Modeling and performance analysis of MAC protocols for WLANs have attracted lots of research efforts because they could help to discover the inherent cause of many problems, give insight into the parameter settings and may even suggest potential novel solutions. Although many analytical models for IEEE 802.11 Distributed Coordination Function (DCF) have been reported, most studies did not consider the dynamics of traffic sources but concentrated on its throughput performance under the saturation condition assuming that there is always a packet available for transmission in each station for the simplicity of analytical modeling and derivation. The comprehensive performance study under nonsaturated traffic situations is still an open problem. In this paper, we propose an analytical performance model for IEEE 802.11 DCF protocol and present how to model the binary backoff scheme under more flexible traffic source. We validate the accuracy of the model by comparing the analytical results with those obtained from simulation experiments.