In this paper, we propose a real time scheduling policy over 802.11 DCF protocol called Deadline Monotonic (DM). We evaluate the performance of this policy for a simple scenario where two stations with different delay constraints contend for the channel. For this scenario a Markov chain based analytical model is proposed. From the mathematical model, we derive expressions of the average medium access delay called service time. Analytical results are validated by simulation results using the ns-2 network simulator.