—Experiments show that IEEE 802.11 DCF system exhibits unstable behavior in the congestion onset load range where the system starts to become saturated. This phenomenon is not well investigated due to the lack of proper models. In this paper, we propose a two-dimensional (2-D) Markov chain model by combining the well-known saturated model with newly created idle states. Closed-form solution to the 2-D Markov chain is derived, and system performances are validated by simulations. With the proposed 2-D model, we are able to characterize the unstable system behavior, and provide insights into 802.11 DCF system dynamics. ∗