6: ZigBee Initialisation and Operation > Route Discovery

Route Discovery

The ZigBee stack network layer supports a “route discovery” facility in which a mesh network can be requested to find the best available route to the destination, when sending a message. Route discovery is initiated when requested by a data transmission request.

Route Discovery Options

There are three options related to route discovery for a mesh network (the required option being indicated in the message):

SUPPRESS route discovery: The message is routed along the tree.

ENABLE route discovery: The message is routed along an already discovered mesh route, if one exists, otherwise the Router initiates a route discovery. Once this is complete, the message will be sent along the calculated route. If the Router does not have the capacity to store the new route, it will direct the message along the tree.

FORCE route discovery: If the Router has the route capacity, it will initiate a route discovery, even if a known route already exists. Once this is complete, the message will be sent along the calculated route. If the Router does not have the route capacity, it will route the message along the tree. Use of this option should be restricted, as it generates a lot of network traffic.

Route Discovery Mechanism

The mechanism for route discovery between two End Devices involves the following steps:

1. A route discovery broadcast is sent by the parent Router of the source End Device. This broadcast contains the network address of the destination End Device.
2. All Routers eventually receive the broadcast, one of which is the parent of the destination End Device.
3. The parent Router of the destination node sends back a reply addressed to the parent Router of the source.
4. As the reply travels back through the network, the hop count and a signal quality measure for each hop are recorded. Each Router in the path can build a routing table entry containing the best path to the destination End Device.
5. Eventually, each Router in the path will have a routing table entry and the route from source to destination End Device is established. Note that the corresponding route from destination to source is not known – the route discovered is unidirectional.

	The choice of best path is usually the one with the least number of hops, although if a hop on the most direct route has a poor signal quality (and hence a greater chance that retries will be needed), a route with more hops may be chosen.

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