Expand description
Keeping a device on the air when the network stops answering.
A network that runs adaptive data rate moves a device to the fastest rate and lowest power that still reach it, which costs the device less battery and the network less air time. The risk is the other direction: a device moved up and then left there, in a spot the network can no longer hear, would go on transmitting into nothing forever.
So a device counts. Every new uplink raises a counter, any Class A downlink at all resets it, and once the counter passes a limit the device starts asking the network to say something. Past that it gives back what the network had tuned away, a step at a time, until either the network answers or the device is back where it started.
The two revisions of the specification step differently, and Backoff follows the one
it is given.
- LoRaWAN 1.0.3, section 4.3.1.1: after the limit plus a delay the device lowers its data rate, and again every delay after that. The request bit is not set at the slowest rate, since nothing could be done about the answer.
- TS001-1.0.4, section 4.3.1.1 and its table 9: the first step restores the default transmit power, each step after that lowers the data rate, and the step after reaching the default data rate re-enables the default channels and sets the repetition count back to one. The request bit is set throughout, and stops once that last step is done.
This is the device half, and it is the half the specification settles. Which rate to move a device to in the first place is a policy the specification deliberately leaves open, and it is not here.
The limit and the delay belong to the regional parameters rather than the link layer,
which names them without values. RP002-1.0.5 section 3.3 recommends 64 and 32 for every
region, and Backoff::recommended starts with those; Backoff::new takes others, for
a device commissioned with different ones.