Maximum number of VoIP telephone calls per 802.11b access point

by Matthew Gast

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Along similar lines to my earlier earlier look at 802.11 throughput, Cisco calculated the theoretical maximum number of telephone calls per 802.11b access point. (802.11b is still relevant because most phones are 802.11b only due to the lower power requirement for the wireless interface chips.) In the appendix of the Cisco Wireless IP Phone 7920 Design and Deployment Guide, there is a detailed two-page calculation that deduces the maximum number of telephone calls for the 64 kbps G.711 codec.

I used the analysis as a starting point for my own calculations. The Cisco calculation neglected the additional overhead of security encapsulation headers for WEP, TKIP, or CCMP, and it did not account for the proper encapsulation of IP within 802.11. However, these results do not materially affect the calculations. In 802.11b, just obtaining access to the medium takes a great deal of time, so the transmission time for a few additional bytes for WEP, TKIP, or CCMP security headers is almost negligible. I threw everything in a spreadsheet so that I could calculate the number of telephone calls per AP for all the different data rates easily, and came up with the following table:

Theoretical Maximum Telephone Calls per 802.11b AP
Codec Header type Operating Rate
11 Mbps 5.5 Mbps 2 Mbps 1 Mbps
G.711 short 13 10 6 3
long 10 8 5 3
G.729 short 15 10 9 6
long 11 10 8 6*

*With security enabled, the number is only 5.

As with my previous analysis, this is highly theoretical. Unlike the previous analysis, the calculation does take into account the medium access time, by using a contention window of about half the maximum value. It, however, does not assume any other contention for the medium. It represents a scenario where QoS is provided by an omniscient medium coordinator who can get all the handsets cooperating.

The other point of interest is that the number of calls drops rapidly as the data rate decreases. If you want to have a conference call between three VoIP telephones, they had better be close to the AP. If they're all far away and operating at the 1 Mbps data rate, there had better not be anybody trying to use the network for data!