AT commands 2025: Essential guide for IoT devices

AT commands (or Hayes commands) form the language used to communicate with cellular modems. Whether you’re developing an IoT device, working with M2M applications, or trying to understand how your device talks to cellular networks, understanding “ATtention” or AT commands is essential.

In this comprehensive guide, I’ll walk you through what AT commands are, how they work, and provide practical examples to get you started.

What are AT commands?

AT commands are a specific command language originally developed for the Hayes Smartmodem in the 1980s. The name “AT” comes from the fact that each command line starts with “AT,” which stands for “Attention.”

AT commands became the standard, text-based way to communicate with modems through the computer via a serial terminal and have evolved to control modern cellular modems in phones, IoT devices, and other connected products.

A typical AT command might look like: AT+CREG? (which checks the network registration status).

Do different manufacturers have different AT commands?

You’re probably looking to execute commands to your manufactured device by SIMCom, Quectel, Gemalto, U-Blox, and Telit, just to name a few.

Good news: there’s a shared standard across the industry. However, the specific command set can vary between manufacturers and models. Examples include AT+QIOPEN for Quectel modules and AT+CIPPING for SIMCom modules.

Additionally, some commands in the standard are optional and may not be present on all modems.

The role of AT Commands in IoT connectivity

If you’re developing any device that needs cellular connectivity (2G, 3G, 4G, LTE, LTE-M, NB-IoT, and even for 5G modems, to name some), AT commands are your primary interface to the modem. They allow you to:

• Configure the modem
• Establish network connections
• Send/receive SMS messages
• Make voice calls
• Transfer data
• Query signal strength and network status
• Perform diagnostics

From setting the Access Point Name (APN), which is a logical name used to select the gateway to the external packet data network, to dialing a data connection, AT commands are the mechanism used to configure and control the modem to get online.

How do AT commands work?

Understanding how AT commands work requires looking at their syntax and interaction flow. Let’s go through them below.

AT commands syntax basics

AT commands have a specific syntax to function correctly, which is typically as seen below:

AT<COMMAND><SUFFIX><DATA>

The basic format is AT followed by the command, a suffix indicating the command type (e.g., setting a value, querying information), and sometimes additional data. Each command line ends with a carriage return, but multiple commands can be strung together on a single line using semicolons.

Core format and structure

• All commands start with "AT" (not case-sensitive) followed by an optional + and then the command:

E.g. AT+CREG (or at+creg? or At+CrEg?) and ATZ

• Arguments for an AT command are comma-separated

E.g. AT+CGDCONT=1,"IP","onomondo"

Setters, getters, and testers

Character/s	What it does	Example
=	Sets a value	AT+CGATT=1
?	Gets the current status	AT+CREG?
=?	Queries supported values/ranges for possible arguments	AT+CREG=?

Chaining Commands

Use of ;: AT commands which are to be executed sequentially right after each other can be delimited with a ; and only the first command should include AT

E.g. AT+CREG?;+GGREG?

AT Command responses

Response	What it means	Example
OK	Successful command execution	AT+CGATT=1
ERROR	Failed command	AT+CREG?
+CME ERROR: <code>	Extended error for mobile equipment
+CMS ERROR: <code>	Error related to SMS functionality
ABORTED	The command has been aborted (applies only to some modems)	AT+CREG=?
Other responses	When the responses above don’t apply, it could prompt for more data

According to U-Blox, before each transmitted AT command the device communicating with the modem should:

• Wait 20 ms
• Flush the buffer

This helps the device handle unsolicited result codes (URCs), which the modem sends for various events if enabled. A typical URC would be the one coming from network registrations both data- and phone/SMS wise.

Here’s what it looks like when you issue a command to register on the network — with URC and without URC:

Example: with URC after having issued a command to register on the network	at+creg=1 // enable URC for circuit-switched network registrations OK at+cops=0 // register to GSM/UMTS/EPS network with automatic network operator selection +CREG: 2 +CREG: 5 +CREG: 5 OK
Example: without URC after having issued a command to register on the network	at+cops=0 // register to GSM/UMTS/EPS network with automatic network selection OK at+creg? // query for circuit-switched network status +CREG: 0,5

2 types of AT commands

All AT commands generally fall into 2 categories: basic and extended.

• Basic AT commands: broadly speaking, if a command does not start with +, this is a basic command. 

• Extended AT commands: Conversely, those starting with + are considered extended commands (and generally, all GSM AT commands are considered extended commands).

