When the wakeup protocol is in-band, it will interact with the MAC protocols
since both are using the same channel. To avoid negatively impacting the
ongoing packet transmission, the wakeup protocol could be augmented with
some basic medium access provisions beyond the simple need to be able to
contact a sleeping device. An obvious example of this solution would be to use
the regular MAC protocol also in the dormant state. However, the drawback
is that these extra medium access provisions do not come for free and reduce
the energy e?±ciency when only very sparse tra?±c is present.
Another approach is to have the wakeup protocol ignore the presence of
other tra?±c and have the MAC protocol absorb the resulting interference
e?®ects. Since node wakeup is by definition a rare event, this solution might
be acceptable. However, in sensor networks, event occurrences are often cor-
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related when they are triggered by physical phenomena being sensed. As a
result, when some part of the network is actively forwarding information, it
is more likely that neighboring areas will need to be woken up and therefore
interfere with the data forwarding. As will be detailed further in Section 5.2,
when the node density is high, this e?®ect might be significant.
Out-of-band solutions avoid this interference by using di?®erent channels
for MAC and wakeup. An example is the STEM protocol [13]. These di?®erent
channels could take the form of di?®erent frequencies or time slots, orthogonal
codes or other established methods of creating disjoint communication channels.
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