Which alternative to use (deploying redundant sensor nodes
or deploying redundant batteries) will require a careful cost-benefit analysis
specific to the application and sensor platforms in consideration, and is out
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of scope for this chapter. But, using fine-grained power management schemes
to extend the lifetime of a WSN is basic. It does not require incurring any
additional expenditure (due to deploying redundant batteries or deploying
redundant nodes). It comes for free.
Applying Fine-grained Power Management
Schemes: In several always-o?® applications, it was possible to extend the
network lifetime by more than 10 times by using fine-grained power management
schemes alone [19, 21]. In the habitat monitoring application [21], sensor
nodes were allowed to sleep for 20 minutes, at the end of which they woke
up for 70 seconds, collected data, transmitted it to a gateway, and then went
back to sleep. By doing so, it was possible to extend the lifetime of a sensor
node from five days to 67 days, a factor of more than 13 times.
It is obvious that the strategy of putting all the sensor nodes to sleep
and have them all wake up periodically to sample the environment will not
work for always-on applications because of a need to continuously monitor the
environment. It has been widely believed that significant lifetime extensions (a
factor of 10 or more) for always-on applications can only be achieved by using
coarse-grained power management schemes [1, 5, 6, 12, 13, 17, 26, 27] that
require deploying more sensor nodes than are absolutely necessary to meet
the monitoring and notification requirements of the application.
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