Load Balancing | Voice Mobility with Wi-Fi

Load balancing is the ability for the network to steer or direct clients towards more lightly loaded access points and away from more heavily loaded ones. Client decides to which access point the client will connect. However, the network has the ability to gently influence or guide the client's decision.

First, let's recap what is meant by wireless load. The previous discussion on admission control first introduced the concept of counting airtime or calls. This is one measure of load—a real-time one. However, this counts only phones in active calls. There is likely to be far more phones not in active calls, and these should be balanced as well. The main reason for balancing inactive phones is that the network has little ability, once the phone starts a call, to transfer the phone to another access point without causing the call to fail going through. To avoid that, load balancing techniques attempt to establish a more even balance up front. The thinking goes that if you can get the phones evenly distributed when the connect to the network, then you have a better shot at having the calls they place equally distributed as well.

Mechanics of Load Balancing

Let's start with the basic mechanics of load balancing. Because the client chooses which access point to associate to, based on scanning operations, the only assured way to prevent a client from associating to an overloaded access point is for that overloaded access point to ignore the client. The access point can do this in a few ways. When the client sends probe requests, trying to discover whether the SSID it wants is still available on the access point, the access point can ignore the request, not sending out a probe response. Hopefully, the client will not enter on the basis of that alone. However, the client may have scanned before, when it could have (but chose not to) enter the access point, and may remember a prior probe response. Or, it can see the beacon, and so it knows that the access point is, in fact, providing the service in any event.

To prevent the client from associating, then, the access point has no choice but to ignore or reject Authentication and Association Request messages from the client. This will have the desired effect of preventing a burdensome load from ending up on the access point, but may not cause the client to choose the correct access point quickly.

Assuming, for the moment, that load balancing is effective in causing clients to distribute their load evenly, we need to look at what the consequences of balancing load are.