The peer-to-peer topology also has a PAN coordinator; however, it differs from the star topology in that any
device is able to communicate with any other device as long as they are in range of one another. Peer-to-peer
topology allows more complex network formations to be implemented, such as mesh networking topology.
Applications such as industrial control and monitoring, wireless sensor networks, asset and inventory
tracking, intelligent agriculture, and security would benefit from such a network topology. A peer-to-peer
network allows multiple hops to route messages from any device to any other device on the network. Such
functions can be added at the higher layer, but they are not part of this standard.
Each independent PAN selects a unique identifier. This PAN identifier allows communication between
devices within a network using short addresses and enables transmissions between devices across
independent networks. The mechanism by which identifiers are chosen is outside the scope of this standard.
The network formation is performed by the higher layer, which is not part of this standard. However, 4.3.1
and 4.3.2 provide a brief overview on how each supported topology can be formed.
4.3.1 Star network formation
The basic structure of a star network is illustrated in Figure 1. After an FFD is activated, it can establish
its own network and become the PAN coordinator. All star networks operate independently from all other
star networks currently in operation. This is achieved by choosing a PAN identifier that is not currently used
by any other network within the radio communications range. Once the PAN identifier is chosen, the PAN
coordinator allows other devices, potentially both FFDs and RFDs, to join its network. The higher layer can
use the procedures described in 5.1.2 and 5.1.3 to form a star network.
4.3.2 Peer-to-peer network formation
In a peer-to-peer topology, each device is capable of communicating with any other device within its radio
communications range. One device is nominated as the PAN coordinator, for instance, by virtue of being the
first device to communicate on the channel. Further network structures are constructed out of the peer-topeer
topology, and it is possible to impose topological restrictions on the formation of the network.
An example of the use of the peer-to-peer communications topology is the cluster tree. The cluster tree
network is a special case of a peer-to-peer network in which most devices are FFDs. An RFD connects to
a cluster tree network as a leaf device at the end of a branch because RFDs do not allow other devices to
associate. Any FFD is able to act as a coordinator and provide synchronization services to other devices
or other coordinators. Only one of these coordinators is the overall PAN coordinator, potentially because
it has greater computational resources than any other device in the PAN. The PAN coordinator forms the
first cluster by choosing an unused PAN identifier and broadcasting beacon frames to neighboring devices.
A contention resolution mechanism is required if two or more FFDs simultaneously attempt to establish
themselves as PAN coordinators; however, such a mechanism is outside the scope of this standard. A
candidate device receiving a beacon frame is able to request to join the network at the PAN coordinator.
If the PAN coordinator permits the device to join, it adds the new device as a child device in its neighbor
list. Then the newly joined device adds the PAN coordinator as its parent in its neighbor list and begins
transmitting periodic beacons; other candidate devices are able to then join the network at that device. If the
original candidate device is not able to join the network at the PAN coordinator, it will search for another
parent device. The detailed procedures describing how a PAN is started and how devices join a PAN are
found in 5.1.2 and 5.1.3.
The simplest form of a cluster tree network is a single cluster network, but larger networks are possible by
forming a mesh of multiple neighboring clusters. Once predetermined application or network requirements
are met, the first PAN coordinator instructs a device to become the PAN coordinator of a new cluster
adjacent to the first one. Other devices gradually connect and form a multicluster network structure, such
as the one seen in Figure 2. The lines in Figure 2 represent the parent-child relationships of the devices and
802.15.4-2006 - Part 15.4: Low-Rate Wireless Personal Area Networks (LR-WPANs)
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