi-Fi,UWBandsoon,showsexcellenttransmissionabilityinlowertransmissionrateandhighlycapacityofnetwork.A.ZigbeeFrameworkFrameworkismadeupofasetofblockscalledlayers.Eachlayerperformsaspecificsetofservicesforthelayerabove.AsshowninFig.1.TheIEEE802.15.4standarddefinesthetwolowerlayers:thephysical(PHY)layerandthemediumaccesscontrol(MAC)layer.TheAlliancebuildsonthisfoundationbyprovidingthenetworkandsecuritylayerandtheframeworkfortheapplicationlayer.Fig.1FrameworkTheIEEE802.15.4hastwoPHYlayersthatoperateintwoseparatefrequencyranges:868/915MHzand2.4GHz.Moreover,MACsub-layercontrolsaccesstotheradiochannelusingaCSMA-CAmechanism.Itsresponsibilitiesmayalsoincludetransmittingbeaconframes,synchronization,andprovidingareliabletransmissionmechanism.B.Zigbee’sTopologyThenetworklayersupportsstar,tree,andmeshtopologies,asshowninFig.2.Inastartopology,thenetworkiscontrolledbyonesingledevicecalledcoordinator.Thecoordinator2isresponsibleforinitiatingandmaintainingthedevicesonthenetwork.Allotherdevices,knownasenddevices,directlycommunicatewiththecoordinator.Inmeshandtreetopologies,thecoordinatorisresponsibleforstartingthenetworkandforchoosingcertainkeynetworkparameters,butthenetworkmaybeextendedthroughtheuseofrouters.Intreenetworks,routersmovedataandcontrolmessagesthroughthenetworkusingahierarchicalroutingstrategy.Meshnetworksallowfullpeer-to-peercommunication.Fig.2MeshtopologiesFig.3isanetworkmodel,itshowsthatsupportsbothsingle-hopstartopologyconstructedwithonecoordinatorinthecenterandtheenddevices,andmeshtopology.Inthenetwork,theintelligentnodesarecomposedbyFullFunctionDevice(FFD)andReducedFunctionDevice(RFD).OnlytheFFNdefinesthefullfunctionalityandcanbecomeanetworkcoordinator.Coordinatormanagesthenetwork,itistosaythatcoordinatorcanstartanetworkandallowotherdevicestojoinorleaveit.Moreover,itcanprovidebindingandaddress-tableservices,andsavemessagesuntiltheycanbedelivered.Fig.3Zigbeenetworkmodel3II.THEGREENHOUSEENVIRONMENTALMONITORINGSYSTEMDESIGNTraditionalagricultureonlyusemachineryandequipmentwhichisolatingandnocommunicatingability.Andfarmershavetomonitorcrops’growthbythemselves.Evenifsomepeopleuseelectricaldevices,butmostofthemwererestrictedtosimplecommunicationbetweencontrolcomputerandenddeviceslikesensorsinsteadofwireconnection,whichcouldn’tbestrictlydefinedaswirelesssensornetwork.Therefore,bythroughusingsensornetworksand,agriculturecouldbecomemoreautomation,morenetworkingandsmarter.Inthisproject,weshoulddeployfivekindsofsensorsinthegreenhousebasement.Bythroughthesedeployedsensors,theparameterssuchastemperatureinthegreenhouse,soiltemperature,dewpoint,humidityandlightintensitycanbedetectedrealtime.Itiskeytocollectdifferentparametersfromallkindsofsensors.Andinthegreenhouse,monitoringthevegetablesgrowingconditionsisthetopissue.Therefore,longerbatterylifeandlowerdatarateandlesscomplexityareveryimportant.Fromtheintroductionaboutabove,weknowthatmeettherequirementsforreliability,security,lowcostsandlowpower.A.SystemOverviewTheoverviewofGreenhouseenvironmentalmonitoringsystem,whichismadeupbyonesinknode(coordinator),manysensornodes,workstationanddatabase.Motenodeandsensornodetogethercomposedofeachcollectingnode.Whensensorscollectparametersrealtime,suchastemperatureinthegreenhouse,soiltemperature,dewpoint,humidityandlightintensity,thesedatawillbeofferedtoA/Dconverter,thenbythroughquantizingandencodingbecomethedigitalsignalthatisabletotransmitbywirelesssensorcommunicatingnode.Eachwirelesssensorcommunicatingnodehasabilityoftransmitting,receivingfunction.InthisWSN,sensornodesdeployedinthegreenhouse,whichcancollectrealtimedataandtransmitdatatosinknode(Coordinator)bythewayofmulti-hop.Sinknodecompletethetaskofdataanalysisanddatastorage.Meanwhile,sinknodeisconnectedwithGPRS/CDMAcanprovideremotecontrolanddatadownloadservice.Inthemonitoringandcontrollingroom,byrunninggreenhousemanagementsoftware,thesinknodecanperiodicallyreceivesthedatafromthewirelesssensornodesanddisplaysthemonmonitors.4B.NodeHardwareDesignSensornodesarethebasicunitsofWSN.Thehardwareplatformismadeupsensornodescloselyrelatedtothespecificapplicationrequirements.Therefore,themostimportantworkisthenodesdesignwhichcanperfectimplementthefunctionofdetectingandtransmissionasaWSNnode,andperformitstechnologycharacteristics.Fig.4showstheuniversalstructureoftheWSNnodes.Powermoduleprovidesthenecessaryenergyforthesensornodes.Datacollectionmoduleisusedtoreceiveandconvertsignalsofsensors.Dataprocessingandcontrolmodule’sfunctionsarenodedevicecontrol,taskscheduling,andenergycomputi