VoIP Capacity over Multiple IEEE 802.11 WLANs

It is well known that IEEE 802.11 WLAN is highly inefficient for transporting voice data. For example, if one simply takes the data rate of 802.11 b, 11 Mbps, and divide it by two times 13.2 Kbps (the bit rate of a typical voice stream in one direction), one comes to the conclusion that more than 400 voice sessions can be supported in an 802.11 b WLAN. As shown in previous work, it turns out that at most 12 sessions can be supported due to various header and protocol overheads inherent in 802.11. This paper points out that the "bad news" does not stop there, and that in practice the number of supportable voice sessions could be lower than 2 sessions per access point (AP)! This is so because as 802.11 WLAN gains popularity, it is common to have many WLANs being deployed in the same geographical area, and these WLANs share the common air medium. Our ns2 simulation experiments, for example, show that the capacity of a 5-by-5, 25-cell IEEE 802.11b WLAN, laid out in a square grid manner, is only 1.6 sessions per AP. The second contribution of this paper is the investigation of techniques to improve the dismal capacity. We show that a systematic call admission mechanism based on clique analysis of a conflict graph can increase the capacity to 2.12 sessions per AP. Adding a "restart mode" to the 802.11 protocol boosts the capacity further to 2.72 sessions per AP. We also briefly discuss the impact of higher data rates (e.g., 54 Mbps in 802.tig and 802.11 a), and careful assignment of available frequency channels (e.g., the three and twelve orthogonal frequency channels in 802.11 b/g and 802.11 a, respectively), on capacity. Although all the above techniques can improve the capacity somewhat, the huge penalty relative to the potential remains. Boosting the voice capacity over multiple WLANs is therefore an area that deserves further attention from the research community.