This paper presents a novel mathematical model for evaluating the performance of infrastructure wireless local area networks (WLANs) carrying voice and data traffic flows. We evaluate the performance of the access point (AP) flows and of the stations that employ IEEE 802.11 Distributed Coordination Function (DCF) as contention mechanism to access the channel. Metrics as throughput, average delay, delay jitter and packet loss rate are considered in order to evaluate the performance of an infrastructure WLAN assuming ideal channel conditions. The paper proposes a bursting feature to the AP to avoid bottle-neck and to increase the capacity of voice stations. The proposed model, which is validated by simulation results, considers the basic access scheme but can be easily applied to the RTS/CTS scheme.