Wi-Fi Multimedia (WMM) Power Save

To provide power saving while the mobile device is in a call, the Wi-Fi Alliance came up with the second power saving technique, WMM Power Save. This technique, based on the quality-of-service additions in the 802.11e amendment to the standard, acts as a parallel scheme to the legacy one, using similar concepts but in a way that avoids having to wait for beacons and can apply on a per-access-category basis.

If you notice, there is nothing in the standard that prevents clients that are using the legacy power save scheme from ignoring beacons, for the most part, and sending PS Polls whenever they want. If the client were sure that there is going to be a packet for it waiting every so often—say, 20 milliseconds—then it could just send PS Polls every 20 milliseconds, collect its data, and have real-time power save. Of course, this doesn't happen for legacy power save, because the client has no guarantee that it won't get some other frames rather than what it is looking for. However, this is the concept that WMM Power Save builds on.

WMM Power Save is optional, and support for it is signaled by the WMM information elements in the Association messages and the beacons. Unlike with legacy power save, WMM Power Save (capitalized, as it is a formal name) is aware of the WMM access categories and can apply to a subset of them. The two subsets are delivery-enabledaccess categories and trigger-enabled access categories.

First, let's start with the polling protocol. The client no longer checks the beacons to see if there is traffic. Instead, it is responsible for knowing that traffic is waiting for it, and how often. For phones, this is not a problem, as voice is bidirectional and consistent. Instead of sending a PS Poll frame, or using the PSNonPoll mechanism, the phone sends data frames in access categories that it has specified to be trigger-enabled. The access point looks for those data frames, and uses that as a trigger—just as it does in legacy with Power Save Poll frames—sending packets in response from the power save buffer. Those packets, however, can only come from the delivery-enabled access categories. Which categories are delivery- and trigger-enabled are usually specified in the Association Request from the client—there, a bitmask specifies which categories are legacy and which are delivery and trigger enabled together—or in TSPEC messages, which we will come to later.

Here's a common example. The phone associates, and tells the access point that it wants the voice category (AC_VO) to be delivery- and trigger-enabled. That means that the other three categories work on the legacy scheme. If packets come in for those other categories while the client is asleep, the TIM bit on the beacon will be set and the client will use legacy power save mechanisms to get the frames. But when a voice packet is sent to the access point, the access point silently holds onto the packet. The only way the client can get the voice packet is to send a voice packet of its own.

When it does, that causes the access point to respond with one or more voice packets in its buffer. Unlike with legacy power save, the client can ask for more than one packet at a time. Using the concept of a service period, which is set at Association time by the client and specifies the number of frames the client wants to get for every trigger (either two, four, six, or all), the access point will send out the correct number of frames. The last frame, whether because the buffer is empty or the service period has been exceeded, will have a special end of service period (EOSP) bit set in the QoS header. Once the client gets that frame, it can go back to sleep.

As you can see, the legacy and WMM Power Save schemes operate simultaneously and independently. The only overlap is that the client goes into to power save mode for both schemes simultaneously. This means that devices that are actively using WMM Power Save should never use the PSNonPoll method during that time, because the client waking up from power save mode will cause the access point to send all frames, whether they are from the legacy or WMM Power Save access categories.

The capability to support WMM Power Save should be considered nearly mandatory for most voice equipment. Some mobile devices use proprietary mechanisms that may or may not be supported by every access point, but the trend is towards using WMM Power Save. 

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.