Can I add a VoIP call?

In this paper, we study the inherent limitations of the 802.11 (a/b) distributed coordination function (DCF) in supporting VoIP calls over a wireless LAN. Specifically, we evaluate the upper bound on the number of simultaneous VoIP calls that can be placed in a single cell of an 802.11 (a/b) network. Making one additional VoIP call in that cell would degrade the quality of all VoIP call. The upper bound is calculated as a function of the choice of VoIP codec and the length of the audio payload. As an example, when a G711 codec with 20 millisecond audio payload is used, an 802.11b cell can support only 3 to 12 simultaneous VoIP calls. The actual number depends on the effective transmission rate of the wireless station, which for 802.11b can be 1 Mbps, 5.5 Mbps and 11 Mbps. We also study the effect of spatial distribution of the wireless stations on the upper bound which is the dominant factor in determining the effective transmission rate of a station.