2: ZigBee Topologies > Node Types
Node Types
This page describes the types of node that are used in a ZigBee network. Reference will be made to the toplogies introduced on the previous page (Star, Tree, Mesh), but these topologies will be described in more detail later in this module.
The ZigBee standard has the capacity to address up to 65535 nodes in a single network. However, there are only three general types of node:
These node types are described below.
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These roles described below exist at the network level – a ZigBee node may also be performing tasks at the application level independent of the role it plays in the network. For instance, a network of ZigBee devices measuring temperature may have a temperature sensor application in each node, irrespective of whether they are End Devices, Routers or the Co-ordinator. |
Co-ordinator
All ZigBee networks must have one (and only one) Co-ordinator, irrespective of the network topology.
- In the Star topology, the Co-ordinator is the central node in the network.
- In the Tree and Mesh topologies, the Co-ordinator is the top (root) node in the network.
This is illustrated below, where the Co-ordinator is colour-coded in dark blue.
At the network level, the Co-ordinator is mainly needed at system initialisation. The tasks of the Co-ordinator at the network layer are:
- Selects the frequency channel to be used by the network (usually the one with the least detected activity)
- Starts the network
- Allows other devices to connect to it (that is, to join the network)
The Co-ordinator can also provide message routing (for example, in a Star network), security management and other services.
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In some circumstances, the network will be able to operate normally if the Co-ordinator fails or is switched off. This will not be the case if the Co-ordinator provides a routing path through the network (for instance, in a Star topology, where it is needed to relay messages). Similarly the Co-ordinator provides services at the Application layer and if these services are being used (for example, Co-ordinator binding), the Co-ordinator must be able to provide them at all times. |
End Device
End Devices are always located at the extremeties of a network:
- In the Star topology, they are perimeter nodes
- In the Tree and Mesh toplogies, they are leaf nodes
This is illustrated below, where the End Devices are colour-coded in light blue.
The main tasks of an End Device at the network level are sending and receiving messages. Note that End Devices cannot relay messages and cannot allow other nodes to connect to the network through them.
An End Device can often be battery-powered and, when not transmitting or receiving, can sleep in order to conserve power.
Router
Networks with Tree or Mesh topologies need at least one Router. The main tasks of a Router are:
- Relays messages from one node to another
- Allows child nodes to connect to it
In a Star topology, these functions are handled by the Co-ordinator and, therefore, a Star network does not need Routers.
In Tree and Mesh topologies, Routers are located as follows:
- In a Tree topology, Routers are normally located in network positions that allow messages to be passed up and down the tree.
- In a Mesh topology, a Router can be located anywhere that a message passing node is required.
However, in all topologies (Star, Tree and Mesh), Router devices can be located at the extremeties of the network, if they run applications that are needed in these locations - in this case, the Router will not perform its message relay function, unless in a Mesh network (see above).
The possible positions of Routers in the different network topologies are illustrated below, where the Routers are colour-coded in red:
Note that a Router cannot sleep.
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