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This is an example based on mbed-os cellular APIs that demonstrates a TCP or UDP echo transaction with a public echo server.

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Example cellular application for Mbed OS

This is an example based on mbed-os cellular APIs that demonstrates a TCP or UDP echo transaction with a public echo server.

(Note: To see this example in a rendered form you can import into the Arm Mbed Online Compiler, please see the documentation.)

Getting started

This particular cellular application uses a cellular network and network-socket APIs that are part of mbed-os.

The program uses a cellular modem driver using an external IP stack (LWIP) standard 3GPP AT 27.007 AT commands to setup the cellular modem and registers to the network.

After registration, the driver opens a point-to-point protocol (PPP) pipe using LWIP with the cellular modem and connects to internet. This driver currently supports UART data connection type only between your cellular modem and MCU.

For more information on Arm Mbed OS cellular APIs and porting guide, please visit the Mbed OS cellular API and contributing documentation.

Download the application

$ mbed import mbed-os-example-cellular
$ cd mbed-os-example-cellular


$ git clone
$ cd mbed-os-example-cellular

Change the network and SIM credentials

See the file mbed_app.json in the root directory of your application. This file contains all the user specific configurations your application needs. Provide the pin code for your SIM card, as well as any other cellular settings, or null if not used. For example:

    "target_overrides": {
        "*": {
            "nsapi.default-cellular-sim-pin": "\"1234\"",

Selecting socket type (TCP, UDP or NONIP)

You can choose which socket type the application should use; however, please note that TCP is a more reliable transmission protocol. For example:

     "sock-type": "TCP",

Turning modem AT echo trace on

If you like details and wish to know about all the AT interactions between the modem and your driver, turn on the modem AT echo trace.

        "cellular.debug-at": true

Turning on the tracing and trace level

If you like to add more traces or follow the current ones you can turn traces on by changing mbed-trace.enable in mbed_app.json

```”target_overrides”: {
“*”: {
“mbed-trace.enable”: true,

After you have defined `mbed-trace.enable: true`, you can set trace levels by changing value in `trace-level`

 ```"trace-level": {
            "macro_name": "MBED_TRACE_MAX_LEVEL",
            "value": "TRACE_LEVEL_INFO"

Board support

The cellular modem driver in this example uses PPP with an Mbed-supported external LwIP stack. It supports targets when modem exists on the Mbed Enabled target as opposed to plug-in modules (shields). For more details, please see our Mbed OS cellular documentation.

Currently supported boards with onboard modem chips can be found under Mbed OS /targets folder. You can find all cellular specific onboard modems by searching an overridden function CellularDevice::get_target_default_instance().

Currently supported modem drivers (for plug-in shields) can be found under cellular /targets folder.

For a cellular shield, you need to define which shield to use with provide-default, and also how the shield is connected to the Mbed OS board. For example, a generic AT/PPP modem would add from the GENERIC_AT3GPP/mbed_lib.json file to your mbed_app.json:

    "target_overrides": {
       "GENERIC_AT3GPP.provide-default": true,
       "GENERIC_AT3GPP.tx": "<tx-pinmap>",
       "GENERIC_AT3GPP.rx": "<rx-pinmap>"

Compiling the application

The master branch is for daily development and it uses the latest mbed-os/master release.

To use older versions update Mbed OS release tag, for example:

mbed releases
 * mbed-os-5.10.4
mbed update mbed-os-5.10.4

You may need to use --clean option to discard your local changes (use with caution).

Use Mbed CLI commands to generate a binary for the application. For example, in the case of GCC, use the following command:

$ mbed compile -m YOUR_TARGET_WITH_MODEM -t GCC_ARM

Running the application

Drag and drop the application binary from BUILD/YOUR_TARGET_WITH_MODEM/GCC_ARM/mbed-os-example-cellular.bin to your Mbed Enabled target hardware, which appears as a USB device on your host machine.

Attach a serial console emulator of your choice (for example, PuTTY, Minicom or screen) to your USB device. Set the baudrate to 115200 bit/s, and reset your board by pressing the reset button.

You should see an output similar to this:

Establishing connection ......

Connection Established.
TCP: connected with server
TCP: Sent 4 Bytes to
Received from echo server 4 Bytes

Success. Exiting


  • Make sure the fields nsapi.default-cellular-sim-pin, nsapi.default-cellular-plmn, nsapi.default-cellular-apn, nsapi.default-cellular-username and nsapi.default-cellular-password from the mbed_app.json file are filled in correctly. The correct values should appear in the user manual of the board if using eSIM or in the details of the SIM card if using normal SIM.
  • Enable trace flag to have access to debug information "mbed-trace.enable": true and "cellular.debug-at": true.
  • Error Message: Assertion failed: iface usually means that a default modem is not defined, e.g. "GENERIC_AT3GPP.provide-default": true
  • If the modem does not respond to (AT) queries, check that UART pins (tx, rx, rts, cts) are connected and defined, e.g. "GENERIC_AT3GPP.tx": "<tx-pinmap>", …
  • It is a common case that a modem seems to connect fine with just USB power, but actually it needs to have an external power supply for a data connection.
  • Try both TCP and UDP socket types.
  • Try both "lwip.ppp-enabled": true and "lwip.ppp-enabled": false.
  • The modem may support only a fixed baud-rate, such as "platform.default-serial-baud-rate": 9600.
  • The modem and network may only support IPv6 in which case "lwip.ipv6-enabled": true shall be defined.
  • The SIM and modem must have compatible cellular technology (3G, 4G, NB-IoT, …) supported and cellular network available.
  • Enable CIoT optimization for NONIP socket control-plane-opt: true.

If you have problems to get started with debugging, you can review the documentation for suggestions on what could be wrong and how to fix it.

License and contributions

The software is provided under Apache-2.0 license. Contributions to this project are accepted under the same license. Please see for more info.

This project contains code from other projects. The original license text is included in those source files. They must comply with our license guide.