Different categories of AT commands

The command types above generally classify them based on how you form the command (with or without + at the beginning), AT command categories classify them further based on what you want to execute on, which are:

• Test commands
• Read commands
• Set commands
• Execution commands

Category	What is it for	Suffix	Syntax	Example
Test	To query the modem about its capabilities or supported parameter ranges	=?	AT<COMMAND NAME>=?	ATD=?
Read	To read the current status or configuration of a setting	?	AT<COMMAND NAME>?	AT+CBC?
Set	To configure parameters or settings on the modem	=	AT<COMMAND NAME>=value1, value2, …, valueN	AT+CSCA=”+123456789”, 120
Execution	To trigger a specific action on the modem, such as dialing a number	none	AT<COMMAND NAME>=parameter1,…, parameterN	AT+CMSS=1,”+123456789”, 120

Practical AT commands examples

If you have a 3GPP/ITU-compatible cellular modem, chances are, these common AT commands are applicable. We will also cover some basic commands that are manufacturer-specific.

1. Standardized list of AT Commands: your cross-modem toolkit

First let’s familiarize ourselves with the AT commands involved in this process, their syntax, their values, and then we’ll see them in action later on in this guide.

A. Establishing a data connection

Let’s break this step down further into choosing, registering, defining PDP, activating/deactivating PDP context, and checking status.

AT+COPS

What is it for: Choose the network operator to use and register on the circuit-switched network.

Setting syntax	AT+COPS=[<mode>[,<format>[,<oper>[,<AcT>]]]
Example response	AT+COPS=?‘s response will be a list (denoted with [element_1, element_2, ..., element_n]) of element values.

Note: Even though it is not documented in the standards, this command also goes ahead and connects to the circuits-switched network of the chosen provider (phone/SMS connection, also called GSM connection in our network logs). As an additional warning from our engineers, telling your modems to attach/detach is not always a good practice.

AT+CREG

What is it for: Circuit-switched network registration status, which is the connection that provides the phone/SMS service.

Testing syntax	AT+CREG=<n>
Example response	+CREG: <n>,<stat>[,[<lac>],[<ci>],[<AcT>][,<cause_type>,<reject_cause>]]

AT+CEREG

What is it for: Registration status of the device for LTE or newer cellular network technologies.

Testing syntax	AT+CEREG=?
Example response	+CEREG: <n>

AT+CGREG

What is it for: Packet-switched network registration status.

Testing syntax	AT+CGREG=?
Example response	+CGREG: OK +CGREG: (0-2) OK

Note: Identical to AT+CREG but for the packet-switched network instead, i.e. data connection

AT+CGDCONT

What is it for: Define packet data protocol (*PDP) context

Setting syntax	AT+CGDCONT=<cid>,<PDP_TYPE>,<APN>
Read syntax	AT+CGDCONT=<cid>,<PDP_TYPE>,<APN>,<PDP_addr>

For both syntaxes the values are:

<cid>: integer type; specifies a particular PDP context definition (you can multiple PDP context)

<PDP_type>: string type; specifies the type of packet data protocol. The default value is manufacturer-specific.
  X.25          ITU-T/CCITT X.25 layer 3 (Obsolete)
  IP            Internet Protocol (IETF STD 5 [103])
  IPV6          Internet Protocol, version 6 (see RFC 2460 [106])
  IPV4V6        Virtual <PDP_type> introduced to handle dual IP stack UE capability. (See 3GPP TS 24.301 [83])
  OSPIH         Internet Hosted Octect Stream Protocol (Obsolete)
  PPP           Point to Point Protocol (IETF STD 51 [104])
  Non-IP        Transfer of Non-IP data to external packet data network (see 3GPP TS 23.401 [82]) Ethernet     
  Ethernet      protocol (IEEE 802.3)
  Unstructured  Transfer of Unstructured data to the Data Network via N6 (see 3GPP TS 23.501 [165])

<APN>: string type; a logical name that is used to select the GGSN or the external packet data network.

<PDP_addr>: string type; identifies the MT in the address space applicable to the PDP.

*A PDP context is a piece of information that is needed inside the mobile core in order for it to set up a data session. From the device point of view, this is where the access point name (APN) is set. The command in itself does not activate any signalling.

For EPS and 5G, this command instead sets the PDN connection and PDU session respectively.

