Some of our customers have asked us for metrics that can be used to evaluate the quality of a given device's connection to their Zigbee network. For example, they would like to monitor all devices on the network and send an alert if a device has been moved out of range of its parent or placed in a challenging RF environment.

The Good News

Zigbee chip vendors typically provide access to LQI and RSSI data, which can be useful for evaluating these things.

Also, the Zigbee standards have been evolving to include more features such as the Diagnostics cluster, to enable the querying of LQI and RSSI data from other devices on the network.  

The Tricky Part

It's difficult to come up with strict rules around "acceptable" values for LQI and RSSI. Consider a few factors:

1. Zigbee has built-in retry mechanisms. Even with a poor LQI value, messages may be sent multiple times and (eventually) delivered properly. 
2. LQI is a calculated value. The formula to calculate LQI is determined by the chip vendor. It can even vary between different chip families from a single vendor. In other words, different devices may report LQI values differently under identical conditions. Silicon Labs offers a helpful explanation here.
3. You'll need to understand the network topology and check the quality of each "hop" from the coordinator to the device in question. For example, let's say that a motion sensor has joined a network as a child of light bulb. Then, the user turns the light switch off. The motion sensor has lost its parent and must find a new one. This may result in a significant drop in link quality.

MMB's Recommendations

If you are building a Coordinator, choose third-party devices with support for the Diagnostics cluster.

Install some Router devices, which serve as "repeaters" to improve the Zigbee mesh. Many mains-powered devices such as smart plugs will include this functionality. Talk to your MMB Account Manager to ask for suggestions that will work for your project.