AQM with Dual Virtual PI Queues for TCP Uplink/Downlink Fairness in Infrastructure WLANs

In this paper the authors address the unfairness problem between uplink and downlink TCP flows in the IEEE 802.11 infrastructure WLANs. It has been shown that TCP flows exhibit resource allocation (e.g., bandwidth) inequalities mainly due to two reasons: TCP´s asymmetric reactions towards data and ACK losses when the downlink buffer overflows at the AP; and the AP´s inability to distinguish itself from other contending stations when accessing the medium with the 802.11 DCF MAC. To overcome this problem, we propose an AQM (active queue management) approach (V2PI AQM) to be implemented at the AP, which utilizes two virtual queues, namely the data queue and the ACK queue, with their lengths controlled by the PI controllers. As a result, data losses can be reduced since the AP´s downlink buffer is no longer overwhelmed by the ACK packets destined to the uplink stations. In addition, when a discrepancy is observed by the AP monitoring the traffic intensity on the wireless link and the lengths of both queues, it uses an AIMD (additive increase multiplicative decrease) approach to adjust its contention window size, which compensates for the unfairness induced by the DCF MAC. We demonstrate using ns-2 simulations that fair bandwidth sharing between uplink and downlink TCP flows can be achieved by the AP´s buffer management in conjunction with contention window adaptation.