E-DOC-CTC-20060609-0001 v2.0
Chapter 4
Wi-Fi Multi Media (WMM)
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4 Wi-Fi Multi Media (WMM)
Introduction
Applications running on wireless devices have equal access rights to transmit data frames. That works well
for data traffic from applications such as Web browsers, file transfers and e-mail. For multimedia
applications, however, this is inadequate. Voice over IP, video streaming, and interactive gaming are highly
sensitive to latency increases and throughput reductions. These applications require Quality of Service (QoS)
functionality. QoS enables Wi-Fi access points to prioritize traffic and optimizes the way shared network
resources are allocated among different applications.
WMM
Wi-Fi Multi Media (WMM), or Wireless Multimedia Extensions (WME), is a Wi-Fi Alliance interpretability
certification, based on the IEEE 802.11e draft standard. It provides basic QoS features to IEEE 802.11
networks.
WMM prioritizes traffic demands from different applications. WMM defines four access categories (voice,
video, best effort, and background) that are used to prioritize traffic so that these applications have access to
the necessary network resources.
To employ WMM functionality in a WLAN, three requirements have to be met:
> The access point is Wi-Fi Certified for WMM and is WMM enabled.
> The wireless station (device) on which the application is running must be Wi-Fi Certified for WMM.
> The source application supports WMM.
WMM access categories
WMM defines four access categories (AC), corresponding to priority levels. While the four ACs were designed
with specific types of traffic (voice, video, best effort, low priority data) and associated priorities in mind,
WMM leaves the network owner free to choose the most appropriate network-wide policy and to decide
which ACs have priority. For instance, a network owner may prefer to give priority to video streaming over
voice. A customized policy for the ACs can be set through an interface in which default priority levels for ACs
can be modified. WMM specifies a protocol used by the access point to communicate the policy to QoS-
enabled wireless stations and by the stations to send transmit requests.
Access Category Description
Voice Highest priority.
Allows multiple concurrent VoIP calls, with low latency and local-call voice
quality.
Video Prioritize video traffic above other data traffic.
Best Effort Traffic from legacy devices, or traffic from applications or devices that lack QoS
capabilities.
Traffic less sensitive to latency, but affected by long delays, such as Internet
surfing.
Background Low priority traffic (file downloads, print jobs) that does not have strict latency
and throughput requirements.
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