Fairness of medium access control protocols for multi-hop ad hoc wireless networks

It is an undisputed fact that fairness is an important element of a well-designed medium access control (MAC) protocol for multi-hop ad hoc networks. However, most popular MAC protocols still fail to attain an acceptable level of fairness in media access although several enhancements have been proposed in the past. These proposed enhancements are effective only in limited scenarios. It is our objective in this paper to do the following: (i) analyze the fairness problem; (ii) identify and analyze the three main causes leading to the fairness problem, namely, the lack of synchronization problem (LSP), the double contention areas problem (DCP) and the lack of coordination problem (LCP); (iii) based on the analysis, propose a new MAC protocol named the extended hybrid asynchronous time division multiple access (EHATDMA) as a solution. For better assessment of fairness, we have designed an index named max-min fairness index, which is scenario-independent and reflects the difference between the fair sharing provided by a protocol and the ideal max-min fair sharing. Comprehensive simulations have been carried out to compare the fairness of our protocol with the existing ones. Simulation results show that although the existing protocols employ various enhancements meant to improve the fairness property, most of them are still strongly biased towards throughput when a conflict between throughput and fairness arises. In addition, the fairness performance of these protocols varies widely from one scenario to another. On the other hand, EHATDMA strikes a good balance between throughput and fairness. It delivers a consistently high level of fairness regardless of network topology, traffic load and radio parameters, yet maintains high throughput whenever possible. Our simulation results also reveal that the most important mechanism affecting the fair sharing of radio channels among flows is the non-work-conserving mechanism.