Once a node has become active, it can participate in data communication.
Consequent channel access is governed by the MAC protocol. Several MAC
protocols that were designed for sensor networks are able to adapt to the
tra?±c load [10], essentially approximating equation (1). As was shown in Figure
3, the MAC protocol operates during bursts of tra?±c activity. In principle,
during periods of very low tra?±c activity, the regular MAC protocol could be
used as well, adjusting its parameters to such low expected rates.
However, as explained earlier through equation (3), if the tra?±c is very
sparse the protocol settings can no longer be governed by the tra?±c rate.
Instead the wakeup latency becomes the determining factor, as given in equation
(4). This state of very sparse tra?±c activity is called the quasi-dormant
state or the ???region of wakeup??? in Figure 3. In this state, the node might have
to perform some checks at certain intervals to make sure it can be contacted
within the delay bound, but the tra?±c rate is so low that in most intervals
the node will not have to wake up. This is simply another way of saying that
the behavior is mainly governed by equation (4) rather than equation (3).
Although in principle, a MAC protocol could be used in this quasi-dormant
state, it is often more e?±cient to utilize a dedicated wakeup protocol. The
reason is that in this state, only a protocol is needed to contact a sleeping
node and tell it to transition to a more awake state.
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