Energy Aware Routing for Low Energy Ad Hoc Sensor Networks Rahul C. Shah and Jan M. Rabaey Berkeley Wireless Research Center University of California, Berkeley Abstract - The recent interest in sensor networks has led to a number of routing schemes that use the limited resources available at sensor nodes more efficiently. These schemes typically try to find the minimum energy path to optimize energy usage at a node. In this paper we take the view that always using lowest energy paths may not be optimal from the point of view of network lifetime and long-term connectivity. To optimize these measures, we propose a new scheme called energy aware routing that uses sub-optimal paths occasionally to provide substantial gains. Simulation results are also presented that show increase in network lifetimes of up to 40% over comparable schemes like directed diffusion routing. Nodes also burn energy in a more equitable way across the network ensuring a more graceful degradation of service with time. I. INTRODUCTION Recently there has been a lot of interest in building and deploying sensor networks – dense wireless networks of heterogeneous nodes collecting and disseminating environmental data. There is a multiplicity of scenarios in which such networks might find uses, such as environmental control in office buildings, robot control and guidance in automatic manufacturing environments, interactive toys, the smart home providing security, identification, and personalization, and interactive museums. Crucial to the success of these ubiquitous networks is the availability of small, lightweight, low-cost network elements, which we call PicoNodes [1]. These nodes must be smaller than one cubic centimeter, weigh less than 100 grams, and cost substantially less than one dollar. Even more important, the nodes must use ultra-low power to eliminate frequent battery replacement. We envision a power-dissipation level below 100 microwatts, as this would enable self-powered nodes using energy extracted from the environment, an approach called energy scavenging or harvesting. Trying to network a large number of such low-power mobile nodes is a challenging problem that has recently been the focus of many researchers. In particular, routing, addressing and support for different classes of service are the primary issues to be tackled at the network layer. In this paper, we’ll concentrate on the routing problem. Current research has focused on protocols that are low power [2][3][11], scalable with the number of nodes [4] and fault tolerant (to nodes that go up or down, or move in and out of This research was supported by DARPA on grant no. F29601-99-1-0169 entitled, "Communication/Computation Piconodes for Sensor Networks” range) [5]. However, we think that a more useful metric for routing protocol performance is network survivability. By this we mean that the protocol should ensure that connectivity in a network is maintained for as long as possible, and that the energy health of the entire network should be of the same order. This is in contrast to energy optimizing protocols that find optimal paths and then burn the energy of the nodes along those paths, leaving the network with a wide disparity in the energy levels of the nodes, and eventually disconnected subnets. If nodes in the network burn energy more equitably, then the nodes in the center of the network continue to provide connectivity for longer, and the time to network partition increases. This leads to a more graceful degradation of the network. This is the idea of survivability of networks. Energy Aware Routing, the protocol that we have developed tries to ensure the survivability of low-energy networks. It is also a reactive protocol such as AODV and directed diffusion; however, the protocol does not find a single optimal path and use it for communication. Rather it keeps a set of good paths and chooses one based on a probabilistic fashion. As we will show later, this means that instead of a single path, a communication would use different paths at different times, thus any single path does not get energy depleted. It is also quick to respond to nodes moving in and out of the network, and has minimal routing overhead. We present simulation results which show the improved performance obtained by our method, and the increase in network lifetime. While the primary metric of interest is network survivability, we also show the performance results with regard to other metrics mentioned above. The rest of the paper is organized as follows. Section II gives background information on sensor networks, their requirements and our ongoing research project, PicoRadio. Some important design choices that we made for PicoRadio are detailed in Section III. Section IV compares existing routing schemes for ad hoc networks and their performance issues. We introduce energy aware routing in Section V and provide qualitative arguments as to its performance. The energy metric used for the protocol is detailed in Section VI. Finally, simulations and results are presented in Section VII, concluding in Section VIII. II. SENSOR NETWORKS AND THE PICORADIO PROJECT Sensor networks typically consist of hundreds of nodes, deployed for the purpose of environment monitoring and control. Let us consider a Smart Building scenario, one of the