AT+CGACT

What is it for: Activate/deactivate PDP context

Setting syntax	AT+CGACT=<state>,<cid>
Example response	(list of) <state>,<cid>

For both syntaxes the values are:

<state>: integer type; indicates the state of PDP context activation.
  0 deactivated
  1 activated

<cid>: integer type; specifies a particular PDP context definition (see the +CGDCONT and +CGDSCONT commands).

If an activation of a PDP context is issued and the modem is not connected to the packet-switched (PS) network then it will first attach to the PS network, this is equivalent to running AT+CGATT=1

AT+CGATT=<state> attaches or detaches to the PS network depending on the <state>

Seeing it in action

In order to get a data connection we have to do the following:

• AT – check the connection to the modem, should respond OK (this step is not strictly needed)
• AT+COPS=0 – choose network operator and register on the circuit-switched network
• Wait for AT+CREG to respond x,5
• AT+CGDCONT=1,"IP","onomondo" – define PDP context (this step could come prior to AT+COPS=0)
• AT+CGACT=1,1 – activate PDP context (will also call AT+CGATT=1, which is a network attach command if not already attached)
• Wait for AT+CGREG to respond x,5

This procedure could be used to test whether or not there are any problems in terms of just getting a connection.

This is the result using the SIM808 modem:

Since the SIM has already been attached at some point to the network operator a new authentication is not needed. We first see new signalling when the modem actually attaches to the packet-switched network, i.e. when AT+CGACT=1,1 is issued.

Why a new GSM/circuit-switched connection, which provides phone/SMS services, is not established is a bit of a mystery. In theory, we should see a new GSM connection once the AT+CREG? command responds with x,5.

B. Establishing a PPP Connection

For this step, most of the AT commands used are similar to what was used for the previous step (establishing a data connection), with the addition of this:

ATD<number>   # Dials the given number

In order to get a PPP connection we have to do the following:

• AT+COPS=0 – choose network operator and register on network
• Wait for AT+CREG and AT+CEREG to respond x,5
• AT+CGDCONT=1,"IP","onomondo" – define PDP context (as an example)
• AT+CGACT=1,1 – activate PDP context (will also call AT+CGATT=1, which is a network attach command if not already attached)
• ATD*99# – dial up provider (*99# is the standard number to dial up the operator)
• Start PPP protocol

This procedure is the only generic way of getting a usable connection from a device. The only issue with this approach is that the device has to support the PPP-, TCP- and UDP protocols – this is not viable for teeny tiny devices, but most microcontrollers which can run Mongoose OS, FreeRTOS, or the likes will be able to use it.

This procedure is actually what is being carried out by Linux’s PPP daemon, pppd, on the deveyes and watchdogs.

Also as a side note then some modems might require that the baud rate used with the serial connection be fixed to a specific value.

C. Getting the modem and SIM information

To get the information about the state of your modem and SIM, these AT commands are nice to know.

Let’s divide them into three:

• Reset commands
• Modem meta-data
• SIM and device identifiers

Resetting the modem profile

There are two ways of getting the modem into a factory setting:

ATZ[value]   # Resetting the modem into a user-specified profile

AT&F[value]  # Factory resetting the modem

The settings that will change are not clearly stated in the standard but check with the settings with your modem. Additionally, the reset will never trigger a change in the baud rate.

Get the modem meta-data

For more information about your hardware, here are some helpful AT commands:

ATI[value]   # Get the modem information  (e.g. SIM808 R14.18)

AT+GMI       # Determine the identity of the modem manufacturer (e.g. SIMCOM_Ltd)

AT+GMM       # Get the modem model (e.g. SIMCOM_SIM808)

AT+GMR       # Get the modem version (e.g. Revision:1418B03SIM808M32)

AT+CGMR      # Get firmware version

SIM and device identifiers

Use these commands to help you identify some basic SIM information:

AT+CIMI      # Get the International Mobile Subscriber Identity (IMSI)

AT+CGSN      # Get the International Mobile station Equipment Identity (IMEI)

AT+CCID      # Get the Integrated Circuit Card Identifier (ICCID) (Not standardized, check manufacturer)

D. Sending an SMS

To send a text message:

AT+CMGF=1                    # Set SMS to text mode (vs. PDU mode)

AT+CSCA="+1234567890"        # Set SMS center number if needed

AT+CSAS                      # Saves SMS settings, SMSC number to the SIM

AT+CMGS="recipient_number"   # Start SMS to recipient

> Your message text here     # Type message after '>' prompt
                             # Press Ctrl+Z to send 

Now, you can try it out using the AT+CMGS=<number> command, e.g.:

AT+CMGS="1234"
> saying hello from modem!

