AT Commands Tutorial

What are AT Commands?

AT Commands, short for 'Attention' Commands, are the foundational protocol for communication between cellular (or RF) modems and their host controllers. These commands are crucial for controlling modem functions, managing network connections, and enabling features like remote diagnostics and firmware updates—key for modern IoT and 5G networks. Any firmware or hardware developer working with these modems will make use of these modem AT commands to essentially send instructions on various functions.

The AT Command set, now a universal standard for controlling cellular modems, was originally developed by Dennis Hayes in 1981 as part of the Hayes Command Set for the ‘Hayes 300 Baud Smart Modem’. This set has since evolved into a key tool in modem communication across a variety of devices and manufacturers.

Use of 3GPP AT Commands for GSM Modems

AT Commands are primarily used to configure, troubleshoot, and establish network connections with carriers across GSM, GPRS, LTE, and 5G modems. As the 5G ecosystem evolves, AT commands continue to play a critical role in managing network parameters like connection quality, roaming status, and advanced network features. These AT Commands for GSM allow developers to send and receive several device and network parameters from the modem such as the current network status indication, roaming, network technology currently being used (4G, NB-IoT, 2G, etc. ), and other such information. The data being transmitted is crucial during any debugging or developing activities.

The AT commands for GSM/IoT modules can be used to access a few crucial services such as:

• Device and SIM information
• SMS & MMS services
• Fax services (if supported)
• Voice & data services

Even today, AT Commands, are still being used to establish and configure network connections on modern 5G modems as well.

How are AT Commands being communicated?

AT Command instructions are single-line commands sent sequentially. The user must wait for the modem's response before sending the next command. In modern modems, these commands can be sent through UART, USB, or even wirelessly over the network for remote diagnostics, providing greater flexibility for IoT applications. To send AT Commands and receive responses, a host device (e.g., microcontroller or single-board computer) typically communicates with the modem through a UART (Universal Asynchronous Receiver/Transmitter) or USB, depending on the modem type. Modern solutions also support wireless communication for remote diagnostics. A UART is a two-wire communication protocol that allows two devices to communicate with each other in a half-duplex fashion. In newer modems, the AT Command instructions can be directly sent across via a USB connection or even wirelessly over the network for remote diagnostics.

List of AT Commands -Syntax and Types

Similar to a programming language syntax, AT Commands also have syntaxes that the user has to follow in order to send valid commands to the device. The general syntax for a command is as follows:

AT<COMMAND><SUFFIX><DATA>

Each AT Command must begin with the prefix “AT” to activate the modem’s command function. This is followed by the actual command, an optional suffix to indicate the command type or mode, and any relevant data (if required). The data field is not always necessary for all commands, making this structure versatile across different modem functions. Additionally, the relevant data (if required) follows the command. Note that the data field is not necessary for all commands.

This structure is known as the command line structure and is terminated by a single carrier return or the user can send multiple commands in a single line separated by a semi-colon.

GSM AT Commands Types

AT Commands can be broadly classified into two sets, Basic Commands, and Extended Commands.

Basic AT Commands

"Basic AT Commands do not begin with the ‘+’ symbol. Examples include commands like 'D' (Dial), 'A' (Answer), 'H' (Hook Control), and 'O' (Return to online data state).

Extended AT Commands

Extended Commands are those that start with the “+” symbol. All AT Commands for GSM fall under this category and a few of the common commands are +CMGS (Send SMS message), +CMGL (List SMS messages), and +CMGR (Read SMS messages).

Further, the AT Commands are of 4 categories primarily - Test Commands, Read Commands, Set & Execution Commands.

