Passive On-Line In-Band Interference Inference in Centralized WLANs

In this paper, we describe a passive on-line method for determining the conflict graph in a centralized wireless LAN. The key insight in our work is to exploit the relative timing between successfully received frames: if two successful frames sent on two different links overlap in time, then the corresponding links do not interfere; if they are consistently adjacent, then those links do interfere. Our scheme is inband and does not require additional monitoring hardware, nor does it require any modifications to clients. In contrast to recent work that has focused on partial on-line estimation of conflict graphs (restricted to downlinks from APs to clients), our approach can detect the complete conflict graph (both uplinks and downlinks). Using extensive experiments on an 802.11 a/b/g indoor testbed we demonstrate the accuracy of our scheme (even under external interference) and its rapid convergence. Moreover, we demonstrate that our interference inference method can be used in an on-line fashion: the inferred conflict graph can be used to dynamically and explicitly allocate airtime to each link, a capability that can avoid TCP flow starvation, solve the rate anomaly problem, and help manage traffic in a centralized WLAN.