Restrict modem (EC21) one of 3G or 4G - ModemManager 1.10.0 (Debian Buster)

[Aleksander Morgado]

Hey,

> >> I'm using a Quectel EC21 modem on a Debian Buster Linux system (using modemmanager 1.10.0).
> >>
> > Please note, MM 1.10.0 is a very very old release, no longer
> > supported, and it's not even the last one inthe 1.10.x series...
>
> Fair enough, but 1.10.0 is comes with Debian Buster, which is the latest
> Stable release of Debian.  However there is a backport of 1.14, from the
> Unstable (next release) distro, for Buster.
>

1.10.0 was released in 2019-01-17, and after that there have been 4
new updates in the 1.10.x series (1.10.2 on 2019-06-10, 1.10.4 on
2019-07-04, 1.10.6 on 2019-09-12, 1.10.8 on 2019-10-30. Then we have
had the whole 1.12.x series up and running (7 releases); 1.12.0 was
released on 2019-11-06 and the last 1.12.12 was released on
2020-06-23. 1.14.0 was released on 2020-06-23. You should probably
poke Debian Buster packagers of ModemManager... they are 12 releases
late in their latest stable release ;)

> >> I want to force it to one of 3G or 4G to stop it changing/swapping between the two.
> >>
> >> I've tried using the `mmcli -m 0 --set-allowed-modes=3g
> >> `.  mmcli seems happy with that.
> >>
> >> However, some time later it swaps over to 4G (lte)
> >>
> >>    --------------------------------
> >>    Status   |                 lock: sim-pin2
> >>             |       unlock retries: sim-pin (3), sim-pin2 (3), sim-puk (10), sim-puk2 (10)
> >>             |                state: connected
> >>             |          power state: on
> >>             |          access tech: lte    <<< 4G !!!
> >>             |       signal quality: 63% (recent)
> >>    --------------------------------
> >>
> >>
> >> Have I got the right expectations of the `--set-allowable-modes` feature?
> >>
> >> Is there something else I need to do?
> >>
> > This may be a known issue with QMI modems where the internal network
> > registration attempt is done with "QMI NAS Register In Network"; this
> > command not only registers in network, it also makes the access tech
> > preference fallback to "any", and MM doesn't consider that in its API.
> >
> > There is a recent patch to avoid this, by avoiding the use of "NAS
> > Register in Network" and use "NAS Set System Selection Preference"
> > instead: https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/commit/c70b3557184fdf1472ff0cb36e9fd937cc7f9024
> >
> > That patch is not yet in any release, though, not even in 1.14.0. I'll
> > backport it to the 1.14.x branch so that it gets into 1.14.2.
> >
> > You can confirm that this issue is happening to you by running this
> > command once the tech went back to the one you didn't want:
> > $ qmicli -d /dev/cdc-wdm0 -p --nas-get-system-selection-preference
>
> Here is the output.
>
> # qmicli -d /dev/cdc-wdm0 -p --nas-get-system-selection-preference
> [/dev/cdc-wdm0] Successfully got system selection preference
>          Emergency mode: 'no'
>          Mode preference: 'umts, lte'
>          Band preference: 'wcdma-2100, wcdma-850-us'
>          LTE band preference: '1, 3, 5, 7, 28'
>          TD-SCDMA band preference: '(null)'
>          Roaming preference: 'any'
>          Network selection preference: 'automatic'
>          Service domain preference: 'cs-ps'
>          GSM/WCDMA acquisition order preference: 'wcdma'
>          Acquisition order preference: lte, umts, gsm, td-scdma,
> cdma-1x, cdma-1xevdo
>
> What are the key indicators that confirm the issue?
>          Network selection preference: 'automatic'???
>          Acquisition order preference: lte, umts, gsm, td-scdma,
> cdma-1x, cdma-1xevdo???
>

Mode Preference (umts + lte) already tells you that the setting was
reseted (assuming you requested 3G-only).
The SSSP patch in network registration will very very likely solve
your problems.

-- 
Aleksander
https://aleksander.es