Upon receiving this request, A
listens to the ???send??? slot of B which is used to communicate the data. Each
node is assigned a send slot such that there are no collisions between data
208
Chapter 8 Wakeup Strategies in Wireless Sensor Networks
Fig. 8. Comparison between S-MAC, T-MAC and wakeup.
packets. In the wakeup regime, where there is very little tra?±c, a node essentially
only wakes up in its assigned wakeup slot. In principle, the time
between wakeup slots could be as large as allowed by the wakeup delay to
minimize the power consumption [25]. However, the actual data transmission
might incur an extra delay waiting for the send slot. Although no provisions
are incorporated in TDMA-W, the number of send slots could also be made
adaptive similar to what is done in TRAMA to accommodate fluctuating traf-
fic demands. With appropriately chosen period, TDMA-W behaves as a true
synchronous wakeup protocol. Its asynchronous counterpart, STEM [13], will
be discussed in Section 5.1. The extra overhead of a synchronous solution is
the need to maintain synchronization. Once data has to be transferred and a
MAC protocol is needed, TDMA is a logical choice as it is both energy e?±-
cient and can leverage the fact that synchronization between nodes is already
maintained.
5 Asynchronous Wakeup
Asynchronous wakeup solutions do not su?®er from the overhead of maintaining
synchronization, and could therefore result in lower power consumption as
compared to synchronous algorithms.
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