The number of VoIP calls that a single AP can support simultaneously depends not only on the product and its configuration, but also on the VoIP protocol, voice encoding (codec), voice sampling rate, and the call quality (mean opinion score) that you're aiming for.
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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.
Mean opinion score (MOS) provides a numerical measure of the quality of human speech at the destination end of the circuit. The scheme uses subjective tests (opinionated scores) that are mathematically averaged to obtain a quantitative indicator of the system performance.
Compressor/decompressor (codec) systems and digital signal processing (DSP) are commonly used in voice communications because they conserve bandwidth. But they also degrade voice fidelity. The best codecs provide the most bandwidth conservation while producing the least degradation of the signal. Bandwidth can be measured using laboratory instruments, but voice quality requires human interpretation.
To determine MOS, a number of listeners rate the quality of test sentences read aloud over the communications circuit by male and female speakers. A listener gives each sentence a rating as follows: (1) bad; (2) poor; (3) fair; (4) good; (5) excellent. The MOS is the arithmetic mean of all the individual scores, and can range from 1 (worst) to 5 (best).