Throughput and Fairness of Collision Avoidance Protocols in Ad Hoc Networks

摘要

Wireless ad hoc networks have received increasing interest in recent years because of their potential to be used in a variety of applications without the aid of any pre-existing network infrastructure. Due to the scarce channel bandwidth available in ad hoc networks, the design of efficient and effective medium access control (MAC) protocols that regulate nodes' access to a shared channel has become the subject of active research in recent years. Many MAC protocols have been proposed to mitigate the adverse effects of hidden terminals through collision avoidance. In Section 1, The authors present an analytical modeling to derive the saturation throughput of these sender- initiated collision avoidance protocols in multi-hop ad hoc networks with nodes randomly placed according to a two-dimensional Poisson distribution. They show that the sender-initiated collision-avoidance scheme achieves much higher throughput than the ideal CSMA scheme with a separate channel for acknowledgments. They also show that the collision-avoidance scheme can accommodate far fewer competing nodes within a region in a network infested with hidden terminals than in a fully connected network, if reasonable throughput is to be maintained. Simulations of the IEEE 802.11 MAC protocol and one of its variants validate the predictions made in the analysis. The simulation results also reveal the fairness problem in IEEE 802.11, which refers to the severe throughput degradation of some nodes due to their unfavorable locations in the network and the commonly used binary exponential backoff (BEB) algorithm that always favors the node that last succeeds. Section 2 introduce a framework to address the fairness problem and propose a topology- aware fair access (TAFA) scheme to realize the framework. Simulation results show that TAFA can solve the fairness problem in UDP-based applications with negligible degradation in throughput. Section 3 concludes this chapter with directions for future work.