n_gsm.c GSM 0710 tty multiplexor HOWTO
===================================================

This line discipline implements the GSM 07.10 multiplexing protocol
detailed in the following 3GPP document :
http://www.3gpp.org/ftp/Specs/archive/07_series/07.10/0710-720.zip

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 */
	sleep(3);

	/* 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 */
	daemon(0,0);
	pause();

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
done

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.

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.

Additional Documentation
------------------------
More practical details on the protocol and how it's supported by industrial
modems can be found in the following documents :
http://www.telit.com/module/infopool/download.php?id=616
http://www.u-blox.com/images/downloads/Product_Docs/LEON-G100-G200-MuxImplementation_ApplicationNote_%28GSM%20G1-CS-10002%29.pdf
http://www.sierrawireless.com/Support/Downloads/AirPrime/WMP_Series/~/media/Support_Downloads/AirPrime/Application_notes/CMUX_Feature_Application_Note-Rev004.ashx
http://wm.sim.com/sim/News/photo/2010721161442.pdf

11-03-08 - Eric Bénard - <eric@eukrea.com>