How Wi-Fi Multimedia (WMM) Works?

It is not easy to directly see what the consequences are by WMM creating multiple queues that act to access the air independently. But it is important to understand what makes WMM works, to understand how WMM—and thus, voice—scales in the network.

Looking at the common WMM parameters, we can see that the main way that WMM provides priority for voice is by letting voice use a faster backoff process than data. The shorter AIFS helps, by giving voice a small chance of transmitting before data even gets a chance, but the main mechanism is by allowing voice transmit, on average, with a quarter of the waiting time that best effort data has.

This mechanism works quite well when there is a small amount of voice traffic on a network with a potentially large amount of data. As long as voice traffic is scarce, any given voice packet is much more likely to get on the air as soon as it is ready, causing data to build up as a lower priority. This is one of the consequences of having different queues for traffic. As an analogy, picture the security lines at airports. Busy airports usually have two separate lines, one line for the average traveler, and another line for first-class passengers and those who fly enough to gain "elite" status on the airlines. When the line for the average traveler—the "best effort" line—is full of people, a short line for first class passengers gives those passengers a real advantage. In other words, we can think of best effort and voice as mostly independent. The problem, then, is if there are too many first-class passengers. For WMM, the problem happens when there is "too much" voice traffic. Unlike with the children of Lake Wobegone, not everyone can be above average.

Let's look at this more methodically. The backoff value is the primary mechanism that Wi-Fi is affected by density. As the number of clients increases, the chance of collision increases. Unfortunately, WMM provides for quality of service by reducing the number of slots of the backoff, thus making the network more sensitive to density. Again, if voice is rare, then its own density is low, and so a voice packet is not likely to collide with other voice packets, and the aggressive backoff settings for voice, compared to data, allow for voice to get on the network with higher probability. However, when the density of voice goes up, the aggressive voice backoff settings cause each voice packet to fight with the other voice packets, leading to more collisions and higher loss.

One solution for this problem is to limit the number of voice calls in a cell, thus ensuring that the density of voice never gets that high. This is called admission control. Another and an independent solution is for the system to provide a more deterministic quality of service, by intelligently setting the WMM parametersaway from the defaults. This exact purpose is envisioned by the standard, but most equipment today expects the user to hand-tune these values, something which is not easy.