# AT commands

AT commands were first introduced by the company Hayes Communications and served to control and configure the Hayes modems. Over the years, these commands have developed as a kind of standard and are used by almost all modems, whatever the manufacturer. Many telephones, telephone systems and GSM phones can interpret and be controlled by some AT commands. These devices are often known as “Hayes-compatible”. If you are using AT commands, you no longer need to use device-specific drivers, because you can control the device functionality with the commands alone.

The term “AT command” has evolved from "attention" to the abbreviation "AT", which prefixes each command. The AT commands are basically divided into four classes:

•  The Basic Command Set
•  The Extended Command Set
•  The Special Command Set
•  The Register Command Set

The class of Basic Commands contains all of the commands for the basic control of modems, such as dialling or hanging up.

To dial a number, the command "ATD987654321" is sent to a modem. Here, the letter "D" behind "AT" stand for "Dial". This is followed by the actual number. In the other classes of the AT commands, among others, hardware-related settings or the reading and setting of register entries are possible.