In most always-on applications, the environment needs to be monitored
continuously. However, keeping all the sensors and the processor continuously
active consumes significant energy. For example, in ExScal, if all the sensors
and the processor were left continuously active, a sensor node would have
lasted less than five days, even if the radio was always turned o?®. If a sensor
can sense the environment without the processor being active, then we can
significantly extend the network lifetime by putting the processor to sleep until
an event is detected. Further lifetime extensions are possible if the following
holds for an always-on application:
??? The sensing platform used is intended to detect multiple types of events.
(In ExScal, the network is required to detect persons on foot and vehicles.)
??? All event types are accompanied by a common simple event. (In ExScal,
every intruder is a moving object so that the simple event is the movement.)
??? A subset of sensors (called wakeup sensors) can detect the simple event.
(In ExScal, PIR sensor detects all moving objects.)
A sensor (or a set of sensors) qualifies as a wakeup sensor, if it has the
following features:
1. It does not need the processor to be active to perform an event detection.
Sampling of the environment, signal processing of the sampled data, and
263 Chapter 11 Maximizing the Lifetime of an Always-On WSN Application
Santosh Kumar, Anish Arora, and Ten H.
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