LTE error codes / call rejections

[Aleksander Morgado]

Hey,

> I'm looking at these two errors:
>
> wwan (31502): error: couldn't connect the modem:
> 'GDBus.Error:org.freedesktop.ModemManager1.Error.MobileEquipment.Unknown: Call
> failed: cm error: lrrc-connection-establishment-failure-not-camped
>
> Tue Feb  8 15:16:38 2022 daemon.notice netifd: wwan (6885): error: couldn't
> connect the modem:
> 'GDBus.Error:org.freedesktop.ModemManager1.Error.MobileEquipment.Unknown: Call
> failed: cm error: lrrc-connection-establishment-failure-rejected
>
> I don't know the exact meaning of "camped" here, but I understand it's part
> of the cell establishment.  In any case, according to:
>
> http://howltestuffworks.blogspot.com/2011/10/rrc-connection-reestablishment-request.html
>
> (Which is a slightly different error), there may be additional information
> in the error - in this case, there's a back off time which is supposed to
> be noticed. Certainly both errors seem to happen when the call is made
> again "too soon".  The are also accompanied by a user UserAuthenticationFailed
> which I imagine is just the fall out error case.
>

Those "cm" errors are reported via QMI on a failed connection attempt.
The QMI protocol, AFAIK, doesn't give more information than the error
codes themselves.

We may be missing QMI error codes, that's for sure, but the ones you
posted are definitely supported, or we would otherwise see them as
numeric errors, not with the error nickname as you're seeing them

And we're definitely missing mapping all QMI error codes to MM error
codes, we currently map the basic ones only, but that's also because
the MM API sticks to 3GPP defined error codes exclusively, and QMI
reports many more error codes.

> I see enumerations of these errors here:
>
> https://forums.quectel.com/uploads/short-url/yWs5zzBa4x2LyGytODTAUu7WvIf.pdf
>
> But I'm not sure MM handles these enumerations (in particular on page 17) which
> might at least be a start to handling this.
>

Those are error codes in +CEER output of the modem; I really doubt
they will give us more info than what we get via QMI, but I may be
mistaken. E.g. I can see "EMM T3417 expired" with value 276, which
would map to QMI_WDS_VERBOSE_CALL_END_REASON_CM_EMM_T3417_EXPIRED for
example.

We only use +CEER on connection attempts done with the AT protocol
(for PPP). If the modem is connected via QMI, we rely on QMI error
codes exclusively.

I also feel that this list of errors is Quectel specific in +CEER, but
I cannot confirm that without digging much more in the 3GPP specs. If
they were Quectel specific we would need to work on
https://gitlab.freedesktop.org/mobile-broadband/ModemManager/-/issues/492
first.

-- 
Aleksander
https://aleksander.es