The IEEE 802.11e draft specification aims to extend the original 802.11 MAC protocol by introducing priority mechanisms able to manage bandwidth and resource allocation according to the QoS needs of real-time applications. Different strategies based on MAC parameter diversifications, such as contention window limits, contention window updating factor and silence monitoring time, can be pursued in order to provide service differentiation, also in the case of distributed access. In this paper, we investigate on the behaviour of each differentiation possibility under different load conditions and traffic requirements. Our results show that the most powerful mechanisms which provide service differentiation are based on inter frame space (IFS) and minimum contention window (CWmin) diversification. We conclude that the best differentiation policy should be based on a joined use of IFS and CWmin differentiation, using the first parameter to adjust access probabilities, and the second parameter to optimize the resource utilization in the network.