This project proposes S-MAC, a medium-access control (MAC) protocol designed for wireless sensor networks. Wireless sensor networks use battery-operated computing and sensing devices. The sensor networks are expected to be deployed in an ad-hoc fashion, with individual nodes remaining largely inactive for long periods of time, but then becoming suddenly active when something is detected. S-MAC uses three novel techniques to reduce energy consumption and support self configuration. To reduce energy consumption in listening to an idle channel, nodes periodically sleep. Neighboring nodes form virtual clusters to auto-synchronize on sleep schedules. S-MAC also sets the radio to sleep during transmissions of other nodes and uses in-channel signaling. Finally, S-MAC applies message passing to reduce contention latency for sensornetwork applications that require store-and forward processing as data move through the network. The implementation of S-MAC is evaluated over a sample virtual sensor node network created on the ns2 platform. The experiment results show that, on a source node, an 802.11-like MAC consumes 2-3 times more energy than S-MAC for traffic load with messages sent every 150 ms. © 2012 IEEE.