This paper proposes an improved two-dimensional Markov chain model for the performance analysis of the IEEE 802.11 DCF protocol. The model is applicable to both saturated and unsaturated traffic conditions. Moreover, it extends the existing models to take into account previously-ignored MAC layer factors such as backoff freezing and limited times of retry. In addition, by deriving the stationary state of the Markov model and considering an M/M/1/K queuing model, we further derive a theoretical model for evaluating performance of the general IEEE 802.11 DCF protocols. Accuracy of the proposed theoretical model is validated with simulation results.