In this paper, we evaluate the impact of the wireless channel and physical layer parameters on the performance of VoIP traffic in IEEE 802.11g networks. The wireless channel is modeled using a finite state Markov chain based on an adaptive modulation and coding scheme. The transition probabilities encompass the effects of the time-varying wireless channel, such as the Doppler frequency and the operating SNR, and the physical layer target packet error rate. This model is implemented in the NS-2 tool and used to analyze the QoS (delay and packet drop rate) of real-time multimedia traffic in realistic wireless channels. Numerical results showing the physical layer impact on the number of supported VoIP calls in an 802.11g network are presented. We also demonstrate that the admission region in the presence of mixed VoIP and data remains linear, even after considering the complex wireless channel effects.