The analyzed MAC mechanism is based on Distributed Coordination Function（DCF） in IEEE802.11 and contains quality of service （QoS）.ln the proposed priority analytical model

retransmission is not used for the real-time traffic

because the real-time traffic is sensitive to the delay.Compared with the best-effort traffic

the real-time traffic has a much better saturation throughput and much less saturation access delay.But the cost is the higher frame-dropping probability for the real-time traffic.To guarantee quality of service（QoS） of the real-time traffic

the paper puts forward a QoS parameter P

0

Qos

to limit frame-dropping probability.After the analysis of results

we can conclude that a optimal backoff window size will be acquired in accord with QoS guarantee and the maximum throughput to implement good performance of MAC mechanism.