How to Send AT commands to SIM900 while pppd is running

Question

I have a Raspberry pi with SIM900 GSM module. currently i am using ttyUSB0 as pppd but i also want to send AT commands (send/receive SMS) to SIM900 while pppd is active. I was created multiple virtual serial ports (like gsmtty1,gsmtty2 etc) but those are not working with screen/minicom.

Answer 1

While these virtual serial ports might work (assuming you are talking about the kernel's 07.10 multiplexing support), your first attempt should be to use the other serial device, eg /dev/ttyUSB1 .

This is assuming the modem provides two serial endpoints, which it is not absolutely required to do, but virtual all modems that support USB does, so I would be very surprised if it really only had one.

Notice that the 07.10 multiplexing protocol requires explicit command and implementation support from the modem and is specified in a 3GPP specification which was created in the 90-ties to cater for the lack of multiplexing in the serial interfaces that were in use at that time (RS-232, IrDA, bluetooth).

But it required dedicated driver support on the operating side, and for windows there was no default drivers supplied so you had to install something additional (and I think the quality of those were not so great either) so 07.10 never got any serious momentum. Today USB's native multiple endpoint support have in many ways obsoleted the 07.10 protocol.