Achieving flow isolation in 802.11 networks with the DTT scheduler

Though the IEEE 802.11 standard has reached wide acceptance, its main access function, the Distributed Coordination Function (DCF), still suffers from some relevant problems coming from the specific features of the wireless channel. By means of simulation, we analyse the performance anomaly and the "inter-flow blocking" problems, highlighting the mechanisms that generate them. Starting from these insights, we propose a simple centralized channel aware scheduling algorithm, named Deficit Transmission Time (DTT). The basic principle under the DTT is measuring the channel quality in terms of frame transmission times. This measurement is then used to take scheduling decisions that guarantee each downlink flow an equal time share of the channel. The proposed scheduler has been developed and deployed in a Linux-based prototype AP to experimentally evaluate its performance. The results clearly show the improvements introduced by the DTT in terms of flow isolation and reduction of the effects of the performance anomaly.