AT Commands in 2025: Your Guide to Cellular IoT Connectivity

From setting up data connections and verifying SIM information to sending SMS messages or debugging connectivity, AT commands form the foundation of communication between your device and the cellular network.

Whether you are building a new IoT solution or managing thousands of deployed devices, mastering AT commands ensures that you can optimize connectivity, reduce downtime, and make the most of your Com4 IoT SIM.

Table of Contents

What Are AT Commands

Why AT Commands Matter for IoT Connectivity

AT Commands in Practice

AT Command Standards and Manufacturer Differences

AT Commands and IoT Standards

How AT Commands Work

Common AT Command Responses

AT Command Quick Reference

Core AT Commands for IoT Connectivity

Security and Access Control

Advanced Debugging with AT Commands

Optimizing AT Command Usage with Com4

The Future of AT Commands in IoT

Final Tips for IoT Developers

What Are AT Commands

AT commands, short for “Attention commands,” were originally designed for Hayes modems in the 1980s. Over time, they evolved into a universal language for controlling cellular modules used in IoT, M2M, and embedded systems.

Each command begins with “AT” to get the modem’s attention, followed by specific instructions.

Example:

AT+CREG?

This command checks whether the device is registered on the cellular network.

AT commands are sent through a serial interface such as UART or USB, and the device responds with simple text outputs like OK, ERROR, or diagnostic codes.

Why AT Commands Matter for IoT Connectivity

For IoT devices, AT commands are the foundation of every cellular action. They allow you to:

• Configure APN and network settings.

• Attach or detach from the cellular network.

• Establish LTE, LTE-M, or NB-IoT data sessions.

• Check signal quality and registration status.

• Send and receive SMS messages.

• Run diagnostic checks and update firmware.

AT commands provide direct, programmable control over how your IoT device behaves on the network.
When using Com4 connectivity, AT commands are essential for validating device behavior, debugging network issues, and automating registration and reconnection processes.

AT Commands in Practice

AT commands are used widely across industries in real IoT environments.

Smart metering: Reboot or query meters remotely when communication timeouts occur.
Fleet management: Monitor SIM status and signal strength across vehicles.
Industrial automation: Perform diagnostics or firmware updates without field visits.
Environmental monitoring: Trigger recalibration or data transmission based on sensor thresholds.

Integrating AT command control into firmware or backend systems improves reliability and reduces operational costs.

AT Command Standards and Manufacturer Differences

Most cellular modules follow these standard sets:

• ITU T V.250, the original modem command set

• 3GPP TS 27.007 and 27.005, which extend support for GSM, UMTS, LTE, LTE M, and NB IoT

Each manufacturer, such as Teltonika, Quectel, Telit, and u blox, may include proprietary commands for specific functions.

Examples:

• Teltonika: AT+UPING for ping tests
• Quectel: AT+QIOPEN for TCP or UDP socket connections

Always refer to the manufacturer’s AT Command Manual for compatibility details and supported features.

AT Commands and IoT Standards

AT commands are closely aligned with global IoT standards such as:

• GSMA SGP.32 for IoT eSIM, enabling remote SIM profile control.
• 3GPP Releases 15 and later, introducing commands for LTE M and NB IoT low power operation.
• OMA LwM2M, integrating AT command functionality for large scale device management.

Understanding these standards ensures long term compatibility and efficient operation across diverse IoT ecosystems.

How AT Commands Work

An AT command typically follows this structure. In general all commands starting with "C" are 3GPP standardised. Other commands are usually vendor proprietary:

Command types:

• Set command (=): sets configuration
• Read command (?): reads current settings
• Test command (=?): lists supported options
• Execution command: performs an immediate action

Example:

AT+CREG?

+CREG: 0,5

OK

Meaning: the device is registered and roaming.

Common AT Command Responses

Unsolicited Responses (URCs):Automatic notifications such as +CREG: 5 inform the host device in real time about changes in network registration or SIM state.

AT Command Quick Reference

Note: The commands AT+URDFLT=1 (enables roaming) and AT+UPING="8.8.8.8" (tests data connectivity) are u-blox-specific and not part of any official AT command standard. These are referring to u-blox modems.

