The effect of contention in CSMA networks: Model and fairness protocol

Multi-hop wireless networks using Carrier Sense Multiple Access (CSMA) suffer from various forms of unfairness. A majority of the existing work focuses on fairness issues due to hidden terminals and the impact on backoff mechanisms. This paper focuses on a more fundamental form of unfairness that arises due to unequal contention opportunities at a node; some nodes rarely observe an idle channel since two or more interferers can transmit together. Contention unfairness is unrelated to hidden terminals. The paper analyzes the impact of contention and provides insight into developing solutions for contention unfairness. We first develop a model–from first principles–for contention in IEEE 802.11 networks. The accuracy of the model is validated through simulations and the results show that such unfairness is a common phenomenon. Based on the insights gained from the model, we propose and evaluate distributed schemes that reduce the effect of unfairness due to contention. We show that contention unfairness occurs frequently in general scenarios. Simulation results indicate that the proposed protocols improve the overall system performance by providing greater fairness with a small reduction in throughput.