fibocom: Minimum PDP context ID

Aleksander Morgado aleksander at
Thu Apr 7 20:32:19 UTC 2022


> On 3/29/22 10:59, Aleksander Morgado wrote:
> > If you used QMI, then the Modem3gpp.SetInitialEpsBearerSettings()
> > method would end up modifying that CID=1.
> > But that doesn't mean you cannot change them using the profile manager
> > APIs also. When using QMI, you can do both things, although if your
> > plan is to change the initial EPS bearer settings, you should better
> > use the specific method for that. The fact that you can use the
> > profile manager APIs is just a side effect of having one profile
> > associated to the initial EPS bearer settings, and ModemManager will
> > not attempt to hide that from the user. We could improve all this and
> > allow the profile management APIs flag specifically which profile is
> > used as initial EPS bearer settings, e.g. by flagging it with
> > MM_BEARER_APN_TYPE_INITIAL (if we're not doing it already! I recall
> > thinking about adding that but not sure if I did).
> Unfortunately, QMI doesn't seem to be an option. Even though, the MA510
> has a QMI USB configuration, it's impossible to bring up the connection
> with that. Also, it's not officially supported.
> Beside the initial EPS bearer issue mentioned before, the MA510 only
> seems to really work with CID=1. Whenever ModemManager configures a
> context with CID!=1, e.g. CID=2, the subsequent +GTRNDIS=1,2 works OK
> and querying the RNDIS state confirms that it used CID=2. However, the
> periodic bearer connection checks using +CGACT? returns
> +CGACT: 1,1
> +CGACT: 0,2
> Which marks the bearer intermediately as not connected and the ECM
> connection is torn down on the host.

Shouldn't we then change the connection check to use the RNDIS state
query instead of +CGACT?

> Querying with +CGCONTRDP confirms
> that the modem is actually only using CID=1.

It could be totally possible that you're seeing IP address reported in
CID=1 due to the initial EPS bearer being active. But that doesn't
explain why you don't see the CID=2 as active as well, if the
connection succeeded (unless it was disconnected right away after a
successful connection, which could also be possible)

> This leads me to wonder what's the best way to force ModemManager to
> always use CID=1 for this particular modem model? Any ideas?

If that were the case, I'd definitely like to understand how we're
supposed to support setups where the initial EPS bearer APN is
different to the Internet data APN. I'm not totally convinced that
this is a limitation of the modem really, although it could very well

What kind of context settings are you using for CID=1 and CID=2?
Does your test operator expect the same settings for the initial EPS
bearer and for the data bearer?
Does the CID=1 output in +CGCONTRDP say it's connected *before* you
attempt to run "+GTRNDIS=1,2" or does it report it's disconnected?
Does the CID=1 output in +CGACT? say it's connected *before* you
attempt to run "+GTRNDIS=1,2" or does it report it's disconnected?


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