IEEE 802.15.4 Networking Topologies

- overview, or tutorial about the basics of the IEEE 802.15.4 mesh networking topologies including the star network and peer to peer network

The IEEE 802.15.4 networking topologies provide a variety of ways in which the different nodes within a, IEEE 802.15.4 network can communicate.

By enabling a variety of IEEE 802.15.4 network topologies, it is possible to provide a high level of flexibility. In this way communication can be effected in the way that is most suitable for a given environment.

These different forms of networking, including mesh networking enable low power devices to link together and communicate over the required distances in a very reliable manner.

IEEE 802.15.4 network topologies

There are two main forms of network topology that can be used within IEEE 802.15.4. These network topologies may be used for different applications and offer different advantages.

The two IEEE 802.15.4 network topologies are:

  • Star topology:   As the name implies the start format for an IEEE 802.15.4 network topology has one central node called the PAN coordinator with which all other nodes communicate.
  • Peer to Peer network topology:   In this form of network topology, there is still what is termed a PAN coordinator, but communications may also take place between different nodes and not necessarily via the coordinator.

It is worth defining the different types of devices that can exist in a network. There are three types:

  • FFD:   Full Function Device - a node that has full levels of functionality. It can be used for sending and receiving data, but it can also route data from toher nodes.
  • RFD :   Reduced Function Device - a device that has a reduced level of functionality. Typically it is an end node which may be typically a sensor or switch. RFDs can only talk to FFDs as they contain no routing functionality. Thee devices can be very low power devices because they do not need to route other traffic and they can be put into a sleep mode when they are not in use.

    These RFDs are often known as child devices as they need other parent devices with which to communicate.
  • Coordinator:   This is the node that controls the IEEE 802.15.4 network. This is a special form of FFD. In addition to the normal FFD functions it also sets the IEEE 802.15.4 network up and acts as the coordinator or manager of the network.

These definitions were originally generated for use in Zigbee, but their use has now been introduced with IEEE 802.15.4 network terminology.

IEEE 802.15.4 star topology

In the star topology, all the different nodes are required to talk only to the central PAN coordinator. Even if the nodes are FFDs and are within range of each other, in a star network topology, they are only allowed to communicate with the coordinator node.

Having a star network topology does limit the overall distances that can be covered. It is limited to one hop.

IEEE 802.15.4 peer to peer topology

A peer to peer, or p2p network topology provides a number of advantages over a star network topology. In addition to communication with the network coordinator, devices are also able to communicate with each other. FFDs are able to route data, while the RFDs are only able to provide simple communication.

The fact that data can be routed via FFD nodes means that the network coverage can be increased. Not only can overall distances be increased, but nodes masked from the main network coordinator can route their data via another FFD node that it may be able to communicate with.

By Ian Poole

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