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n_gsm.c 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
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 */
/* 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 */
4- 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
5- 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
6- first close all virtual ports before closing the physical port.
Additional Documentation
More practical details on the protocol and how it's supported by industrial
modems can be found in the following documents :
11-03-08 - Eric BĂ©nard - <>