Proposal: identifying modems and mobile broadband cards

Dan Williams dcbw at
Fri Feb 8 12:56:02 PST 2008


While adding mobile broadband card support to NetworkManager, it turned
out that we needed a more specific way of finding modems.  Most of these
devices present themselves as a few serial ports, either /dev/ttyUSBx
or /dev/ttyACMx.  The first serial port is the communications port,
which acts just like a modem and supports AT commands.  When the data
connection is open though, you must talk to the device (for signal
strength, etc) on the other serial ports because interrupting the data
stream for an AT command is a Bad Thing.

We can already search for the 'serial' capability, but any serial port
(including COM ports) has that capability, and we certainly don't want
to go around poking the ports to see what's there.

Second, we need to differentiate between CDMA and GSM based mobile
broadband cards, and the best way to do that is through FDI files too.


- The interface that has the 'serial' capability acquires the 'modem'
capability if that serial interface is indeed known to be a modem

- A property "modem.type" gets added with the value "at" if the modem
supports Hayes-compatible AT commands

- A property "modem.phy_type" gets added that contains a value like
'gsm', 'cdma', or 'wimax', maybe even 'pots' for old-school modems

- Eventually we could put AT commands for common operations into the FDI
files and clients like NM could retrieve them from HAL



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