This paper proposes S-MAC, a medium-access control (MAC) protocol designed for wireless sensor networks. Wireless sensor networks use battery-operated computing and sensing devices. A network of these devices will collaborate for a common application such as environmental monitoring. We expect sensor networks 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. These characteristics of sensor networks and applications motivate a MAC that is different from traditional wireless MACs such as IEEE 802.11 in almost every way: energy conservation and self-configuration are primary goals, while per-node fairness and latency are less important. 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. Inspired by PAMAS, S-MAC also sets the radio to sleep during transmissions of other nodes. Unlike PAMAS, it only uses in-channel signaling. Finally, S-MAC applies message passing to reduce contention latency for sensor-network applications that require store-andforward processing as data move through the network. We evaluate our implementation of S-MAC over a sample sensor node, the Mote, developed at University of California, Berkeley. The experiment results show that, on a source node, an 802.11-like MAC consumes 2--6 times more energy than S-MAC for traffic load with messages sent every 1-10s.

We consider a cluster of sensors that are interrogated by a mobile node that is passing by or loitering overhead. The mobile node functions as a clusterhead that collects one packet of sensed data from each sensor node. A broadcast from the mobile node is used to synchronize the motes in its cluster to within 1 microsecond. We exploit this physical layer characteristic to improve the retransmission algorithm in CSMA/CA-based MAC protocols. The result is a Synchronized, One-Stage-Backoff Retransmission Algorithm (SOSBRA) that is easy to implement and is shown by analysis and simulation to have fewer collisions than binaryexponential-backoff retransmission algorithms. We show via simulation that a SOSBRA-based 802.11 DCF protocol performs significantly better- in terms of energy usage, delay, and throughput- than both 802.15.4 (ZigBee) and 802.11 DCF (WiFi). A comparison between SOSBRA- and TDMAbased MAC protocols is also provided that shows SOSBRA is fair and more fault-tolerant. 1.