Hi!

To explain droid 4 modems:

** Modem description on Droid 4

Ok, so to explain.. situation on Droid 4 is complex:

              /--- Audio---\
             /              \
           CPU---serial--3G modem
           / \----USB----/
          /   
       LTE modem

I'll ignore LTE modem for now.

There are two physical connections between CPU and 3G modem: serial
one and USB link. There are many virtual links over both serial and
USB connections.

3G modem presents itself as

/dev/ttyUSB4 -- fairly standard AT commands, but poorly
implemented/tested. It is easy to support this with ofono, with small
patches. "at" branch.

There are more virtual links over USB, one of them is QMI interface;
it might be useful but I did not investigate it.

But USB have great disadvantage: power consumption.

3G modem does packet communication over serial, exposing virtual links:

/dev/motmdm1 -- basic support, calls, on/off

/dev/motmdm3 -- send sms interface

/dev/motmdm9 -- receive sms interface

These serial interfaces kind of look like AT commands, but they are
very different in details, and normal AT parser is unusable
there. There are better designed than standard AT commands, but way
harder to support in ofono. This is "d4-1.xx" branch.

Additional advantage is that power consumption is lower when USB link
can be powered down.

I hope this clears up the confusion?

Best regards,

                                Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html