+CMGS: 11
OK

In Onomondo, the SMS will then appear in your webhook.

2. Manufacturer-Specific AT Commands (SIMCom and Quectel)

Other common commands, such as pinging a server and looking into TCP/UDP communication varies depending on the modem manufacturer. Our examples below are for SIMCom SIM808 and Quectel EC25-E and might not work for all of their modems but should at least work for modems on the same series.

A. Pinging a server

• For SIMCom modems:

In order to use any of the subsequent SIMCom’s AT commands an active PDP context is needed, but it has to be established using SIMCom’s own special AT commands12. Here’s the procedure:

AT+CSTT="your_apn"   # Start task with APN

AT+CIICR             # Bring up wireless connection

AT+CIFSR             # Get local IP address (mandatory)

AT+CIPPING=<host>[,<retryNum>[,<dataLen>[,<timeout>[,<ttl>]]]]
 # Ping Google's DNS server

The values for the AT+CIPPING syntax are:

<host> Address of the remote host, string type. This parameter  can  be  either: 

  - IP address in the format:”xxx.xxx.xxx.xxx”
  - Host name solved by a DNS query

<retryNum> The number of Ping Echo Requset to send
  1-100 Default: 4

<dataLen> The length of Ping Echo Request data
  0-1024 Default: 32

<timeout> The timeout, in units of 100 ms, waiting for a single Echo Reply
  1-600 Default: 100(10  seconds)

<ttl>  Time to live
  1-255 Default:  64

• For Quectel modems:

For these Quectel specific AT commands to work, an activated PDP context is needed, i.e. an active data session, which was discussed in the “establishing a data connection” part. Then you can run the commands accordingly.

AT+QPING=<contextID>,<host>[,<timeout>[,<pingnum>]]
 # Pings a remote server

The values for the AT+QPING syntax are:

<contextID> Integer type. The context ID. The range is 1-16.

<host> The host address in string type. The format is a domain name or a dotted decimal IP address.

<timeout> Integer type. Set the maximum time to wait for the response of each ping request. The range is 1-255, and the default value is 4. Unit:second.

<pingnum>Integer type. Set the maximum number of time for sending ping request. The range is 1-10, and the default value is 4.

B. TCP/UDP communication

To send data over TCP (again, varies by manufacturer):

• SIMCom example:

AT+CIPMODE=<mode>
 # Set transparent (1) or non-transparent mode (0)

AT+CIPSTART=[n,]<mode>,<host>,<port> 
 # The setting syntax. Connect to server on port 80 ([0…5]<”TCP” or “UDP”>,<remote server IP address>,<Remote server port>

AT+CIPSEND
 # Send data through TCP or UDP. AT+CIPSEND[=<length>|] (in case of multiple connections a <n> variable also need to be set.

> GET / HTTP/1.1
# Your data (followed by Ctrl+Z)
Host: server.example.com

AT+CIPSERVER=<mode>[,<port>]
 # Set up a TCP server. `port` is not needed when closing it =<mode> 0 Close server, 1 Open server, <port>  1..65535 Listening port

• Quectel example:

To open a socket service:

AT+QIOPEN=<contextID>,<connectID>,<service_type>,<host>,<remote_port>[,<local_port>][,<access_mode>] 

Here are its variables:

<contextID> Integer type. The context ID. The range is 1-16. (the PDP context ID)

<connectID> Integer type. The socket service index. The range is 0-11.

<service_type> String type. The socket servicetype.
  “TCP” Start a TCP connection as a client
  “UDP” Start a UDP connection as a client
  “TCP LISTENER” Start a TCP server to listen to TCP connection
  “UDP SERVICE” Start a UDP service

<host> String type. If <service_type> is TCP or UDP, it indicates the IP address (could also be a domain name) of remote server, such as “220.180.239.212”. If <service_type> is TCP LISTENER or UDP SERVICE, please enter “127.0.0.1”..

<remote_port> The  port  of  the  remote  server,  only  valid  when  <service_type> is “TCP” or “UDP”. The range is 0-65535.

<local_port> The local port. The range is 0-65535. If <service_type> is “TCP LISTENER” or “UDP SERVICE”, this parameter must be specified. If <service_type> is “TCP” or “UDP”, if <local_port> is 0, then the local port will be assigned automatically. Otherwise the local port is assigned as specified.

