At the start of a beacon interval, coordinators
send out beacons that advertise to their associated leaf nodes whether
tra?±c is pending for them. Upon hearing these beacons, the leaf node can
request data from its coordinator. The network device is not required to wake
up each beacon interval, and it can elect to skip listening to some of the
beacons to further lower its energy consumption. By imposing synchronized
operation in this beacon enabled mode, coordinators can also save power by
restricting communication to an active period (superframe duration) that is
only a fraction of the beacon interval.
At present, the inter-coordinator aspects of this mode are not covered in
the standard. Especially, synchronization between coordinators could lead to
issues similar to those discussed for IEEE 802.11, where synchronization is
hard to maintain for multi-hop ad hoc networks. The current power saving
provisions in the IEEE 802.15.4 standard are therefore mainly geared towards
specific network topologies. In star-like topologies, network devices are leaf
nodes and can be extremely low power. In multi-hop ad hoc networks, also
known as mesh networks, the backbone nodes need to be coordinators and
could be less energy constrained.
Fig. 7. IEEE 802.15.4 beacon enabled mode.
207
Curt Schurgers
4.2 Sensor Network Proposals
Several synchronized MAC protocols have been proposed specifically for sensor
networks.
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