GSM 0710 tty multiplexor HOWTO

This line discipline implements the GSM 07.10 multiplexing protocol detailed in the following 3GPP document:

This document give some hints on how to use this driver with GPRS and 3G modems connected to a physical serial port.

How to use it

  1. initialize the modem in 0710 mux mode (usually AT+CMUX= command) through its serial port. Depending on the modem used, you can pass more or less parameters to this command,
  2. switch the serial line to using the n_gsm line discipline by using TIOCSETD ioctl,
  3. configure the mux using GSMIOC_GETCONF / GSMIOC_SETCONF ioctl,

Major parts of the initialization program : (a good starting point is util-linux-ng/sys-utils/ldattach.c):

#include <linux/gsmmux.h>
#define N_GSM0710     21      /* GSM 0710 Mux */
#define DEFAULT_SPEED B115200
#define SERIAL_PORT   /dev/ttyS0

      int ldisc = N_GSM0710;
      struct gsm_config c;
      struct termios configuration;

      /* open the serial port connected to the modem */
      fd = open(SERIAL_PORT, O_RDWR | O_NOCTTY | O_NDELAY);

      /* configure the serial port : speed, flow control ... */

      /* send the AT commands to switch the modem to CMUX mode
         and check that it's successful (should return OK) */
      write(fd, "AT+CMUX=0\r", 10);

      /* experience showed that some modems need some time before
         being able to answer to the first MUX packet so a delay
         may be needed here in some case */

      /* use n_gsm line discipline */
      ioctl(fd, TIOCSETD, &ldisc);

      /* get n_gsm configuration */
      ioctl(fd, GSMIOC_GETCONF, &c);
      /* we are initiator and need encoding 0 (basic) */
      c.initiator = 1;
      c.encapsulation = 0;
      /* our modem defaults to a maximum size of 127 bytes */
      c.mru = 127;
      c.mtu = 127;
      /* set the new configuration */
      ioctl(fd, GSMIOC_SETCONF, &c);

      /* and wait for ever to keep the line discipline enabled */
  1. create the devices corresponding to the "virtual" serial ports (take care, each modem has its configuration and some DLC have dedicated functions, for example GPS), starting with minor 1 (DLC0 is reserved for the management of the mux):

    MAJOR=`cat /proc/devices |grep gsmtty | awk '{print $1}`
    for i in `seq 1 4`; do
       mknod /dev/ttygsm$i c $MAJOR $i
  2. use these devices as plain serial ports.

    for example, it's possible:

    • and to use gnokii to send / receive SMS on ttygsm1
    • to use ppp to establish a datalink on ttygsm2
  3. first close all virtual ports before closing the physical port.

    Note that after closing the physical port the modem is still in multiplexing mode. This may prevent a successful re-opening of the port later. To avoid this situation either reset the modem if your hardware allows that or send a disconnect command frame manually before initializing the multiplexing mode for the second time. The byte sequence for the disconnect command frame is:

    0xf9, 0x03, 0xef, 0x03, 0xc3, 0x16, 0xf9.