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IEEE 802.11 is world widely established as the most popular protocol for Wireless Local Area Networks (WLANs). Voice over IP (VoIP) applications are widely spread due to their relatively low cost. Applying VoIP traffic on IEEE 802.11 WLANs introduces significant delays in the wireless medium due to the small size of voice packets. In this paper, we develop an analytical model for the IEEE 802.11 Distributed Coordination Function (DCF) considering voice and data stations transmitting simultaneously. Our new model calculates IEEE 802.11 DCF performance in terms of average delay, delay variation (jitter) and drop probability for voice packets. Metric values that constitute good voice quality are identified and based on these we develop a methodology for assessing the network's capacity in terms of good quality voice calls. The proposed analytical model is validated by comparing analytical results against simulation outcome. Finally, we study the impact of data transmissions on voice capacity and show that for every data session (a data station and a TCP-ACK station) added to a 802.11b WLAN, its voice capacity drops by 2 voice sessions (4 voice stations).