Physical layer impact on the design and performance of routing and broadcasting protocols in ad hoc and sensor networks

• Authors: I. Stojmenovic;A. Nayak;J. Kuruvila;F. Ovalle-Martinez;E. Villanueva-Pena
• Venue: Computer Communications
• Year: 2005

Existing routing and broadcasting protocols for ad hoc networks assume an ideal physical layer model, where two nodes communicate if and only if they are at distance at most R, where R is the transmission radius. This article surveys our efforts to consider a more realistic physical layer model, and its impact on routing and broadcasting. We apply the log normal shadow fading model to represent a realistic physical layer to derive (accurately and approximately) the probability p(x) for receiving a packet successfully as a function of distance x between two nodes. We define the transmission radius R as the distance at which p(R)=0.5. We consider routing algorithms with and without acknowledgements, with messages consisting of one or more packets. For single packet routing with hop by hop acknowledgements, we propose a MAC layer protocol where receiver node acknowledges packet to sender node u times, where uxp(x)~1. The expected hop count (EHC) between two nodes under this protocol is (1/p(x)^2)+(1/p(x)) (for u=1). We show that forwarding to neighbor closest to destination is suboptimal, and that optimal forwarding distance is one that minimizes EHC per progress made, that is, minimizes h(x)=(1/p(x)^2+1/p(x))(1/x). Depending on the power attenuation factor @b (between 2 and 6), the optimal forwarding radius is between 0.7R and 0.8R. The function h(x) can be used to derive hop count optimal route discovery based routing. During route discovery, each node, receiving message from a neighbor at distance x (distance can be estimated from received signal strength), will set timeout proportional to h(x), before retransmitting. The destination will respond to the first message received. Neighbor discovery is not straightforward since hello messages are not received by all neighbors. We proposed several localized routing schemes for the case when position of destination is known, optimizing expected hop count (for hop by hop acknowledgement), or maximizing the probability of delivery (when no acknowledgements are sent). We then considered localized power aware routing schemes under realistic physical layer, when nodes can adjust their transmission powers. Finally, we discuss broadcasting in ad hoc network with realistic physical layer, and propose new concept of dominating sets to be used in broadcasting process.