Classification of AT Commands

The response given by the modem is known as the ‘Result Code’. This code lets the user know the status of the given command response. An ‘OK’response signifies that the command has been executed successfully while an ‘ERROR’response may be returned for the following reasons:

• Syntax of the AT Command is Incorrect
• The modem is incompatible with the AT Command issued
• The modem is currently unable to process the command (eg. when trying to change settings while the modem is active and transmitting)

Test Commands

These are primarily used to check whether the given command is supported by the modem or not. Depending on the specific modem, some AT Commands may or may not be supported due to hardware or software limitations. The test commands allow the user to confirm the support of the modem before pushing the instruction.

ATD stands for Dial Command and ATD=? Is used to check if the modem supports Dialing functionality.

Read Commands

Read Commands are used to extract information from the modem. This information can be regarding certain parameters or settings that have been configured in the modem or even network-related parameters.

Set Commands

Set Commands are used to modify and configure modem parameters/settings. This is mostly used during the initial setup of the modem

Execution Commands

Execution Commands are used to perform an operation with or on the modem. This command is the most common type of command.

Most Commonly used AT Commands Lists

AT Command with Examples

Although different vendors have different lists of AT Commands for their specific modems, there are two common AT Command sets that are used by all

• Universal AT Commands:Common commands can be used on any modem.
• Proprietary AT Commands:Specific commands developed by the modem’s vendor for use with their modems.

There are 7 basic universal AT Commands that can be easily familiarized with for any modem.

AT

This is the most basic AT command that lets the user check whether a connection has been established between the modem and the host system. If the interface is correct, the modem returns “OK” else, it will return an “ERROR” if there is a misconfiguration in the connection, and in cases where the physical connection is not correct, no response would be received.

+CMGF

The AT+CMGF command configures the SMS mode of the modem. The mode can be set to ‘0’ for PDU mode or ‘1’ for text mode, enabling text-based or hexadecimal encoding for SMS operations. The text-mode operation is quite simple with a direct textual interface but is quite limited by its capabilities. PDU on the other hand allows the user to gain more detailed access to the SMS service with HEX values being used in place of plain text.

+CMGW

The +CMGW command is used to store a message in the SIM card of the modem. After executing the command, the ‘>’ sign will be displayed and the user can enter the message to be stored. These messages are stored in a serial fashion.

CMGS

The CMGS command allows the user to send a saved SMS message from the SIM card.

ATD

ATD is the simplest command to dial a provided number.

ATA

ATA is a command used to answer any incoming calls to the modem. The call will be denoted by a message “RING” which is repeated at every ring of the incoming call. If the user does not answer/the call ends, “NO CARRIER” will be displayed.

ATH

ATH is a command used to disconnect a remote user connects to the modem.

List of GSM Modem AT Commands

Major AT Commands for GSM modems areas listed below. Not all commands are supposed by all modems due to hardware or software limitations

AT Commands for Testing

AT Commands for Call Control

AT Commands for Data Card Control

AT Commands for Computer Data Interface

AT Commands for Service

AT Commands for Network Communication Parameter

Miscellaneous AT Command Examples

AT Commands for SMS Text mode

AT Commands for SMS PDU mode

New AT Commands for 5G and IoT Modems

With the rise of 5G and NB-IoT, new AT commands are evolving to manage advanced features like low-power modes, secure connections, and optimized network usage. Examples of new commands include:

• AT+NRIND

  Retrieves 5G New Radio (NR) signal strength and connection status.

• AT+CSQ5G

  Measures 5G signal quality.

• AT+CGACT

  Activates/deactivates PDP contexts for NB-IoT or LTE-M devices.

Wireless AT Commands and Cavli Hubble IoT Platform

Over the years of modem technology, AT Commands and their implementations have mostly stayed stagnant, and the requirement of physical access in order to push the commands to modems has been proven cumbersome in situations where the device has already been deployed.

Cavli’s SmartModules now feature the ability to receive AT Commands remotely via the Cavli Hubble IoT platform, enabling device configuration and diagnostics over cellular networks, even after deployment. This allows developers to send AT Commands to deployed devices across the world through the Hubble platform which allows for easy and quick device diagnostics and configuration through an easy-to-use terminal interface on the Hubble platform.