h2. AT

The AT protocol comes from the "Hayes command set":https://en.wikipedia.org/wiki/Hayes_command_set used in early Internet modems.

It has been standardized by various standard bodies like the ITU, which then handed it over to the 3GPP. It's now the 27.007 standard at the 3GPP.

In practice device manufacturers often don't respect the standard, and instead do custom changes.

However as their implementation is still based on the standard it's still relatively easy to adapt the userspace modem stack to various modem from various manufacturers.