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n_gsm: add a documentation

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* this documentation gives some details on how to get the n_gsm
line discipline to work with modems supporting 07.10 basic option.

* it was tested on Telit and Simcom modems.

Signed-off-by: Eric Bénard <eric@eukrea.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
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Eric Bénard 2011-03-09 19:24:48 +01:00 committed by Greg Kroah-Hartman
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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 :
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 succesful (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.
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>