AT commands

AT commands (also known as the Hayes command set ; AT comes from the English attention - “attention”) - a set of commands developed in 1977 by Hayes for its own design, the Smartmodem 300 baud modem . A command set consists of a series of short text strings that combine to form complete operation commands, such as dialing, starting a connection, or changing connection parameters.

In order for the modem to recognize these commands, they must be recorded in a specific form. Each team always begins with the letters AT or at (from English AT tention , for which they got their name), supplemented by one or more commands and terminated at the end by pressing the ↵ Enter key. Commands are accepted by the modem only when it is in “command mode” or offline.

AT commands are usually sent to the modem via communication software, but can also be entered manually by the user from a computer keyboard.

The command set turned out to be a very successful solution and as a master setting of a Hayes-compatible modem ^[1] , it is used for its optimal functioning for various purposes, under different conditions: with different telephone line conditions, frequency response of the line , noise, frequent spark interference etc.

The standardization of the Hayes (and AT-commands) instruction set was expressed in a document called Data Transmission Systems and Equipment - Serial Asynchronous Automatic Dialing and Control, known as TIA / EIA- 602. Further, ITU-T has released a V.250 document (also known as V.25ter), containing all the TIA / EIA-602 commands and several additional ones. After which TIA / EIA-602-A was released containing only references to V.250 and a small historical background ^[2] .

To configure (activate, deactivate and reconfigure settings) modem, Hayes commands can use different modem profiles . The set of commands and the architecture turned out to be quite successful and were repeatedly expanded and supplemented. ETSI has released a number of standards describing the management of mobile phones and GSM modems, such as GSM07.05 and GSM07.07. Some manufacturers of communication equipment complement the standard set of AT-commands with their own extensions.