When deploying a WLAN it’s easy to fall into the trap of enabling features you might not need, just because, well, you paid for them and they are cool. Often times a KISS approach results in better performance, but hey, look at this cool new thing it can do it!
Load balancing is one of those features. While it seems harmless enough, there are some scenarios that can get you into trouble.
For the uninitiated, WLAN load balancing is a feature that encourages clients to associate with the least-loaded nearby AP. Typically, a client will attempt to associate with the loudest nearby AP, without regard to how many clients are already associated (most AP’s don’t share that information anyway, but some do by using the BSS Load element within management frames). Most load balancing algorithms work by suppressing probe and association responses from heavily loaded AP’s so that a client either won’t know that it is there, or it will fail to associate with it. Hopefully the client will then attempt to associate with a different AP that has more capacity available to clients.
There are a couple problems with this to keep in mind. The most important problem is one the affects all clients. The AP has a very different view of the RF environment than the client does. What a highly sensitive, enterprise grade AP is capable of hearing is quite different from what a low-cost, consumer-grade Wi-Fi chipset can, and they are of course not listening from the same location either. It gets worse if that client radio is part of a smartphone tucked into a pocket or purse. In this example, the AP may think it’s safe to ignore probe and association requests from that client because it’s aware of three other nearby AP’s that are less-loaded, but the reality is that the smartphone can’t hear any AP but the one that is ignoring it.
And not all load balancing algorithms do what you think they do. Some operate by simply limiting the total number of associated clients an AP radio will accept, even some that are described as “airtime-based” from my experience. The problem here is that this doesn’t take into account the actual airtime utilization, the truest measure of the load on an AP radio. Often, the airtime utilization is quite low when an algorithm decides the AP is too loaded and should push clients elsewhere. Say 30 clients are associated to one of the AP’s radios. If they are all idle, there is still plenty of capacity for others to associate as well, as very little airtime is being used. Make sure you know exactly how your WLAN’s load balancing works. Test it to make sure it does what it claims it does, and set your limits high.
Here are some examples where load balancing can causes problems:
- A high school classroom fills with students. As they enter the room, their smartphones, which were already configured to join the WLAN, automatically roam to the loudest nearby AP. The teacher asks the students to get out their laptops as part of her lesson. The laptops now try to connect, but the nearby AP already has 30 smartphones associated to it, so it ignores the probe and association requests from the laptops. The best case scenario is that the laptops are able to associate with another nearby AP, albeit at a lower data rate than the louder AP. The worst case scenario is that client’s Wi-Fi radio drivers won’t budge, and continually fail to associate with the loudest AP (which is ignoring them), or the neighboring AP that the loaded-AP is trying to push new clients to is actually too distant for the new clients to hear. But the smartphones are all nearly idle, so it would have been better for the laptops to associate with the louder AP.
- The school media center is used to store several carts of iPads. The iPads are not powered-down before being stored, so they all associate with the media center AP. Visitors to the media center have difficulty connecting to the network in the media center, because the algorithm believes it is heavily loaded and ignores requests to associate to the media center AP. The media center AP can hear another AP well, but most visiting clients in the media center cannot. The visiting clients cannot connect to the WLAN, yet in this case as well, the AP is actually not loaded at all. The iPads are completely idle and using almost no airtime.
Here is the where load balancing makes sense:
- In areas where you can reasonably anticipate that a single AP radio may become overloaded, such as a cafeteria, gym, or performance space.
- In areas where multiple AP’s are very close to one and other and create tightly overlapping coverage cells. This helps mitigate the problem of clients and AP’s having a differing view of the RF.
- Nowhere else. Only use load-balancing when both of the above criteria are met.