On Spatial Fairness of the 802.11 DCF Protocol and the Role of Directional Antenna

The IEEE 802.11 DCF protocol suffers from the spatial fairness problem where different nodes achieve different throughput due to their topological distributions. In this paper we extend Bianchi's Markov model of the DCF protocol to multihop WLANs with some simple topologies and study the behavior of the binary exponential backoff (BEB) algorithm under heavy load conditions. Our study is focused on the interaction between the BEB processes of nodes at different locations. We identified several courses of the fairness problem, and found the BEB processes of some nodes have significant advantage over some other nodes. The BEB algorithm provides an undesired positive feedback and worsens the fairness problem. We further study directional antenna as a means to improve network fairness, and find that fairness can be improved significantly with directional antenna without changing the BEB algorithm.