Enterasys RoamAbout RBT-4102 - Wi-Fi Multimedia (WMM) Configuration; WMM Operation

Radio Interface
RoamAbout RBT-4102 Wireless Access Point Configuration Guide 4-65
Wi-Fi Multimedia (WMM) Configuration
Wirelessnetworksofferanequalopportunityforalldevicestotransmitdatafromanytypeof
application.Althoughthisisacceptableformostapplications,multimediaapplications(with
audioandvideo)areparticularlysensitivetothedelayandthroughputvariationsthatresultfrom
this“equalopportunity”wirelessaccessmethod.Formultimedia ap plications
torunwellovera
wirelessnetwork,aQualityofService(QoS)mechanismisrequiredtoprioritizetraffictypesand
providean“enhancedopportunity”wirelessaccessmethod.
TheaccesspointimplementsQoSusingtheWi‐FiMultimedia(WMM)standard.UsingWMM,the
accesspointisabletoprioritizetrafficandoptimize
performancewhenmultipleapplications
competeforwirelessnetworkbandwidthatthesametime.WMMemploystechniquesthatarea
subsetofthedevelopingIEEE802.11eQoSstandardanditenablestheaccesspointtointeroperate
withbothWMM‐enabledclientsandotherdevicesthatmaylackanyWMMfunctionality.
WMM
definesfouraccesscategories(ACs):voice,video,besteffort,andbackground.These
categoriescorrespondtotrafficprioritylevelsandaremappedtoIEEE802.1Dprioritytags(refer
toTable 4‐6).ThedirectmappingofthefourACsto80 2.1Dprioritiesisspecificallyintendedto
facilitateinteroperabilitywithotherwired
networkQoSpolicies.WhilethefourACsarespecified
forspecifictypesoftraffic,WMMallowstheprioritylevelstobeconfiguredtomatchany
network‐wideQoSpolicy.WMMalsospecifiesaprotocolthataccesspointscanuseto
communicatetheconfiguredtrafficprioritylevelstoQoS‐enabledwireless
clients.
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.
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.
ForeachACqueue,thecollis ionresolutionmechanismisdependentontwotimingparameters:
•AIFSN(ArbitrationInter‐FrameSpaceNumber),anumberusedtocalculatetheminimum
timebetweendataframes
•CW(ContentionWindow),anumberusedtocalculatea
randombackofftime
Table 4-6 WMM Access Categories
Access
Category
WMM
Designation
Description
802.1D
Tags
AC_VO
(AC3)
Voice Highest priority, minimum delay. Time-sensitive data such
as VoIP (Voice over IP) calls.
7, 6
AC_VI
(AC2)
Video High priority, minimum delay. Time-sensitive data such as
streaming video.
5, 4
AC_BE
(AC0)
Best Effort Normal priority, medium delay and throughput. Data only
affected by long delays. Data from applications or devices
that lack QoS capabilities.
0, 3
AC_BK
(AC1)
Background Lowest priority. Data with no delay or throughput
requirements, such as bulk data transfers.
2, 1