Voice transmitted over wireless LAN faces serious challenge because of the fluctuation of the wireless channel. In this paper, the transmission of voice over wireless LAN is firstly analyzed. Taking collision probability into account, the stochastic analysis based on a Markov chain to calculate the upper bound number of simultaneous VoIP calls that can be supported in a single cell of an IEEE 802. llb/a/g network is proposed. The upper bound capacities of G. 711, G. 729 and G. 723.1 code transmitted over 802. llb/a/g are figured out. The simulation results from NS2 simulator have validated the analysis values.