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¶
- 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,
- switch the serial line to using the n_gsm line discipline by using TIOCSETD ioctl,
- configure the mux using GSMIOC_GETCONF / GSMIOC_SETCONF ioctl,
- obtain base gsmtty number for the used serial port,
Major parts of the initialization program : (a good starting point is util-linux-ng/sys-utils/ldattach.c):
#include <stdio.h> #include <stdint.h> #include <linux/gsmmux.h> #include <linux/tty.h> #define DEFAULT_SPEED B115200 #define SERIAL_PORT /dev/ttyS0 int ldisc = N_GSM0710; struct gsm_config c; struct termios configuration; uint32_t first; /* 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); /* get first gsmtty device node */ ioctl(fd, GSMIOC_GETFIRST, &first); printf("first muxed line: /dev/gsmtty%i\n", first); /* and wait for ever to keep the line discipline enabled */ daemon(0,0); pause();
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
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.
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 Bénard - <firstname.lastname@example.org>