4.1 Key Assumptions Needed in Lifetime Analysis
In analyzing the lifetime of any WSN, some key assumptions need to be made
based on the expected use of the network. We make the following assumptions
for ExScal, with the goal of deriving a conservative estimate of its lifetime:
1. The lifetime of the network is determined by the heaviest-loaded XSM.
This is an XSM close to the base station. This is conservative because
ExScal network will continue to meet its requirements even if all XSMs
within one hop of a base station fail.
2. Every hour, six intrusion events occur in the vicinity of the heaviestloaded
XSM5. With equal probability, the event can be the intrusion of a
person or that of an SUV. Further, the intruders are assumed to follow the
least-covered path through the network. With this last assumption, the
number of sensors detecting an intruder in the ExScal network is given by
the values in Table 3.
3. Every time an event occurs, the heaviest-loaded XSM keeps its processor
as well as all three of its sensors active for an average of 10 seconds. This
is because, on average, a slow moving target (a person on foot) will spend
an average of 10 seconds in the sensing range of a sensor.
4. One-eighth of the event detection messages generated in the vicinity of
the heaviest-loaded XSM are routed by the heaviest-loaded XSM6.
5 This event rate is higher than what ExScal was required to handle.
6 This is because grid routing [25] is used in ExScal, which balances the routing
load on multiple routes and because of the topology used in ExScal [16].
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