These power numbers were obtained from product data
sheets and reported measurement values [3] [5] [6]. Four di?®erent operational
modes can be distinguished: Tx (transmit), Rx (receive), idle (the radio is on,
but nothing is received or transmitted) and sleep (the radio is turned o?®).
Fig. 1. Radio power consumption numbers.
An important observation is that for these typical radios, which have relatively
short transmission range compared to other systems such as cellular
phones, the transmit, receive and idle power are comparable. In other words,
whenever the radio is on, the power consumption does not vary much and the
only way to reduce it significantly is to turn the radio o?®. In sensor networks,
the node spends a large fraction of time in a state where it is not actively participating
in data communication. If it would keep its radio on throughout,
it would spend a majority of its lifetime in the idle listening mode. In typical
scenarios, the energy associated with this idle listening could overwhelmingly
dominate the overall communication energy consumption. Cases where nodes
spend more than 90% or more of their time in idle listening have been reported
[7] [8] [9]. Substantial node lifetime improvements can be obtained by
transitioning the radio into the much more power e?±cient sleep state instead.
This realization has driven most of the research in energy-e?±cient protocol for
sensor networks, such as for medium access control (MAC) [10] or topology
management.
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