We propose to provide QoS differentiation at the IP level based on the DiffServ model over the standard distributed coordination function (DCF) method of 802.11. By constraining the output rate of each host in a cell via DiffServ scheduling and traffic shaping mechanisms, we can keep the 802.11 network in the state of nonsaturation so that the time-sensitive expedited forwarding (EF) class benefits from stable short delays. We analyze the performance of 802.11b to find how we should constrain the rate of DiffServ sources and present extensive measurements that show how the QoS differentiation is achieved.