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>