Rare Event Detection and Propagation in Wireless Sensor Networks.

Rarely occurring events present unique challenges to energy constrained systems designed for long term sensing of their occurrence or effect. Unlike periodic sampling or query based sensing systems, longevity cannot be achieved simply by adjusting the sensing nodes’ duty cycle until an equitable balance between data density and network lifetime is established. The low probability of occurrence and random nature of rare events makes it difficult to guarantee duty cycled battery powered sensing nodes will be energised when events occur. Equally, it is usually considered impractical to leave the sensing nodes energised at all times if the network is to have an acceptably long operational life. In the past decade and a half, wireless sensor network research has addressed this aspect of rare event sensing by investigating techniques including synchronised duty cycling of redundant nodes, passive sensing, duplicate message suppression, and energy efficient network protocols. Researchers have also demonstrated the efficacy of harvesting energy from the environment to extend operational life. Here we survey existing rare event detection and propagation techniques, and suggest areas suitable for continued research.