Experimental evaluation of synchronization and topology control for in-building sensor network applications

While multi-hop networks consisting of 100s or 1000s of inexpensive embedded sensors are emerging as a means of mining data from the environment, inadequate network lifetime remains a major impediment to real-world deployment. This paper describes several applications deployed throughout our building that monitor conference room occupancy and environmental statistics and provide access to room reservation status. Because it is often infeasible to locate sensors and display devices near power outlets, we designed two protocols that allow energy conservation in a large class of sensor network applications. The first protocol, Relay Organization (ReOrg), is a topology control protocol which systematically shifts the network's routing burden to energy-rich nodes, exploiting heterogeneity. The second protocol, Relay Synchronization (ReSync), is a MAC protocol that extends network lifetime by allowing nodes to sleep most of the time, yet wake to receive packets. When combined, ReOrg and ReSync lower the duty cycle of the nodes, extending network lifetime. To our knowledge, this paper presents the first experimental testbed evaluation of energy-aware topology control integrated with energy-saving synchronization. Using a 54-node testbed, we demonstrate an 82-92% reduction in energy consumption, depending on traffic load. By rotating the burden of routing, our protocols can extend network lifetime by 5-10 times. Finally, we demonstrate that a small number of wall-powered nodes can significantly improve the lifetime of a battery-powered network.