Forming a ZigBee Network
The Co-ordinator is responsible for starting a ZigBee network. Network initialisation involves the following steps: 1. Search for a Radio Channel
The Co-ordinator first searches for a suitable radio channel (usually the one which has least activity). This search can be limited to those channels that are known to be usable - for example, by avoiding frequencies in which it is known that a wireless LAN is operating. 2. Assign PAN ID
The Co-ordinator starts the network, assigning a PAN ID (Personal Area Network identifier) to the network. The PAN ID can be pre-determined, or can be obtained dynamically by detecting other networks operating in the same frequency channel and choosing a PAN ID that does not conflict with theirs. At this stage, the Co-ordinator also assigns a network (short) address to itself. Usually, this is the address 0x0000. 3. Start the Network
The Co-ordinator then finishes configuring itself and starts itself in Co-ordinator mode. It is then ready to respond to queries from other devices that wish to join the network. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%Joining a ZigBee Network Once the network has been created by the Co-ordinator, other devices (Routers and End Devices) can join the network. Both Routers and the Co-ordinator have the capability to allow other nodes to join the network. The join process is as follows: 1. Search for Network
The new node first scans the available channels to find operating networks and identifies which one it should join. Multiple networks may operate in the same channel and are differentiated by their PAN IDs. 2. Select Parent
The node may be able to ‘see’ multiple Routers and a Co-ordinator from the same network, in which case it selects which one it should connect to. Usually, this is the one with the best signal. 3. Send Join Request
The node then sends a message to the relevant Router or Co-ordinator asking to join the network. 4. Accept of Reject Join Request
The Router or Co-ordinator decides whether the node is a permitted device, whether the Router/Co-ordinator is currently allowing devices to join and whether it has address space available. If all these criteria are satisfied, the Router/Co-ordinator will then allow the device to join and allocate it an address.
| Typically, a Router or Co-ordinator can be configured to have a time-period during which joins are allowed. The join period may be initiated by a user action, such as pressing a button. An infinite join period can be set, so that child nodes can join the parent node at any time.
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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,...
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