The calculation for wireless VoIP is more complex, depending on an enterprise's existing network and building architecture, as well as user requirements. For decades, the telecom industry has relied on a set of equations known as the Erlang formulas to estimate how many access lines a PBX will need for a given traffic load and QoS. Erlang calculations usually require dedicated software, though Web-based versions are now available at sites such as www.erlang.com. The equivalent for wireless networks is much worse, as these calculations also need to factor in radio interference and the need to provide coverage in areas that only contain users some of the time. This makes it a process of trial and error.
In the graph, access lines are overprovisioned to handle twice the average external call load, and AP voice connections are overprovisioned to handle 10 times the average total call load. This can seem high, but there are still fewer APs than users, as each AP is assumed to be able to handle five simultaneous calls, and most users spend only a small proportion of their total time on the phone.
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Risk Assessment: Wi-Fi Telephony
Enterprise VoIP is immature. Wi-Fi is immature. Put them together and you get twice the hype and more than twice the problems to be worked out. Most Wi-Fi phones don't yet support 802.11i or EAP, and the IEEE is still arguing over a QoS standard.
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