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Wireless Features



Does Wi-Fi MultiMedia (WMM) Really Do Anything?

You probably have seen references to WMM (Wi-Fi Multimedia) in the specs and feature descriptions of many wireless LAN products. Typically, they say something like "Support for WMM provides improved QoS over wireless connections for better video and voice performance".

But have you ever seen "better video and voice performance" from using WMM? Or even known if you were using it? Well, if your answer is "No", then we're in the same boat. However, I'm finally going to try to answer the question posed up there at the top of the page. But first, a little background.

WMM Basics

WMM is a subset of IEEE 802.11e (MAC Enhancements for Quality of Service) that came into being in 2004 as 11e was slowly wending its way to becoming a standard (which happened in mid-2005). The Wi-Fi Alliance took a subset of 11e—more accurately a profile of 11e—created a Certification test suite and called it Wi-Fi Multimedia or WMM.

The Alliance's Wi-Fi CERTIFIED for WMM white paper presents a good overview of the topic and I have drawn most of the following explanation from it. It also contains this important starting point for understanding WMM.

The WMM Checklist

To take advantage of WWM functionality in a Wi-Fi network, three requirements have to be met:
(1) the access point is Wi-Fi CERTIFIED for WMM and has WMM enabled;
(2) the client (device) that the application is running on must be Wi-Fi CERTIFIED for WMM; and
(3) the source application supports WMM. [emphasis mine]

As indicated by the bold italics, it turns out that the third point is the weak link in the path to all the QoS goodness promised by WMM. But let's come back to that later.

According to the white paper, WMM defines four access categories (ACs) derived from 802.1d, which correspond to priority levels (Table 1). The 802.1d tags are also used by 802.1p.

WMM Access Categories

Table 1: WMM Access Categories
from Wi-Fi CERTIFIED for WMM

WMM specifies a protocol used by the AP to communicate the policy to QoS-enabled clients and by the clients to send transmit requests. WMM does not, in itself set the priority policy;that is the job of application or device that is sending the data.

The white paper contains a handy visual aid (Figure 1) to illustrate how three properly-prioritized data streams should behave with and without WMM. These throughput vs. time plots should be familar to regular SNB readers. They are similar to the IxChariot plots we use to show wireless product throughput stability and other performance parameters.

WMM in action

Figure 1: WMM in action

The top plot in Figure 1 shows WMM maintaining a smooth 10 Mbps rate for a video stream when data streams with lower priority cause total bandwidth to be exceeded. The Video stream is left with as much bandwidth as it needs, while the lower-priority streams are slowed to provide the required bandwidth.

The lower plot, which shows what happens when WMM is not in effect, shows all streams dropping back in speed when the third stream again causes available bandwidth to be exceeded.

The Wi-Fi white paper goes into a bit more detail on the mechanics of how all this is accomplished. But the explanation above is enough for us to know what we should be looking for to see if WMM is working.

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