23 June 2011

Dealing with SMS storage on a GSM modem

I should have called this post "History repeATs": I would have never imagined that I would reuse my Hayes AT knowledge after ... 20 years. From dialup to GSM modems, technology has evolved but not the way to communicate with the DCE: the AT command set.

This is all part of my GSM remote control project, that has abandoned the idea of using an unmodified GSM cellphone.

Problem: a GSM modem kept blinking a LED, meaning there were unread messages. Since the SIM card was empty, the message(s) had to be inside the modem:

+CPMS: "SM",0,50,"SM",0,50,"MT",2,250
There they are, 2 of them. How to get to them? Given the AT command documentation, "SM" stands for SIM and "ME" is the Mobile Equipment. What is "MT"? In my opinion it is the whole Mobile Terminal, so MT=SM+ME.
Still, the AT+CMGL (List SMS) and AT+CMGR (Read SMS) commands do not accept a parameter to select the message storage (SM vs ME).

The trick is then to select a different default message storage, "ME" in this case, and proceed to read/delete those messages:
AT+CPMS="ME","ME","MT"  # switch to ME message storage
AT+CMGL # list messages
#... do your SMS actions ...
AT+CPMS="SM","SM","MT" # switch back to SIM if you want to

It took me a while to figure out this procedure and the product documentation didn't help much.

If you landed on this blogpost looking for GSM modems for a M2M application or the like, known models are: Nokia 12, Nokia 30; Siemens M20t, Siemens TC35; Telit GM862/GM864; etc etc You better look for their "terminal" version, that usually comes with a manageable interface like RS232 instead of a small 80 pin connector.