CMUX AT commands

This page describes CMUX-related AT commands.

The GSM 0710 multiplexer protocol (CMUX) enables multiplexing multiple data streams through a single serial link, setting up one channel per data stream. For example, it can be used to exchange AT data and have a Point-to-Point Protocol (PPP) link up at the same time on a single UART. Serial Modem implements the basic option of the CMUX protocol with only UIH frames as described in the 3GPP TS 27.010 specification. The maximum length of the information field in UIH frames is configurable using the CONFIG_MODEM_CMUX_MTU Kconfig option, which defaults to 127 bytes.

Note

To use the nRF91 Series SiP as a cellular dial-up PPP modem, see Cellular PPP modem.

CMUX is enabled in Serial Modem by compiling it with the appropriate configuration files, depending on your use case. See the Configuration files section for more information.

Note

  • Only basic mode (mode 0) is supported.

  • Only UIH frames are used.

  • The speed used is the configured baud rate of Serial Modem’s UART.

  • No system parameters (N1, T1, T2, T3, k, m) are configurable.

CMUX setup +CMUX

The AT+CMUX command starts the CMUX multiplexer. It is defined in 3GPP TS 27.007 (section 5.7) and the underlying protocol is specified in 3GPP TS 27.010.

Only basic mode (<mode>=0) with subset 0 is supported. All other parameter values are rejected.

Set command

The set command allows you to start the CMUX multiplexer.

An OK response is sent before CMUX is started, after which only CMUX framing is accepted on the serial link.

Syntax

AT+CMUX=<mode>[,<subset>]

  • The <mode> parameter selects the operation mode. Only 0 (basic mode) is supported.

  • The <subset> parameter selects the subset of mode 0. Only 0 is supported. Default value is 0.

Read command

The read command returns the current multiplexer configuration.

Syntax

AT+CMUX?

Response syntax

+CMUX: <mode>,<subset>

  • The <mode> parameter is always 0 (basic mode).

  • The <subset> parameter is always 0.

Test command

The test command returns the supported parameter ranges.

Syntax

AT+CMUX=?

Response syntax

+CMUX: (0),(0)

Example

AT+CMUX=?

+CMUX: (0),(0)

OK
AT+CMUX?

+CMUX: 0,0

OK
AT+CMUX=0

OK
// CMUX is now started. Open the CMUX channels to continue communication.
AT+CMUX=0,0

OK
// Equivalent to AT+CMUX=0.

CMUX setup #XCMUX

Note

AT#XCMUX is a compatibility command for Serial Modem v1.x.x and is not recommended for new designs. Use the standard AT+CMUX=0 command instead.

The #XCMUX command manages the configuration of CMUX over the serial link.

Set command

The set command allows you to start CMUX and assign the CMUX channels.

The CMUX link is closed down automatically when the remote end sends the CMUX Multiplexer Close Down (CLD) sequence. The CMUX link can be closed manually with the AT#XCMUXCLD command.

Syntax

AT#XCMUX[=<AT_channel>]

The <AT_channel> parameter is an integer used to indicate the address of the AT channel. The AT channel denotes the CMUX channel where AT data (commands, responses, notifications) is exchanged. If specified, it must be 1, unless PPP is enabled. If PPP is enabled, it can also be 2 (to allocate the first CMUX channel to PPP). If not specified, the previously used address is used. If no address has been previously specified, the default address is 1.

Note

If there is more than one CMUX channel (such as when using PPP), the non-AT channels will automatically get assigned to addresses other than the one used for the AT channel. For example, if PPP is enabled and CMUX is started with the AT#XCMUX=2 command, the AT channel will be assigned to address 2 and the PPP channel to address 1.

An OK response is sent if the command is accepted, after which CMUX is started. This means that after successfully running this command, you must set up the CMUX link and open the channels appropriately. The AT channel will be available at the configured address. You can send an empty AT command to make sure that the protocol is set up properly.

Caution

Using AT#XCMUX to switch the AT command channel at runtime is error-prone and not recommended. The channel switching might run out of sync with automatic PPP startup and recovery enabled by AT#XPPP=1.

Read command

The read command allows you to read the address of the AT channel and the total number of channels.

Syntax

AT#XCMUX?

Response syntax

#XCMUX: <AT_channel>,<channel_count>

  • The <AT_channel> parameter indicates the address of the AT channel. It is between 1 and <channel_count>.

  • The <channel_count> parameter is the total number of CMUX channels. It depends on what features are enabled (for example, PPP).

Example

Without PPP:

AT#XCMUX?

#XCMUX: 1,1

OK
AT#XCMUX

OK
// Here, CMUX is started and communication can now happen only through it (until a reset).
// Open the AT channel, which is the only one, to continue exchanging AT data.
AT

OK

With PPP:

AT#XCMUX?

#XCMUX: 1,2

OK
AT#XCMUX=2

OK
// Start up CMUX and open the channels. The AT channel is now at address 2.
AT#XCMUX?

#XCMUX: 2,2

OK

CMUX close down #XCMUXCLD

The #XCMUXCLD command closes down the CMUX service.

This command can be used on host devices that do not support sending the CMUX Multiplexer close-down sequence. Once CMUX is closed down, the serial link returns to AT command mode.

Set command

The set command allows you to close down the CMUX link.

Syntax

AT#XCMUXCLD

An OK response is sent in the CMUX channel if the command is accepted, after which CMUX is closed down.

Read command

The read command is not supported.