Such a
radio, called radio-triggered wakeup radio, was proposed in [11]. However, it
is not yet available on current sensor platforms.
Low Power Listening (LPL) proposed in [20] is an approximation to the
radio-triggered wakeup. It allows a sensor node to put its radio (and the
processor) to sleep mode for a certain interval and wake it up periodically to
sample the channel. If the radio detects preamble bytes, it stays awake and
extracts the entire packet. Otherwise, it returns to sleep. This feature was
implemented on the sensor platform used in ExScal.
One downside of using the low power listening feature is that the sender
has to send a preamble at least as long as the sleep period of the radio. So,
there is a trade-o?® in choosing the sleep period, as was pointed out in [20].
We will analyze the e?®ect of this trade-o?® in Section 4. Despite this trade-o?®,
the low power listening feature can significantly extend the lifetime of WSNs
deployed for always-on applications because communication is rare (less than
10 packets every minute) in most of them. In Section 4.4, we show that by
using LPL we can extend the lifetime of ExScal by 2.6 times.
2.2 Hierarchical Sensing
The concept of hierarchical sensing was originally introduced in [9] under the
name of energy-quality hierarchy. Here, we provide a more general definition
of the concept and identify its defining characteristics so that it can be used
in other always-on applications.
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