A common challenge of this solution is that
the paging device does not always have the same transmission range as the
main data radio. This often limits the applicability of this technique in multihop
networks, but it may o?®er a valuable solution if infrastructure support is
available. On-demand paging is discussed in detail in Section 3 of this chapter.
An alternative to having a low-power paging device is duty cycling a regular
radio, essentially using the setup that was shown in Figure 2 earlier. When
a radio is in the sleep mode most of the time and only occasionally checks to
see if someone wants to communicate with it, this can e?®ectively emulate an
average low power behavior. This principle has formed the basis for both the
other two classes of schemes: synchronous and asynchronous. The di?®erence
between these two is whether neighboring nodes maintain some form of time
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Chapter 8 Wakeup Strategies in Wireless Sensor Networks
synchronization or not. This categorization has a parallel in MAC protocols
where designs are schedule-based or contention-based.
In general, synchronous wakeup schemes allow easier in-band integration
with MAC since this can be done in the time domain. The drawback is
that maintaining synchronicity under various levels of clock drift causes synchronization
overhead, which somewhat reduces the energy-e?±ciency of the
wakeup scheme. On the other hand, asynchronous solutions, although avoiding
synchronization overhead, cannot use the time dimension to limit interaction
with the MAC protocol.
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