Matthew Gast delves into the mathematics of capacity calculations in his latest O’Reilly article How Many Voice Callers Fit on the Head of an Access Point? If you’re engineering a VoIP network you should take a look at Matthew’s analysis of the theoretical maximum capacity of access points to carry VoIP traffic over 802.11a, b, and g. Matthew is the author of 802.11 Wireless Networks: The Definitive Guide.
Over the summer, I came across a capacity calculation in the manual for a Cisco voice over IP phone detailing the number of simultaneous calls that can be supported on an access point. Intrigued, I extended the analysis. Voice and data on wireless LANs require opposing preconditions for good performance. High-quality voice requires that frames containing voice data can be transmitted very quickly after arrival, and they need to be transmitted on a very regular schedule with tight timing requirements. Good data throughput comes from stuffing the transmission queue as full as possible. Individual frames might suffer long delays, but the overall capacity is high. Voice quality is often very sensitive to network load.