Quality of service (QoS) is a key problem in wireless environments where bandwidth is scarce and channel conditions are time varying and sometimes highly loss. Although IEEE 802.11 wireless LAN (WLAN) is the most widely used WLAN standard today, and the upcoming IEEE 802.11e QoS enhancement standard exists and introduces the QoS for supporting multimedia applications. This paper compares the propositions of standard IEEE 802.11e with the standard IEEE 802.11 without QoS, a simulation of these standards is performed by using the NS simulator. A discussion is presented in detail using simulation-based evaluations and we let us confirm the QoS 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. We finish with a conclusion.