Selecting parents in a ZigBee Network

[#1]

When having several ZigBee Networks in the same area, and therefore several potential parents, it may become necessary to join one of them and discard the rest.

While having a mechanism to only accept joining devices when desired is the best method (like using a button to trigger the joining), it might not always be possible since the parent nodes could be commercial devices or another vendor’s product without this feature. Below are some mechanisms that could be used for this purpose.

In general, when searching for suitable parents, the process is as follows:

• ZDO of device to join sends a MAC scan request.
• The MAC layer starts scan.
• For every beacon it receives, it sends a beacon notify indication that is processed in ParseBeaconNotifincaiton() function from AppStackImpl.c
• The ParseBeaconNotifincaiton() function will add the relevant information in the discovery table and for this it needs a free entry, so it calls GetFreeEntryInDiscoveryTable() function with reuse parameter as FALSE.
• If the table is full, it will call GetFreeEntryInDiscoveryTable() with reuse set to TRUE to literally re-use low priority entries.
• When the MAC scan has finished, it will send a MAC scan confirm.
• When this reaches ZDO, the SearchForSuitableParent() function is called.

At this point, there are several approaches that could be used:

1. Use a specific Extended PAN ID to search only for a specific parent node
2. Use a specific PAN ID to prioritize the network’s ID
3. Search in a specific Channel where network is supposed to be operating in

All these parameters are configurable in ApplicationConf.h file of the project’s Configure Folder and used in SearchForSuitableParent() function to filter Discovery table entries. Nevertheless, those solutions are not always the best for all applications since it may require hard-coding the network’s parameters. Fortunately, BeeStack leaves all this open for any modification in case it is necessary.

In brief, if the discovery table gets full with suitable parents that you DO NOT want to use, you should update the "if(reuse)" statement of the GetFreeEntryInDiscoveryTable() function to replace an entry. In other words, if you think that the desired parent is not present in the discovery table (due to its size limitation or other reason), you should update the GetFreeEntryInDiscoveryTable() function to make sure discovery table contains only devices that are of interest to your node. Please note that the criteria used to select the desired parents is totally application specific.

As mentioned, it is always best having a way to trigger the joining such as a button so the rest of parents have permit join set to FALSE and therefore join only to the desired parent without having to implement custom code. Anyway, you may select the solution that meets your application’s requirements the most.