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1、ZigBeeReliabilityandSecurityWhatWillILearninthisModule?Oncompletingthismodule,youwillbeabletodescribe:·reliabilitymeasuresemployedtoensurethatZigBeemessagesarecommunicatedwithoutcorruptionorlossofdata·securitymeasuresemployedtopreventhostileaccesstoZigBeecommunications
2、ReliabilityInawirelessnetwork,itisimportanttoensurereliablecommunications-thatis,tomakesurecommunicationsreachtheirdestinationsuncorrupted.Corruptionorlossofmessagescouldresult,forexample,fromradiointerferenceorfrompoortransmission/receptionconditions.Thereliabilitymea
3、suresemployedbyZigBeeallowawirelessnetworktooperateinaprotectedenvironment,evenwhenthereareotherZigBeenetworksnearbyoperatinginthesamefrequencyband.AdjacentZigBeenetworkswillnotinterferewitheachother.Inaddition,ZigBeenetworkscanoperateintheneighbourhoodofnetworksbasedo
4、notherstandards,suchasWi-FiandBluetooth.ThetechniquesemployedbyZigBeetoensurereliablecommunicationsaredescribedonthenextpage.ReliabilityMeasuresZigBeeoffersarangeoftechniquestoensurereliablecommunications.Thesearedescribedbelow.ListenBeforeSendThetransmissionschemeused
5、inZigBeeavoidstransmittingdatawhenthereisactivityonthechosenchannel–thisisknownasCarrierSense,MultipleAccesswithCollisionAvoidance(CSMA-CA).Putsimply,thismeansthatbeforebeginningatransmission,anodelistensonthechanneltocheckwhetheritisclear.Ifactivityisdetectedonthechan
6、nel,thenodedelaysthetransmissionforarandomamountoftimeandlistensagain.Ifthechannelisnowclear,thetransmissioncanbegin,otherwisethedelay-and-listencycleisrepeated.AcknowledgementsAnacknowledgementmechanismisbuiltintoZigBeetoensurethatmessagesreachtheirdestinations.Whenam
7、essagearrivesatitsdestination,thereceivingdevicesendsanacknowledgementtosaythemessagehasbeenreceived.Ifthesendingdevicedoesnotreceiveanacknowledgementwithinacertaintimeinterval,itresendstheoriginalmessage(itcanresendthemessageseveraltimesuntilthemessagehasbeenacknowled
8、ged).AlternativeRoutesInaMeshtopology,thenetworkhasbuilt-inintelligencetoensurethatmessagesreachtheirdestinations.Ift
