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. 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 BĂ©nard - <eric@eukrea.com>