In the paper we introduce and evaluate the performance of the Deficit Transmission Time (DTT) scheduler, a centralized channel aware scheduling algorithm for IEEE 802.11 networks. The scheduler has been designed to face two inconveniences of 802.11: the performance anomaly and the "inter-flow blocking" problem. DTT splits outgoing frames into multiple queues and uses gross transmission times as a measure of the link quality. DTT guarantees each downlink flow an equal time share of the channel, which translates into the desirable properties of proportional fairness and flow isolation. The performance of the scheduler has been measured in terms of the number of VoIP calls that can coexist in the system with an acceptable perceived speech quality. This assessment has been done on the basis of the E-model, an ITU-T standardized computational tool. The results definitely point out the improvement over the plain FIFO policy.