Re: Interaction with ModemManager

[John Whitmore <arigead gmail com>]

On Thu, May 28, 2015 at 03:07:12PM -0500, Dan Williams wrote:
> On Thu, 2015-05-28 at 15:30 +0100, John Whitmore wrote:
> > I could probably send this message to the ModemManager list as well but I'll
> > start here. 
> >
> > I was on last week about connecting 2 4G LTE USB Dongles to a RaspberryPi. I
> > got one working but had a few issues with the second (vodafone) one. Every
> > time I edited the connection, the "vodafone" password had disappeared from the
> > connection. I'm away from the unit at the moment, taking a break 'till
> > tomorrow, then back at it. I ended up moving from Raspbian to Ubuntu Mate on
> > the RPi as Raspian has such old versions of NM and MM. Actually had serious
> > problems with Ubuntu Mate and Serial ports both the GPIO serial on the RPi and
> > USB - Serial cables. SO think I might be moving on the Arch Linux. There's a
> > first time for everything.
> >
> > Anyhow that's not my question. When I'd added a dongle that was correctly
> > modeswitched and recognised by ModemManager I was able to create a new network
> > connection in the NetworkManager. My question is with regards to entering the
> > ISP details into the NetworkManager. If I'd an operator SIM, say "three" in
> > the dongle and create the network connection but then pull the dongle and
> > change the SIM to "vodafone" then the NetworkManager will attempt to connect to
> > the "vodafone" network with the "three" APN details.
>
> Correct.  The APN is stored in the network connection details itself, so
> the connection is really tied to a specific provider.  We intend to add
> a way to lock a connection to a specific device/SIM in the future, but
> that won't even really solve the SIM swapping thing.
>
> > Now I've not tested that and what happens so forgive my possibly premature
> > question. It seems more logical to me that the APN details would be entered
> > into the ModemManger and NetworkManager don't need to know it, but just
> > requests a connection from the ModemManager. Ideally the ModemManager could
> > look at the current SIM and say what that means in terms of APN details.
>
> In recent versions of NetworkManager you can leave the APN blank (just
> don't specify it when creating the connection) and then NM+MM will
> request the "default subscribe APN" it the network supports it.  But, of
> course, you're at the mercy of whether the network supports it, and what
> is configured at the provider's end which may not always be correct.
>
> > New to the party so I'm sure there's a good reason why it is the way it is,
> > but just wondered. I'm sure that the data required for a Data connection must
> > be on the SIM becasue if I have an open unlocked phone and insert any SIM I've
> > never been asked for APN details. Phone just knows what to do. Well that's
> > been my expierence.
>
> The data is stored on the device, actually, not the SIM.  Devices/modems
> have a cached list of "PDP contexts" (GSM/UMTS) or "EPS contexts" (LTE)
> that includes the APN.  When you swap the SIM that list doesn't change
> unless something actively changes it, like software on the computer.
>
> What happens on the phone is that there is software to inspect the SIM's
> IMSI, extract the MCC/MNC of the provider, and reconfigure the default
> APNs stored on the device to something in a hardcoded list.  Or, after
> registering with the network, the provider can send an SMS that includes
> the correct APNs and the phone will automatically provision.
>
> NetworkManager doesn't do that kind of thing, it's currently up to the
> callers of NetworkManager to update the configuration/connections as NM
> is just a mechanism and doesn't actually enforce policy like this.  The
> various UIs that talk to NM do look at the "mobile broadband provider
> database" that is maintained on gnome.org, that does include a list of
> MCC/MNC and APNs that a client could use to match up the IMSI with a
> suggested APN though.
>
> Does that help?  In short, the SIM swapping problem is always solved at
> a level higher than ModemManager, and probably higher than
> NetworkManager too.

Been mulling that over in my head over the weekend and, at the risk of sounding
like a fan boy, it's brilliant. What I think that means is that I could write
a higher level piece of code, in python and talk to ModemManager via the DBus
interface to ask it about the installed modem and then fire a request to the
NetworkManager, again over DBus, to request a connection. 

The idea was always to manage two modems so given that, I can use the higher
level to ask ModemManager who has the stronger signal strength and then ask the
NetworkManager to establish a connection accordingly. Like I say Brilliant.



> Dan