3.1 PROBLEMS, MODELS, AND ASSUMPTIONS

One of the fundamental issues in wireless sensor networks (WSNs) is the sensor coverage problem. Sensor coverage is to deploy a set of sensor nodes in an area of interest for monitoring and/or tracking. Sensor nodes are normally densely deployed in WSNs. To prolong the network lifetime, sensors should sleep as much as possible. Ideally, they should wake up only when they are really needed. However, this may not be possible since additional hardware may be required for such ability. For example, a radio-triggered hardware component was introduced by Gu and Stankovic (2004). Since the events of interest often contain energy, their energy can be used to trigger the added hardware component which then in turn initiates the transition of the system from sleep mode to wake-up mode. Existing sensors, however, are not equipped with such hardware. In these cases, when sensors decide to enter sleep mode, they set their clock for waking at a predetermined time, regardless of events nearby. Wireless sensor networks and wireless sensor actuator networks (WSANs) employ collaborative mechanisms for scheduling wake-up and sleep periods.

Wake-up and sleep periods exist for both sensing and communication hardware components. Normally, a portion of nodes is required to be active (with respect to certain hardware) to perform the given tasks, while other nodes could sleep to save energy. Both sensor and actuator networks have node redundancies for communication ...

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