<access_mode> Integer type. The data access mode of the socket service.
  0 Buffer access mode
  1 Direct push mode
  2 Transparent access mode

AT+QIRD – to retrieve the received data from a socket

AT+QIRD=<connectID>[,<read_length>]
 # Retrieve the received data from a socket =<socket service index from 0-11>[,<0-1500 byte range>]  

Advanced features and debugging tips

Establish a connection to the network: Signalling investigation

In order to understand what is going on with each AT commands the generic procedure for establishing a data connection, as discussed earlier, was carried out on Quectel’s EC25-E modem. The test was done with a new network operator so that the authorization signalling and so on would show:

AT+COPS=? // check for available network operators
+COPS: (2,"TDC","TDC","23801",2),(1,"TDC","TDC","23801",0),(1,"3 DK","3 DK","23806",2),(1,"Telenor DK","TelenoDK","23802",2),(1,"Telia DK","Telia","23820",2),()

OK

AT+COPS=1,0,"3 DK" // manually select 3 DK network
OK

AT+CGDCONT? // check PDP contexts
+CGDCONT: 1,"IP","","0.0.0.0",0,0,0,0 // nothing, this means the device does not have a data connection

OK

AT+CGACT? // check defined PDP contexts' status
+CGACT: 1,0 // PDP context 1 is not activated

OK

AT+CGATT? // check packet-switched (PS) network status
+CGATT: 0 // the modem is not connected to the PS network

OK

AT+CREG? // check circuit-switched (CS) network status
+CREG: 0,5 // URC disabled, roaming on CS network

OK

AT+CGREG? // check packet-switched (PS) network status
+CGREG: 0,2 // URC disabled, searching

OK

In Onomondo, we have a feature that allows you to see the network logs. When the commands above were executed, here’s what the logs show:

Then the following was executed:

AT+CGDCONT=1,"IP","onomondo" // define PDP context
OK

AT+CGACT=1,1 // activate PDP context (will also do a AT+CGATT=1)
OK

AT+CGREG? // check packet-switched network status
+CGREG: 0,5 // URC disabled, roaming (this is good we are connected)

OK

Which resulted in the following new network logs:

Here’s a recap of what happened during this procedure:

Adding a network preference list

Most networks have a list of preferred networks their SIMs can attach to, this is called the PLMN list.

Using AT commands allow you to change your prefered networks with these 2 commands:

• AT+CPOL (the simple way)
• AT+CSIM (the complex way)

We created a full tutorial to use these AT commands in this PLMN list blog, that also includes a template you can use to help you manage your list.

Debugging tips

1. Enable verbose error reporting with AT+CMEE=2 to get detailed error messages
2. Check signal quality with AT+CSQ before attempting connections
3. Enable network registration notifications with AT+CREG=1 to receive real-time updates
4. Verify APN settings with AT+CGDCONT?
5. Check if the modem is attached to the packet network with AT+CGATT?

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References

• This guide originally appeared on the author’s GitHub.
• https://www.itu.int/rec/T-REC-V.250/en
• https://portal.3gpp.org/desktopmodules/Specifications/SpecificationDetails.aspx?specificationId=1515
• https://www.u-blox.com/sites/default/files/AT-CommandsExamples_AppNote_(UBX-13001820).pdf
• Exploratory Analysis of a GGSN’s PDP Context Signaling Load, http://downloads.hindawi.com/journals/jcnc/2014/526231.pdf
• https://www.cika.com/soporte/Information/GSMmodules/Quectel/AppNotes/Quectel_GSM_PPP_Application_Note.pdf
• https://zoomtel.zendesk.com/hc/en-us/articles/212403243-Mobile-broadband-service-provider-settings
• https://simcom.ee/documents/SIM7100E/SIM7100_AT%20Command%20Manual_V1.01.pdf
• https://www.telit.com/wp-content/uploads/2018/03/Telit-LN940-SERIES-AT-Command-User-Guide-REV1.4.pdf
• http://www.python-exemplary.com/download/sim800_series_tcpip_application_note_v1.00.pdf
• https://simcom.ee/documents/SIM900/SIM900_PING_AT%20Command%20Manual_V1.00.pdf (even though it is for the SIM900’s series it still works for the SIM800 series)
• https://www.elecrow.com/wiki/images/2/20/SIM800_Series_AT_Command_Manual_V1.09.pdf
• https://sixfab.com/wp-content/uploads/2018/09/Quectel_EC2xEG9xEM05_TCPIP_AT_Commands_Manual_V1.0.pdf