3.1.2. IEEE 802.15.4 communications system
According to the IEEE 802.15.4 standard, two main topologies
may be considered: the star and the peer-to-peer topology. In the
star topology, the communication is established between nodes
and a single central controller, called the PAN Coordinator (from
Personal Area Network Coordinator). The PAN Coordinator is the
primary controller of the PAN network. In the peer-to-peer topology,
every network node can communicate with any other device
as long as they are in coverage. This topology may also be used
to interconnect independent networks. One special case of peerto-
peer topology is the cluster-tree network, which is the one
selected in our network deployment. It consists of clusters, each
one having a node denoted as Coordinator acting as a cluster head
with multiple devices depending on it. A PAN Coordinator initiates
the network and takes the master role. Then, the network is
formed by parent–child relationships, where new devices associate
as children with the existing PAN Coordinator. In peer-to-peer and
cluster-tree networks, some devices work as Coordinators which
may communicate with any other Coordinators within their range,
and propagate messages that are routed to Coordinators outside
their range. This is the reason why these nodes are always awake.
Fig. 5 illustrates our cluster-tree network topology scenario.