6: ZigBee Initialisation and Operation > Message Propagation

Message Propagation

The way that a message propagates through a ZigBee network depends on the network topology. However, in all topologies, the message usually needs to pass through one or more intermediate nodes before reaching its final destination. The message therefore contains two destination addresses:

• Address of the final destination
• Address of the node which is the next “hop”

The way these addresses are used in message propagation depends on the network topology, as follows:

Star Topology

All messages are routed via the Co-ordinator.

Both addresses are needed and the “next hop” address is that of the Co-ordinator.

Tree Topology

A message is routed up the tree until it reaches a node that can route it back down the tree to the destination node.

Both addresses are needed and the initial “next hop” address is that of the parent of the sending node. The parent node then resends the message to the next relevant node - if this is the target node itself, the “final destination” address is used. The last step is then repeated and message propagation continues in this way until the target node is reached.

Mesh Topology

In this case, the propagation path depends on whether the target node is in range:

• If the target node is in range, only the “final destination” address is used.
• If the target node is not in range, the initial “next hop” address is that of the first node in the route to the final destination. The message propagation continues in this way until the target node is reached.

	Application programs in intermediate nodes are not aware of the relayed message or its contents - the relaying mechanism is handled by the ZigBee stack.

The message propagation methods for the different topologies are illustrated by the animations below.

Star Topology

Tree Topology

Mesh Topology

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