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

[Daniele Palmas]

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