  Prior work indicates that 802.11 is extremely inefficient for VoIP transport.
Only 12 and 60 VoIP sessions can be supported in an 802.11b and an 802.11g
WLAN, respectively. This paper shows that the bad news does not stop there.
When there are multiple WLANs in the vicinity of each other, the already-low
VoIP capacity can be further eroded in a significant manner. For example, in a
5-by-5, 25-cell multi-WLAN network, the VoIP capacities for 802.11b and 802.11g
are only 1.63 and 10.34 sessions per AP, respectively. This paper investigates
several solutions to improve the VoIP capacity. Based on a conflict graph
model, we propose a clique-analytical call-admission scheme, which increases
the VoIP capacity by 52% and 37% in 802.11b and 802.11g respectively. If all
the three orthogonal frequency channels available in 11b and 11g are used, the
capacity can be nearly tripled by the call-admission scheme. This paper also
proposes for the first time the use of coarse-grained time-division multiple
access (CoTDMA) in conjunction with the basic 802.11 CSMA to eliminate the
performance-degrading exposed-node and hidden-node problems. We find that
CoTDMA can further increase the VoIP capacity in the multi-WLAN scenario by an
additional 35%.
