Achieving fairness in IEEE 802.11 DFWMAC with variable packet lengths

The medium access control (MAC) protocol through which mobile stations can share a common broadcast channel is essential in an ad-hoc network. Due to the existence of hidden terminals and partially-connected network topology, contention among stations in an ad-hoc network is not homogeneous. Some stations are at a disadvantage in access to the shared channel and can suffer severe throughput degradation when load to channel is high. This is known as the "fairness problem". Existing MAC protocols like IEEE 802.11 distributed foundation wireless medium access control (DFWMAC) may exacerbate this problem as it uses the binary exponential backoff (BEB) algorithm in contention resolution, which always favors the last succeeding station. This paper reviews the "fairness index", which is a metric to quantify fairness, and proposes a new estimation based backoff algorithm for the IEEE 802.11 DFWMAC protocol. The new algorithm can support the case when packet lengths are variable, which is a typical scenario of IEEE 802.11 compliant implementations that include both the basic CSMA/CA access method and the RTS/CTS access method. Simulation results show that the fairness problem can be very severe with the original BEB algorithm when packet length is variable and our new backoff algorithm can achieve far better fairness without adding much in complexity.