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Matthew Gast runs the numbers to find out how many simultaneous calls are practical per access point: VoWLAN depends on having a great availability of access points in dense areas so that callers receive preferably wireline “dial tone” availability, or, at worst, cellular availability. Gast walks through the requirements for major coder/decoder (codec) routines used for VoIP. He provides tables and graphs for the maximum possible number of calls that could theoretically run across a network assuming no contention and all slots filled.
Gast shows the inherent benefit of 802.11a over 802.11g: 802.11a has no older standard to worry about; 802.11g must contend with (pun intended) 802.11b. Even a nominally all G network invokes protection whenever B packets are encountered, thus significantly reducing network throughput. Gast calculates that this protection of G packets in mixed B/G environments drops theoretical call capacity by one-quarter to one-third.
Matthew Gast crunches and improves Cisco’s simultaneous VoIP over WLAN call estimates for 802.11b: Gast, author of the recently released 802.11 Wireless Networks: The Definitive Guide, 2nd Edition, uses Cisco’s calculations for overhead and throughput to figure out how many VoWLAN calls could be made at the same time using 802.11b, which is still considered a standard for Wi-Fi VoIP on the receiver end.
The results aren’t pretty: if all 802.11b devices are running at 11 Mbps, you can achieve from 11 to 15 calls depending on voice encoding factors. This is why a lot of institutions are considering or deploying 802.11a for VoIP: they’ll be able to handle dozens of calls on a single AP and deploy APs densely in the same area without overlap.