Lai
thresholding (for detecting an abrupt change in the environment due to an
event) can be done in the sensor hardware without involving the processor.
2. It has the hardware circuitry to raise an interrupt that can wake up a
sleeping processor.
3. It can detect the common simple event. (This feature is necessary because
otherwise some events can be missed by the network.)
4. It has the longest sensing range of all the sensors mounted on a sensor
node. (Again, this feature is necessary because otherwise some events can
be missed by the network.)
A sensor platform is said to have the Hierarchical Sensing feature if it has
at least one wakeup sensor. If a platform has the hierarchical sensing feature,
it can just keep the wakeup sensor active continuously and put the processor
and all the other sensors to sleep. In case an event is detected by the wakeup
sensor, it will wake up the processor, which will further process the sensor
data to determine if a real event has occurred, and if so, it will wake up all
the sleeping sensors to detect other properties of the event using di?®erent
sensing modalities. For example, in ExScal, the wakeup sensor can detect the
presence of an intruder and the sleeping sensors can help classify the type of
the intruder.
For the hierarchical sensor scheme to work, the choice of wakeup sensor is
critical. The wakeup sensor should not wake up the processor very frequently
due to false alarms.
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