What happens when it fails?

Start with the operation that failed

A sensor stopped reporting, or a new device will not join. Those are different failures, and the readings a controller keeps tell them apart. Last seen is when the device was last heard, which is not the same as healthy. A route says whether the network knows a way to the device. Retries count frames that had to be sent again. Signal strength and link quality are two readings, not one, and neither is a diagnosis.

Four faults, four patterns

If permit-join is closed, a new device is refused while every existing reading is normal. The network is healthy and is simply not accepting joins. Open the window and try again.

If a sleepy device's parent is offline, the child's data has nowhere to go. On the bench, last seen goes old, there is no route and retries climb, because the child keeps trying a parent that does not answer.

If the battery is flat, the device stops polling. On the bench, last seen goes old and there is no route, but there are no retries, because nothing is being sent. The last report is the last evidence.

If the channel is noisy, the signal strength looks fine. Link quality is low and retries pile up, while the route is still present. A reader who looks only at signal strength calls the link good.

Why distance is not the answer

Distance and signal strength describe how much of the transmitted power arrives. They say nothing about what else is on the channel, whether the parent is awake or whether the battery has any charge left. On the bench every one of the four faults looks acceptable by signal strength alone. The numbers are examples, and the pattern is the point.

An orphaned child that has lost its parent looks for another one. That takes time, needs a router with room to accept it, and is not guaranteed, which is why a device can be missing for a while and then return without anyone touching it.

Common mistakes

• Reading a strong signal as a working link. Interference raises signal strength at the receiver and lowers link quality at the same time.
• Treating last seen as a health check. It is the time of the last frame heard, and a flat battery and a lost parent look the same on it.
• Moving a device closer as the first fix. Distance is one possible cause among several, and the other readings say which.