Effects of Multi-rate in Ad Hoc Wireless Networks

An ad hoc wireless network is an autonomous selforganizing system of mobile nodes connected by wireless links where nodes not in direct range communicate via intermediate nodes. Modern wireless devices, such as those that implement the 802.11b, 802.11a, 802.11g, and HiperLAN2 standards, utilize multiple transmission rates in order to accommodate a wide range of channel conditions. The use of multiple rates presents a significantly more complex challenge to ad hoc routing protocols than the traditional single rate model. The hop count routing metric, which is traditionally used in single rate networks, is sub-optimal in multi-rate networks as it tends to select short paths composed of maximum length links. In a multi-rate network, these long distance links operate at the slowest available rate, thus achieving low effective throughput and reduced reliability due to the low signal levels. In this work we explore the lower level medium access control and physical phenomena that affect routing decisions in multi-rate ad hoc networks. We provide simulation results which illustrate the impact of these phenomena on effective throughput and show how the traditional minimum hop routing strategy is inappropriate for multi-rate networks. Our results motivate the need for additional work in this area to fully develop rate aware routing protocols, and stresses the importance of inter-layer communication for optimal performance in wireless networks.