Analysis of the 802.11 CSMA/CA mechanism has received considerable attention recently. Bianchi [3] presents an analytic model under a saturated traffic assumption. Bianchi's model is accurate, but typical network conditions are non-saturated heterogenous. We present an extension of his model to a non-saturated environment. The model's predictions, validated against simulation, accurately capture many interesting features of non-saturated operation. For example, the model predicts that peak throughput occurs prior to saturation. Our model also allows stations to have different traffic arrival rates, enabling us to address the question of fairness between competing flows.