VoIP calls don't necessarily require a lot of bandwidth -- for example, 90 Kbps may yield a very good call, but...
a lower quality call could get by with a fraction of that bandwidth. Here are some links that you can use to estimate VoIP call bandwidth requirements, based on these and other input variables:
However, call bandwidth isn't the most important factor to consider when designing your WLAN. VoIP calls require frequent, consistent access to the channel, because VoIP applications send a large number of small frames for the duration of the call. As the number of VoIP calls increase, channel contention grows, causing more latency and jitter. Call quality will gradually decline, and eventually calls will be dropped.
Therefore, what you really want to calculate is the maximum number of calls each AP must support to achieve your desired call quality (measured as mean opinion score: MOS). You can find a good explanation of these metrics and a Voice-over-WLAN calculator at Connect802.
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