1.2 Wakeup and Medium Access
In sensor networks, many applications are characterized by very sporadic traf-
fic and therefore contain substantial periods of radio inactivity. In general, the
data communication rate is low compared to the available bandwidth. To ensure
energy-e?±cient operation, sensor nodes should therefore switch between
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Chapter 8 Wakeup Strategies in Wireless Sensor Networks
a state in which they communicate, the length of which is an active communication
epoch Tactive, and the radio sleep state. The average amount of
information transferred during such communication epochs, L (in bits), can
be expressed as a function of the average time between such epochs Tperiod
(in seconds) and the required communication data rate Rdata (in bits/second)
of a node:
L
Tperiod
?? Rdata. (1)
In principle, this equation deals with time averages. One particular instantiation,
assuming a periodic arrangement, is shown in Figure 2.
Fig. 2. Abstract representation of a basic transmission scheme.
Numerous medium access control (MAC) protocols have been proposed for
sensor networks which in addition to managing access to the wireless medium,
try to minimize the energy spent [10]. Combined with reducing collisions,
overhearing and overhead, this is achieved by minimizing the time spent idle
listening. Ideally, this means that expression (1) holds with equality. Even
then, L and Tperiod could still be varied over a large range, as long as their ratio
remains the same.
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