2 Classification
A substantial number of alternatives to implement the wakeup functionality
have been proposed for sensor networks. They all deal with the fundamental
question that was discussed in the previous section: What is the required functionality
such that sleeping nodes can be contacted with a bounded delay in the
case of very low tra?±c demand while minimizing the energy consumption?
Some solutions have been cast specifically as a wakeup protocol, working
in this low tra?±c regime of the network. Others have been proposed as more
generic MAC protocols, which gracefully move into conditions of very low
tra?±c and where they therefore can be considered as operating as wakeup
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Chapter 8 Wakeup Strategies in Wireless Sensor Networks
Fig. 4. Normalized energy - delay tradeo?®.
protocols. In this chapter, both these types are discussed jointly. However,
two important ways of classifying the di?®erent approaches are introduced first
in the next two subsections.
2.1 In-band versus Out-of-band
A first way to classify wakeup schemes is as either in-band or out-of-band.
In this case, the division refers to wakeup operating in the same band as the
regular MAC protocol or not. Often, not all nodes are always in a dormant
state at the same time. If one node is awake, it is running a MAC protocol,
while its dormant neighbor could rely on a wakeup protocol instead. Whether
both operate in the same logical channel (in-band) or utilize di?®erent channels
(out-of-band) critically a?®ects their mutual interference.
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