Achieving Uplink/Downlink Fairness in WLANs via Multiple Backoff Timers

The IEEE 802.11 Distributed Coordination Function (DCF) scheme suffers from a well-known uplink/downlink throughput imbalance problem when operating in the infrastructure mode. A common existing approach is to assign higher priority to the access point by properly tuning the backoff parameters (e.g., the contention windows size and the inter frame space) of each station so that the access point obtains a larger share of network bandwidth. However, it is quite difficult to map and adjust these parameters to accurately offset arbitrary uplink/downlink throughput imbalance. Recently, the IEEE 802.11e standard additionally provides several parameters that can be properly adjusted to effectively offset the imbalance problem. However, this simple solution also takes away some of the IEEE 802.11e capacity to support flexible prioritized services. In this work, we propose a new simple and effective technique to solve the uplink/downlink throughput imbalance problem in an infrastructure WLAN. The idea is to have the access point employ multiple independent backoff timers, each of which is associated with the downlink traffic flows for each client station. The proposed solution requires minimal changes in the access point firmware, but no change in the client device is necessary. Simulation results show that the proposed mechanism efficiently and effectively solves the uplink/downlink throughput imbalance problem in an infrastructure WLAN.