These standards
not only introduce some of the common design choices that appear in other
synchronous solutions as well, but also relate to some of the asynchronous
schemes that will be treated in Section 5.
4.1 Standards
The IEEE 802.11 standard specifies both the physical and medium access
control sublayers for local area networks [20]. As such, it is not directly geared
towards sensor networks. However, its use has been proposed for more general
ad hoc networks, and the basic design principles are important background
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Curt Schurgers
for sensor network-specific solutions such as the ones that will be discussed in
the next subsection.
Figure 6 illustrates the basic behavior of the power saving mode of the
IEEE 802.11 standard, for the case of an ad hoc network without access point
support. Nodes do not keep their radio on all the time, but wake up periodically,
with a period called the beacon interval, which is very similar to
the principle shown in Figure 2. In such a beacon interval, the nodes are
only awake for a short interval called the ATIM (announcement tra?±c indication
message) window [20]. Since nodes are assumed to be synchronized,
their ATIM windows start at approximately the same time. At the beginning
of the ATIM window, nodes contend to send out a beacon frame, used for
synchronization purposes. After the beacon, a node, for example B, may instruct
another device C to remain active during the entire beacon interval for
a regular data exchange.
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