This is done via an ATIM frame. Possible optimizations,
which are not part of the current standard, are dynamic adjustment of
the ATIM window size to the channel contention level, and allowing nodes to
return to the sleep state after finishing their data transfer [21].
This power saving protocol critically relies on maintaining synchronization
between nodes. In dense multi-hop ad hoc networks, where not all nodes can
hear each other, the desired level of synchronization is hard to achieve [22].
This has led to synchronous variants of this standard, which will be discussed
in Section 5.1.
Fig. 6. IEEE 802.11 power saving mode without access point support.
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Chapter 8 Wakeup Strategies in Wireless Sensor Networks
Another relevant standard is IEEE 802.15.4, which targets low-rate wireless
personal area networks [23]. Two di?®erent types of devices are considered
here: FFDs (full function devices), which can act as coordinators, and RFDs
(reduced function devices), which can only be regular network devices and not
take on coordinator functionalities. Network devices are allowed to sleep and
decide independently when to wake up and poll for data from their coordinator.
This allows for very power e?±cient operation for these network devices,
but they can only act as leaf nodes in a network topology. Two di?®erent
modes are defined in the standard: beacon enabled and non-beacon enabled.
The beacon enabled mode allows coordinator devices to save energy through
duty cycling, as shown in Figure 7.
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