Core AT Commands for IoT Connectivity

Checking Connectivity and Signal Strength

AT             // Communication test  of the AT interface 

AT+CSQ         // Signal check  

AT+CREG?       // GSM registration  

AT+CEREG?      // LTE registration  

Setting Up a Data Connection

Define APN:

AT+CGDCONT=1,"IP","com4"

Attach to network:

AT+CGATT=1

Activate data session:

AT+CGACT=1,1

Verify connection:

AT+CGATT?

Expected response: +CGATT: 1

Reading Modem and SIM Information

Note: The commands AT+GMM (returns the module model, e.g., TRB255) and AT+GMI (returns the manufacturer, e.g., Teltonika Networks) are Teltonika-specific implementations of standard AT commands. Their output may vary depending on the device and should be referenced only in the context of Teltonika modems.

Pinging a Server

AT+UPING="8.8.8.8"

Note: The command AT+UPING="8.8.8.8" is u-blox-specific and not part of any official AT command standard. It is used to test data connectivity by sending a ping to a specified IP address (in this case, 8.8.8.8). This command should only be referenced when working with u-blox modems.

Used to test data connectivity and network reachability.

TCP and UDP Socket Communication

AT+USOCR=6     // Create TCP socket  

AT+USOCO=0,"example.com",80  

AT+USOWR=0,20,"GET / HTTP/1.1\r\nHost: example.com\r\n\r\n"  

AT+USORD=0,100  

Note: The commands used for TCP and UDP socket communication — for example,

AT+USOCR=6 (create TCP socket), AT+USOCO=0,"example.com",80 (connect to a remote host), AT+USOWR=0,20,"GET / HTTP/1.1\r\nHost: example.com\r\n\r\n" (send data), and AT+USORD=0,100 (read data) — are u-blox-specific AT commands. These are not part of any official AT command standard and should only be referenced when working with u-blox modems.

Security and Access Control

• Restrict remote AT access to authorized hosts
• Encrypt communication using TLS or SSL where possible
• Disable sensitive AT operations without authentication
• Use Com4 SIM level access control for enhanced protection

Advanced Debugging with AT Commands

If connectivity issues occur, follow this diagnostic sequence:

AT+CMEE=2  

AT+CSQ  

AT+CEREG?  

AT+CGDCONT?  

AT+CGATT?  

Common issues and actions:

Optimizing AT Command Usage with Com4

Com4’s IoT connectivity platform enhances AT command operations through real time SIM and signal monitoring. These capabilities reduce downtime and simplify IoT fleet management.

The Future of AT Commands in IoT

As IoT evolves with 5G, iSIM, and edge computing, AT commands will continue to expand their capabilities. Com4 ensures that customers remain ready for emerging technologies and evolving global standards.

Common Mistakes To Avoid

Even experienced engineers can face issues when handling AT commands, especially in large IoT deployments. These mistakes often cause unstable connectivity, higher power usage, or silent failures that are hard to trace.

• Sending the next command before receiving OK
  Many devices push commands too quickly without waiting for a clear OK response. This can confuse the modem and lead to failed registrations or broken data sessions.
• Using fixed delays instead of checking registration status
  Relying on time based delays rather than commands like AT+CEREG? can result in unreliable behaviour. Network response times vary, especially in roaming or low coverage areas.
• Ignoring ERROR responses
  Some firmware skips over ERROR replies instead of reacting to them. This prevents proper recovery and makes troubleshooting difficult in the field.
• Not logging AT command responses
  Without storing raw AT responses, diagnosing issues becomes guesswork. Clear logs help teams identify whether the problem lies in the SIM, network, or device logic.

Final Tips for IoT Developers

• Add retry logic and small delays between commands.

• Monitor unsolicited responses for live status updates.

• Always use the correct APN (com4).

• Keep device firmware updated.

• Automate regular signal and registration checks.

AT commands remain the universal interface for managing cellular IoT connectivity. By mastering them, developers can configure modems, optimize performance, and ensure reliable communication at scale.

With more than a decade of experience, Com4 provides the tools, expertise, and infrastructure needed for secure, high performance, and future ready IoT connectivity.

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