User's Manual

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Chapter4 | WirelessSettings 
RadioSettings 
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WMMOperation—WMMusestrafficprioritybasedonthefourACs;Voice,  Video,
BestEffort,andBackground.ThehighertheACpriority,thehigherthe probability
thatdataistransmitted.
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Whentheaccesspointforwardstraffic,WMMaddsdatapacketstofour
independenttransmitqueues,oneforeachAC,dependingonthe802.1D priority
tagofthepacket.Datapacketswithoutaprioritytagarealwaysadded
 totheBest
EffortACqueue.Fromthefourqueues,aninternal“virtual”collision
 resolution
mechanismfirstselectsdatawiththehighestprioritytobegranteda transmit
opportunity.Thenthesamecollisionresolutionmechanismisused
 externallyto
determinewhichdevicehasaccesstothewirelessmedium.
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ForeachACqueue,thecollisionresolutionmechanismisdependentontwo 
timingparameters:
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◆AIFSN(ArbitrationInter‐FrameSpaceNumber),anumberusedto
calculate theminimumtimebetweendataframes
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◆CW(ContentionWindow),anumberusedtocalculatearandombackofftime
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Afteracollisiondetection,abackoffwaittimeiscalculated.Thetotalwaittime is
thesumofaminimumwaittime(ArbitrationInter‐FrameSpace,orAIFS)
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determinedfromtheAIFSN,andarandombackofftimecalculatedfromavalue 
selectedfromzerototheCW.TheCWvaluevarieswithinaconfigurablerange.  It
startsatCWMinanddoublesaftereverycollisionuptoamaximumvalue,
 CWMax.
Afterasuccessfultransmission,theCWvalueisresettoitsCWMin
 value.
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Figure30: WMMBackoffWaitTimes 
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Forhigh‐prioritytraffic,theAIFSNandCWvaluesaresmaller.Thesmallervalues 
equatetolessbackoffandwaittime,andthereforemoretransmit  opportunities.
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High Priority
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Low Priority
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Time
CWMin CWMax
A
IFS Random Backof
f
Minimum Wait Time
Random Wait Time
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CWMin CWMax
A
IFS
Random Backof
f
Minimum Wait Time Random Wait Time