I know that the USB 3G modems connects to computer by not one serial ports, one of it for diagnostics and for send AT commands and receive answers.
I looking for library for C/++ under linux which universal supports many USB 3G modems and give me possibilities to send AT-commands to modem while it connected via pppd.
If this library not exists I looking for standards which describes rules of creation and using this serial ports.
The special characters device files in /dev (e.g. /dev/ttyUSB0) are created at run-time these days. You can set up an udev rule that would match the modem's ID to make some sort of reliably persistent symbolic link. If you make sure you won't interfere with pppd in a wrong manner, you should be able to just write the special file belonging to the device using standard open() / write() calls (unless it is locked, of course).