This program enables GSM 0710 multiplexing using linux n_gsm line dicipline.

This program :

• Opens modem device on serial line
• Enables modem CMUX
• Attaches n_gsm line dicipline
• Creates virtual TTYs
• Daemonizes and waits for signal
• Removes the virtual TTYs

In order to activate the CMUX mode the program sends some AT commands. Theses commands are mainly vendor specific and should be adapted to your own modem. The example in this code works for the Quectel M95 GSM module.

• In order to run this program you should have n_gsm module built for your linux kernel and loaded. Use modprobe n_gsm to load it. If it fails you probably have to build the kernel module by yourself.
• Change the defined options in cmux.c:

/* serial port of the modem */
#define SERIAL_PORT	"/dev/ttyS1"
/* line speed */
#define LINE_SPEED	B115200

• Change the AT commands set to fit your modem in cmux.c.
• Make and run the program.

make
./cmux

It will create four devices:

• /dev/ttyGSM1
• /dev/ttyGSM2
• /dev/ttyGSM3
• /dev/ttyGSM4

You should now be able to access your modem with four TTYs and both use ppp and send AT commands to your modem.

This program is mainly an actual implementation of GSM 0710 tty multiplexor HOWTO