Force 2G/3G/4G modes (when they are not reported as supported)

[Dan Williams]

On Wed, 2015-10-28 at 17:04 +0100, José wrote:
> Hi, I am working with a Sierra MC7710, and this is a output example from
> ModemManager:

...

> As you can see, there is only one supported mode (allowed: 2g, 3g, 4g;
> preferred: none) which (as far as I know) offers me no contl about
> which network technology is being used. When using this modem with
> other SIM cards or other protocols (this modem also supports QMI,
> direct IP and serial) the supported modes varies.
> 
> My understanding is that the supported modes are provided by the modem
> to ModemManager. Which variables affect the supported modes reported?
> 
> Is there a way to force ModemManager to use only 2g/3g/4g despite of
> the supported modes reported by the modem?

I don't recall why it was hardcoded in MM specifically to disallow mode
switching on LTE-capable devices.  Aleksander would probably know more.

In any case, I just tried setting various modes with my UML290 using
qmicli, and it allows setting various combinations:

cdma-1x|cdma-1xevdo
gsm
umts
lte
gsm|umts
gsm|umts|lte
cdma-1x|cdma-1xevdo|lte
gsm|umts|lte|cdma-1x|cdma-1xevdo

The UML290 does not support these combinations:

gsm|lte
umts|lte
gsm|umts|cdma-1x|cdma-1xevdo
gsm|lte|cdma-1x|cdma-1xevdo
gsm|cdma-1x|cdma-1xevdo

But with a Huawei E392 (which has no CDMA/EVDO), I *can* set umts|lte
and gsm|lte.

As a workaround, you can poke the device with:

sudo qmicli -p -d /dev/cdc-wdm0
--nas-set-system-selection-preference="<mode>"

where <mode> is a combination like what I've listed above.

-----


      *     (5) If current capabilities [GSM/UMTS + LTE]:
+     *       [2G only]
+     *       [3G only]
+     *       [4G only]
+     *       [2G + 3G]
+     *       [2G + 4G]
+     *       [3G + 4G]
      *       [2G + 3G + 4G]
      *
      *     (6) If current capabilities [CDMA/EVDO + LTE]:
+     *       [4G only]
+     *       [2G + 3G]
      *       [2G + 3G + 4G]
      *
      *     (7) If current capabilities [GSM/UMTS + CDMA/EVDO + LTE]:
-     *       [2G + 3G + 4G]
+     *       [4G only]
+     *       [2G + 3G + 4G] (all 3GPP and 3GPP technologies)
      */


Dan