The goal in the ExScal application was to deploy a wireless sensor network
over a large region to monitor intrusion activities. The network was required
to detect di?®erent types of intruders breaching the perimeter of the protected
region, classify them into some predetermined categories (e.g., person, soldier,
car, tank), and track their trajectories of intrusion. The network was also
required to notify the nearest base station of an intrusion event in less than
two seconds.
The key issues in ExScal were to minimize the cost of coverage, minimize
the power consumption to maximize the network lifetime, provide accurate
(i.e., low false alarm rate) and timely (i.e., less than two seconds from the
occurrence of the event) detection of intrusion events in the face of unavoidable
hardware and software failures, and do all of this with low human involvement.
Minimizing the cost of coverage required minimizing the number of sensor
nodes needed (which for our purpose means not deploying any more sensor
nodes than are absolutely necessary to meet the monitoring and notification
requirements of the application). This, in turn, required deploying nodes in
an optimal topology4. We refer the readers to [16] for details on the layout of
sensor nodes that was used in ExScal. Minimizing the cost of coverage also
required finding o?®-the-shelf sensors with the largest sensing ranges and using
radios that provided the largest communication range with the lowest energy
consumption, while keeping the cost low.
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