Voice Mobility with Wi-Fi Capacity

How the Capacity is Determined

Through either admission control scheme, the network needs to keep track of how much capacity is available. From the previous discussions on the effects of RF variability and cellular overlap, you can appreciate that this is a difficult problem to completely solve. As devices get further away from the access points, data rates drop. Changing levels of interference, from within the network or without, can cause increasing retransmissions and easily overrun surplus bandwidth allowances.

In the end, networks today adopt one of two stands, and may even show both to the user. The more complicated stand for the network—but simpler for the user—is for the network to automatically take the variability of RF into account, and to determine its own capacities. In systems that do this, there is no notion of a static maximum number of calls. Instead, the system accepts however many calls as it can handle. If conditions change, and fewer calls can be handled in the system, the network reserves the right to proactively end a client's reservation, often in concert with load balancing.

The other stand, simpler for the network but far more complicated for the user, is for the administrator to be required to enter the maximum number of calls per access point (or some other static metric). The idea here is that the administrator or installer is assumed to have gone through a planning process to determine how many calls can besafely allowed per access point, while still leaving room for best effort data. That number is usually far lower than the best-case maximum capacity, and is designed to be a low water mark: barring external changes, the network will be able to achieve that many calls most of the time. This number is then manually input into the wireless network, which then counts the number of calls. If the maximum number of calls is reached on that access point, the system will not let any more in. These static metrics may be entered either as the number of calls, or a percentage of airtime. Systems that work as a percentage of airtime can sometimes take in a padding factor to allow for calls that are roaming into the network.

Setting these values can be fraught with difficulty. Pick a number that's too low, and airtime is being wasted. Pick a number that's too high, however, and sometimes call quality will suffer. Even percentage of airtime calculations are not very good, because they may not take into account airtime that is unusable because of variable channel conditions or co-channel interference that the access point cannot directly see, such as client-to-client interference 

All in all, you might find vendors recommending setting the values to a low, safe value that allows for voice to work even if there is plenty of variability in the network. This works well for networks that are predominantly data-oriented, but voice-only networks cannot usually afford that luxury.