Some questions about qmi/wwan

[Dan Williams]

On Fri, 2014-07-25 at 19:17 +0200, Marco wrote:
> Hello,
> 
> I've been able to start a broadband connection with a gobi modem using
> qmicli/qmi-network + dhcp client. I was able to get it working by reading
> http://sigquit.wordpress.com/2012/08/20/an-introduction-to-libqmi/ and
> hacking around a bit (for example to validate the PIN before starting the
> network).
> Everything works, however this is all new to me and I have some questions:
> 
> - I didn't load any firmware whatsoever. Is it needed or not?

Not always.  Older Gobi modems (1000/2000) didn't store firmware onboard
and require gobi_loader.  Gobi 3000+ does store firmware onboard, and
will often have images pre-programmed from the factory.  Most
non-Gobi-branded devices that use Qualcomm chipsets just have a single
image flashed on the device, and don't implement multiple firmwares.

> - Once the kernel module is loaded, I get two /dev/cdc-wdm devices (0 and
> 1). What are those for? The one that seems to be working with qmicli is
> /dev/cdc-wdm1. I also get two network interfaces, wwp0s20u6 and
> wwp0s20u6i3, the second one is the one that I can use dhcp client with and
> get an IP. Which is the purpose of the other one?

I'll leave that to Bjorn and Aleksander, but on some devices there is a
second non-operative QMI configuration.  Could be a firmware quirk.

> - Sniffing with tcpdump or wireshark shows ethernet frames. Don't broadband
> connections use PPP anymore? How is authentication performed? According to
> the man, PAP/CHAP authentication is still an option.

No, they don't use PPP anymore.  PPP was only used between the host and
the modem firmware anyway (well, except for CDMA).  But it turns out PPP
is just too much overhead, and you can't get HSPA rates using PPP, so
everyone is using "psuedo ethernet" or custom interfaces these days.
With DHCP, the modem firmware implements a mini-DHCP server to return
the IP configuration it already received from the network back to the
host.

In reality, ethernet frames are overhead too, since what you *really*
get from the modem is a pure IP-level stream.  Ethernet emulation only
exists to make life easier for driver writers and users.

Authentication details are set via custom APIs, either AT commands or
via proprietary protocols like QMI and MBIM.

> - The device has an integrated GPS (PDS in qmi-speak). I understand that,
> currently, with libqmi it is not possible to use it (or at least I see no
> pds-* options in qmicli 1.10.0, which is the version I'm using). Is that
> right?

qmicli doesn't have support for that, but programs like ModemManager use
libqmi to provide it.

Dan