The most widespread Wireless Local Area Networks (WLANs) are based today on the IEEE 802.11 standard and its various versions, especially the IEEE 802.11b. In this article we first briefly explain the IEEE 802.11 architecture and the possible implementations of the Data Link Layer. We then present and discuss the results from simulation experiments we performed in order to evaluate the protocol performance. We studied cases of integrated MPEG teleconference and computer data transmissions, over the IEEE 802.11b WLAN. Our simulation results clearly demonstrate the difficulty of the protocol to support time sensitive applications with a large number of wireless users under the same Access Point.