This is illustrated in Figure 10. Within each
period, a node has a set of listen slots, at the beginning of which it may send
a beacon to contact another node. The vertical arrow in the figure denotes
the start of a wakeup event, i.e., node B has a packet destined for sleeping
node A. The slots are chosen such that A can hear a beacon from B at
least once within one period, for any o?®set between the node schedules. A
number of variations have been proposed, which di?®er in the way the listen
slots are selected [14] [22] [27]. The goal is to minimize the listen time within
each period while still guaranteeing the minimum overlap to hear at least one
beacon irrespective of the mutual schedule o?®set.
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Chapter 8 Wakeup Strategies in Wireless Sensor Networks
Fig. 10. Rendezvous-based asynchronous wakeup.
Sender-based asynchronous wakeup shifts the wakeup burden to the sender,
i.e., the node that wants to contact a sleeping neighbor. Figure 11 illustrates
the basic principle. A sleeping node, e.g., node A, periodically listens to the
channel for a short duration. If node B wants to contact A, it continuously
sends beacon messages until it receives a response from A, which can at most
take one period [13] [28]. The node that periodically wakes up therefore has
to listen long enough such that it can surely hear one beacon. The benefit of
using beacons is that it is possible to selectively wake up a specific node.
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