This
solution is adopted in STEM-B [13].
An alternative solution replaces the succession of beacons by a continuous
tone. Once a node A detects this tone, it wakes up, but unless it has a separate
channel available, it cannot respond to B. Node B therefore has to continue
the tone until A has surely heard it, i.e., for the duration of one period.
Also, nodes are not addressed individually with a tone, and all neighbors of
B therefore wake up. On the other hand, the listen duration to detect a tone
can be significantly shorter than that of a beacon [13], such that the sleeping
node only has to wake up for a very short amount of time [29]. This tonebased
approach is used in STEM-T [13]. It also forms the basis of pre-amble
sampling [30] [31] [32]. In preamble sampling, a packet is proceeded by a
long preamble, fulfilling the role of the wakeup tone. The receiver periodically
wakes up to sample the channel and detect the presence of this preamble.
alternative to TDMA-W, which is discussed in Section 4.2 and illustrated in
Figure 9. In both cases, sleeping nodes wake up periodically to check for a
beacon or tone. In the synchronous case, the sender knows the listen slot
of the receiver and therefore can limit the contacting overhead, however at
the cost of maintaining synchronization. Whether the reduction of contacting
overhead on the part of the sender outweighs the synchronization overhead
depends on the expected frequency of wakeup events.
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