CMUX AT commands
This page describes CMUX-related AT commands.
The CMUX protocol 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.
Note
To use the nRF91 Series SiP as a standalone modem in Zephyr, see nRF91 Series as a Zephyr-compatible modem.
CMUX is enabled in SLM by compiling it with the appropriate configuration files, depending on your use case. See the Configuration files section for more information.
Note
The maximum recommended baud rate is 460 800. At higher baud rates (921 600, 1 000 000), it is possible for bytes to come in faster than the chip is able to handle, which causes the buffer space to run out if it goes on for too long. UART RX is not disabled in that case, which results in data loss and communication failures.
At a baud rate of 460 800, the maximum throughput is slightly below that of the nRF91 Series modem when using LTE-M.
Note
SLM does not have an equivalent to the AT+CMUX command described in 3GPP TS 27.007.
Here is how SLM’s implementation of CMUX relates to the standard command’s parameters:
Only UIH frames are used.
The speed used is the configured baud rate of SLM’s UART.
The maximum frame size is
2100(defined bySLM_AT_MAX_RSP_LENfound inslm_defines.h).
CMUX setup #XCMUX
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.
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.
Note
This command can be run when CMUX is already running to change the address of the AT channel. However, this is not allowed when PPP is running.
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 between1and<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