Quality of service is a key problem in wireless environments where bandwidth is scarce and channel conditions are time varying and sometimes implies highly packet losses. IEEE 802.11b/g/a wireless LAN are the most widely used WLAN standards today, and the IEEE 802.11e quality of service enhancement standard exists and introduces quality of service support for multimedia applications. This paper compares the propositions of standard IEEE 802.11e with the standard IEEE 802.11 without quality of service, a simulation of these standards is performed by using the network simulator simulator. We test also the mobility and the roaming of stations. A discussion is presented in detail using simulation-based evaluations and we let us confirm the quality of service of IEEE 802.11e compared to IEEE 802.11, but we have detected some weaknesses of 802.11e. It starves the low priority traffic in case of high load, and leads to higher collision rates, and did not make a good estimate of weight of queues, so there is an unbalance enters the flows with high priorities.