I. INTRODUCTION
Sensor networks have recently emerged as an effective tool for monitoring large-scale environments, and have been successfully deployed to solve problems as diverse as detecting floods, controlling the temperature in office buildings, and monitoring hospital patients [1]. In order to deploy sensor networks in such large environments, often with hundreds of nodes, wireless sensors must be low-cost and affordable. Hence, wireless sensors are typically highly limited in terms of sensing, computation, communication, battery life, and the actions they can perform.