Users Guide

Dell PowerConnect ArubaOS 5.0 | User Guide Voice and Video | 565
Dynamic WMM Queue Management
Traditional wireless networks provide all clients with equal bandwidth access. However, delays or reductions in
throughput can adversely affect voice and video applications, resulting in disrupted VoIP conversations or
dropped frames in a streamed video. Thus, data streams that require strict latency and throughput need to be
assigned higher traffic priority than other traffic types.
The Wi-Fi Alliance defined the Wi-Fi Multimedia (WMM) standard in response to industry requirements for
Quality of Service (QoS) support for multimedia applications for wireless networks. WMM anticipates the
ratification of the IEEE 802.11e standard that is currently in development.
WMM requires:
z The access point is Wi-Fi Certified and has WMM enabled
z The client device is Wi-Fi Certified
z The application supports WMM
Enhanced Distributed Channel Access
WMM provides media access prioritization through Enhanced Distributed Channel Access (EDCA). EDCA
defines four access categories (ACs) to prioritize traffic: voice, video, best effort, and background. These ACs
correspond to 802.1d priority tags, as shown in Table 124 on page 565.
While the WMM ACs designate specific types of traffic, you can determine the priority of the ACs. For example,
you can choose to give video traffic the highest priority. With WMM, applications assign data packets to an AC.
In the client, the data packets are then added to one of the transmit queues for voice, video, best effort, or
background.
WMM is an extension to the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) protocol’s
Distributed Coordination Function (DCF). The collision resolution algorithm responsible for traffic
prioritization depends on the following configurable parameters for each AC:
z arbitrary inter-frame space number (AIFSN)
z minimum and maximum contention window (CW) size
For each AC, the backoff time is the sum of the AIFSN and a random value between 0 and the CW value. The
AC with the lowest backoff time is granted the opportunity to transmit (TXOP). Frames with the highest-priority
AC are more likely to get TXOP as they tend to have the lowest backoff times (a result of having smaller AIFSN
and CW parameter values). The value of the CW varies through time as the CW doubles after each collision up
to the maximum CW. The CW is reset to the minimum value after successful transmission. In addition, you can
configure the TXOP duration for each AC.
On the controller, you configure the AC priorities in the WLAN EDCA parameters profile. There are two sets of
EDCA profiles you can configure:
z AP parameters affect traffic from the AP to the client.
z STA parameters affect traffic from the client to the AP.
Table 124 WMM Access Categories and 802.1d Tags
WMM Access Category Description 802.1d Tag
Voice Highest priority 7, 6
Video Prioritize video traffic above other data traffic 5, 4
Best Effort Traffic from legacy devices or traffic from applications or devices that do
not support QoS
0, 3
Background Low priority traffic (file downloads, print jobs) 2, 1