Problem using an APN with 4G not available

[Giovanni Parodi]

Thank you, you are right, I verified today.
There are 2 different reply from APN: “unsubscribed option” when I use a
SIM that doesn’t support LTE option (didn’t pay for it) or “unknown-apn”
when the sim could connect to lte, but the APN doesn’t support it. I should
support it downgrading connectivity to umts|gsm and everything works fine.
Stupid error from my side, I was sure there should be an automatic
downgrading to LTE as it happens when you have a not powerful enough
signal, but this a different issue, and it should be managed by the SW
developer. Thank you for your great help

Il giorno sab 7 nov 2020 alle 17:59 Aleksander Morgado <
aleksander at aleksander.es> ha scritto:

> Hey,
>
> > Good morning, I'm a user of libqmi and qmicli tool and I need some help.
> > I'm using a SIM of an Italian provider (TIM), configured on a standard
> private APN (the SIM and APN are of a customer of my company, he pays for a
> custom APN to the TIM company).
> >
> > When using the standard public APN (ibox.tim.it), with a modem that
> supports LTE connectivity (Quectel EC25), I can go online and get a
> connection to the network.
> > If I configure the custom private APN (that uses the same network
> infrastructure), I'm able to connect to the (private) network and to go
> online, pinging the private customer server, only if a set the modem as 3G
> using the command line:
> >
> > qmicli -d /dev/cdc-wdm0 --nas-set-system-selection-preference="umts|gsm"
> >
> >
> > If I configure the SIM as:
> >
> > qmicli -d /dev/cdc-wdm0
> --nas-set-system-selection-preference="lte|umts|gsm"
> >
> >
> > I receive the error:
> >
> > <<<<<<   type       = "Call End Reason" (0x10)
> > <<<<<<   length     = 2
> > <<<<<<   value      = F5:03
> > <<<<<<   translated = gsm-wcdma-unknown-apn
> > <<<<<< TLV:
> > <<<<<<   type       = "Verbose Call End Reason" (0x11)
> > <<<<<<   length     = 4
> > <<<<<<   value      = 06:00:1B:00
> > <<<<<<   translated = [ type = '3gpp' reason = '27' ]
> >
> >
> > Here is my problem: if I use the SIM in a smartphone, the smartphone
> fails connection in lte mode but after a few seconds (about 30) goes to
> UMTS mode and gets its address.
> > My problem is that, even if I select to support umts,LTE,GSM, when the
> modem tries connecting, it tells me unknown-apn, and does not switch to
> UMTS connectivity option.
> > If I use 3G mode, everything works fine.
> > So here is my question: how should I manage this kind of problem?
> > How can I force qmicli to manage switching at connection time different
> mode search of APN?
>
> If you know in advance that you're going to hit the situation, I
> assume you could run the qmicli commands to prefer (or even limit) 3G
> depending on the specific APN to use? Or, otherwise, just assume that
> if you ever get the "gsm-wcdma-unknown-apn" call end reason you should
> think of falling back manually to 3G and retrying.
>
> > I know this may be a simple issue for expert people, but I'm really
> confused.
>
> I am no expert on the inner LTE stack details, so I cannot comment
> much more I'm afraid. This could also be a lack of planning in the
> operator side, for what it's worth; i.e. why does the APN get reported
> as unknown when using the LTE infrastructure? Is that a known
> limitation of that APN?
>
> --
> Aleksander
> https://aleksander.es
>
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