Garbled description with --nas-get-serving-system

[Paul Gildea]

Hi Daniele,

I've tried it with regular carrier and it worked ok. It didn't work for two
different brand of private networks/EPC's that we tried here.

--
Paul

On Mon, 2 Dec 2019 at 22:31, Daniele Palmas <dnlplm at gmail.com> wrote:

> Hi all,
>
> Il giorno lun 2 dic 2019 alle ore 17:27 Aleksander Morgado
> <aleksander at aleksander.es> ha scritto:
> >
> > Hey!
> >
> > > > Hi, I was wondering had any knowledge of an issue we are seeing. We
> > > > are
> > > > connecting to private networks we are running on two different brands
> > > > of
> > > > EPC. Then, we are using libqmi to query the modem for the network
> > > > info via
> > > > --nas-get-serving-system
> > >
> > > 0A:      D4:F2:9C:EE:2C:D3:EF:6F:F9:1A
> > > ^length  ^ data
> > >
> > > So length is 10 bytes, but in ASCII "TestNetwork" is 11 bytes long.
> > > Hmm. What do we know packs more than one character into each byte...
> > >
> > > The string is most likely GSM7 encoded and libqmi is expecting all
> > > strings in UTF8, which is what almost everything in QMI is reported as.
> > >
> > > So my best guess is that the Telit modem firmware is not correctly
> > > converting the network name to Unicode for us. Every other QMI
> > > implementation we've seen does :)
> > >
> > > ModemManager has some special code to ignore the description when it's
> > > not UTF-8 safe, and libqmi only does it for printable stuff like what
> > > qmicli outputs. Perhaps it could work around the Telit firmware
> > > somehow.
> > >
> >
> > Other messages, like "Get Home Network" (0x0025), have a special TLV
> > indicating the encoding of the network description with the following
> > supported values:
> > • 0x00 – Octet, unspecified
> > • 0x02 – 7-bit ASCII
> > • 0x04 – Unicode
> > • 0x09 – GSM 7-bit default
> >
> > I don't know of any such TLV in "Get Serving System", but maybe
> > there's a new one we don't know of. There isn't any unknown one in the
> > provided message as far as I can see.
> >
> > As Dan said, ModemManager has UTF-8 validity checks before using the
> > strings, but the printable TLV descriptions in libqmi and the qmicli
> > output lack that logic.
> >
> > @Daniele Palmas @Carlo Lobrano do you know if this is a Telit specific
> > issue or a more generic one? Is the firmware preferring the GSM-7
> > output for this field over Unicode when the characters fit in GSM-7?
> >
>
> let me check this, it will take a few days.
>
> @Paul, is this happening only with your test network or also with
> normal carriers? I'm not seeing the issue in normal usage.
>
> Regards,
> Daniele
>
> > --
> > Aleksander
> > https://aleksander.es
>
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