The smart modem

I think I've mentioned occasionally that various devices, mostly cellular modems, just use the Hayes or AT command set. Recently I obtained a GPS tracking device (made by Queclink) that is, interestingly, fully configured via the Hayes command set. It's an example of a somewhat newer trend of converging the functionality of IoT devices into the modem baseband. But what is this Hayes command set anyway?

The Hayes command set is a fascinating piece of technology that's been hanging around for far longer than most likely even its creators thought it would.