Sift: A MAC Protocol for Event-Driven Wireless Sensor Networks

Abstract

Nodes in sensor networks often encounter spatially-correlated contention, where multiple nodes in the same neighborhood all sense an event they need to transmit information about. Furthermore, in many sensor network applications, it is sufficient if a subset of the nodes that observe the same event report it. We show that traditional carrier-sense multiple access (CSMA) protocols like 802.11 do not handle the first constraint adequately, and do not take advantage of the second property, leading to degraded latency and throughput as the network scales in size. We present Sift, a medium access protocol for wireless sensor networks designed with the above observations in mind. Sift is a randomized CSMA protocol, but unlike previous protocols, does not use a time-varying contention window from which a node randomly picks a transmission slot. Rather, to reduce the latency for the delivery of event reports, Sift uses a fixed-size contention window and a carefully-chosen, non-uniform probability distribution of transmitting in each slot within the window. We show using simulations that Sift can offer up to a 7-fold latency reduction compared to 802.11 as the size of the sensor network scales up to 500 nodes. We then analytically prove bounds on the best latency achievable by a decentralized CSMA-based MAC protocol for sensor networks where one report of each event is enough, and show that Sift comes close to meeting this bound.