Writing a GSM emulator for use with Android

Question

For some work I'm doing, I want to have an emulated GSM modem which will communicate with an Android-x86 virtual machine over a Unix socket. The VM should see the emulator as a real modem and use it to send SMS (as the first pass of functionality).

So far, I've put something together which handles some AT commands and just replies "OK" to all the rest. For some commands, like "AT+CRSM", I just have a table of responses gathered from running the official Android emulator and looking at the radio log. For others, I maintain some state and construct answers; those commands include:

CFUN?
CPIN?
CGREG?
CGREG?
COPS?
CGREG=
CREG=
CPIN=
COPS=
CRSM=
CSMS=
CGSN
CIMI
CSQ

Android will boot, and send commands to my emulated modem, which answers, however it doesn't seem to be fully functional. Android doesn't detect any radio signal strength, for instance, just showing an "x" in the signal bar.

Does anyone know of a similar project, or just what AT commands are absolutely necessary to get some basic functionality?

Answer 1

Well, I'll answer another of my own questions, it's easier that way.

I ended up implementing an emulator which maintains a little bit of state, such as whether or not unsolicited CREG and CGREG messages are enabled, the network registration status & network name format (for the COPS command), and a message reference counter for SMS.

It supports the following commands:

CFUN?
CPIN?
CGREG?
CREG?
COPS?
CGREG=
CREG=
CPIN=
COPS=
CRSM=
CSMS=
CMGS=
CGSN
CIMI
CSQ

All other messages just get an "OK" response.

With the stock AOSP Android source running in an emulator with the "-radio unix:/tmp/phone" switch, I can send SMS messages and decode the binary PDUs into real messages. I will continue to add functionality so SMS messages can be injected back to Android, and hopefully open-source the code at some point.