Single RS232 port Telit and CMUX mode (GSM 27.010 / n_gsm)

[Tim Small]

Hello,

I'm trying to get a (GPRS / 2.5G) Telit GE910-Quad V3 working in CMUX
mode.  There's only a single RS232 port connection between the modem and
the host computer, so I'd like to use GSM 27.010 ("CMUX") mode.  The
Telit module does have another physical port, but there isn't a spare
UART on the host (and I can't use USB), so I'm stuck with the one RS232
connection only.

So far I've hacked about with things as a proof of concept - an external
binary attaches the line discipline and I've manually setup calls using
pppd and chat scripts on one port, whilst monitoring signal quality etc.
on another multiplexed.

I've then hacked around with the ModemManager source to force it to
probe a couple of  /dev/gsmtty, ModemManager then identifies two
separate Telit modems:

root at nmtest:~# mmcli -L

Found 2 modems:
        /org/freedesktop/ModemManager1/Modem/1 [Telit] GE910-QUAD
        /org/freedesktop/ModemManager1/Modem/0 [Telit] GE910-QUAD

/org/freedesktop/ModemManager1/Modem/0
  -------------------------
  Status |          state: 'registered'
         | signal quality: '58' (recent)
         |          bands: 'egsm, dcs'
         |    access tech: 'unknown'
  -------------------------
  3GPP   |   registration: 'roaming'
         |  operator code: '23415'
         |  operator name: 'vodafone UK'
         |   subscription: 'unknown'

root at nmtest:~# mmcli -m 1 --simple-status

/org/freedesktop/ModemManager1/Modem/1
  -------------------------
  Status |          state: 'registered'
         | signal quality: '58' (recent)
         |          bands: 'egsm, dcs'
         |    access tech: 'unknown'
  -------------------------
  3GPP   |   registration: 'roaming'
         |  operator code: '23415'
         |  operator name: 'vodafone UK'
         |   subscription: 'unknown'


I can also get nm and mm to set up a ppp connection over one of them.

I'm not sure what my next steps should be to implement this fully, and
end up with a reasonably clean implementation (and a single modem of
course).  Any hints appreciated.

Thanks,

Tim.