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The AT command set is a text-based interface for controlling a modem or mobile device: a host sends commands beginning with AT, and the equipment returns a response or status. In cellular devices, the phrase can cover both standardized basic modem commands and cellular-specific extensions; the exact commands available depend on the standards and the device’s implementation.
What is the AT command set?
“AT” introduces a command sent from terminal equipment—such as a computer or embedded host—to a modem or mobile equipment. The device interprets the text and may configure a feature, report a value, or perform an operation. It then returns a response; common final responses include OK and ERROR, although command-specific responses and error forms vary.
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For cellular equipment, this is a host-to-device control interface, not a universal list of commands that every modem must support. ETSI describes 3GPP TS 27.007 as a profile for controlling Mobile Equipment functions and GSM network services from Terminal Equipment through a Terminal Adapter. That is the useful mental model: the host sends command text through an interface, and the device responds with results or status.
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Which standards define AT commands?
The phrase can refer to multiple related command families. The standards divide responsibility rather than defining one all-purpose command set.
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| Standard | What it covers |
|---|---|
| ITU-T Recommendation V.250 | Standardized basic modem commands. |
| 3GPP TS 27.007 | AT command profile for cellular User Equipment (UE), including cellular-specific extended commands. Its Release 17 specification says standardized basic commands are found only in ITU-T Recommendation V.250. |
| 3GPP TS 27.005 | A separate specification referenced for Short Message Service (SMS) commands; SMS commands should not be attributed wholesale to TS 27.007. |
The 3GPP specification portal identifies TS 27.007 as “AT command set for User Equipment (UE)” and notes that it is under change control. Standards define profiles and syntax, but device firmware determines which commands and parameters a particular product actually implements.
How do extended AT command forms work?
Extended commands use forms for configuring a feature, reading a value, checking supported parameters, or performing an action. These forms are common patterns, not a guarantee that every command has every form; use the applicable standard or the device’s command reference for exact syntax.
| Form | Purpose | Typical ending |
|---|---|---|
| Set | Supplies parameter values to configure a feature. | =<values> |
| Read | Requests the current value of a parameter-type command. | ? |
| Test | Checks whether a command is supported and asks what parameter types it accepts. | =? |
| Action | Performs an operation rather than storing parameter values; an action command has no read form. | Command-specific; no read form |
For example, a command may have a set form to change a setting, a read form ending in ? to report its current value, and a test form ending in =? to return supported parameter types. These are illustrative patterns only: consult the relevant command definition before sending one to equipment.
Why command support and responses vary
A standards profile is not proof that a modem implements every command or parameter in it. Manufacturers may support only a subset, interpret optional features differently, or add vendor-specific extensions. Check the modem’s own AT command reference and distinguish standardized commands from vendor-specific ones before relying on a command in software.
Response formats can also evolve. The Release 17 specification warns that future versions may add response parameters. A robust parser should process all returned parameters it understands and tolerate additional ones, rather than assuming every response will always contain exactly the same number of fields. This makes software less likely to fail when firmware or a standards version changes.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What security risks should you consider?
Some AT commands can carry or reveal sensitive information, including passwords, subscriber identities, location, equipment identifiers, or security-related values. ETSI TS 127 007 V17.6.0 (3GPP TS 27.007 version 17.6.0, Release 17; August 2022) flags these concerns; its examples are not exhaustive.
Quick Recap
- Restrict access to serial ports and software interfaces that can send commands to a modem.
- Avoid recording sensitive command text or returned values in application logs unless there is a clear need and appropriate protection.
- Review both commands sent and responses received when assessing what an interface or diagnostic tool may expose.
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