However, since nodes are not aware of
each others??™ schedule, the cost of contacting a sleeping node might be higher.
In addition, care has to be taken that wakeup does not aversely a?®ect the
medium access protocol or vice-versa, given the fact that time division cannot
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Curt Schurgers
Fig. 9. Operation of TDMA-W.
be used to create an out-of-band mechanism in this case, as this would require
inter-node synchronization.
5.1 Design Alternatives
A variety of protocols have been proposed that can be classified as asynchronous
wakeup solutions. It is possible to cast these into a unified framework
consisting of three basic design alternatives: rendezvous-based, sender-based
and receiver-based asynchronous wakeup. In this subsection, these three alternatives
are discussed from a high-level point of view to highlight their
fundamental behavior and mutual di?®erences. Specific instantiations of these
alternatives also di?®er in how wakeup is integrated into the MAC protocol,
the discussion of which is the subject of Section 5.2.
Rendezvous-based wakeup evolved as an asynchronous version of the IEEE
802.11 power saving mode, which was discussed in Section 4.1 [22]. It was
observed that for relatively dense multi-hop ad hoc networks, such as most
sensor networks, the synchronization required in IEEE 802.11 power saving
mode is hard to maintain. Instead, the times a node sends its beacon and
listens to the channel should be chosen carefully in such a way that nodes can
hear each others??™ beacons within a certain bounded period of time, even if they
are completely non-synchronized.
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