Performance tuning of Infrastructure-Mode wireless LANs

Conventional wisdom about 802.11 WLANs dictates that as the number of active users increases, the contention windows (CW) of all the contending users needs to increase to prevent collision losses. However, given the increasing data rates of 802.11 standards, the fraction of time spent not transmitting information (as a result of back-off) is also increasing, leading to reduced throughput when compared to the theoretical maximum. While there appears to be an inherent conflict between reducing collisions by increasing the CW and reducing overhead by decreasing the CW, we demonstrate in this paper that this conflict can be eliminated in Infrastructure-Mode wireless LANs (I-WLAN) where downlink TCP and UDP flows dominate. Unlike the traditional trend of WLAN performance analysis that assumes that all the stations are greedy, we consider the case in which only the access point (AP) is greedy while the mobile users in the cell react to the information sent to them. This setting captures the traffic patterns in I-WLANs more accurately as users typically utilize their WLAN connections predominantly for downloading information through TCP (i.e., web browsing) and UDP (i.e., video streaming) connections. We show via analysis and extensive experimentation that by changing the minimum CW at the APs and the users, we can achieve 25-30% greater throughput than the typically recommended 802.11 settings. We achieve this without requiring any changes to the MAC or PHY, and utilizing only the standard features present on current-generation 802.11 chip-sets.