[ublox] A problem with the network connection over LTE on TOBY-L2

[Ulrich Mohr]

Hey Aleksander,

Thank you for the clarifiction, this helps a lot. There is one point 
open though, that I did not fully get, see comments below.

Am 05.11.2018 um 09:54 schrieb Aleksander Morgado:
> On 11/5/18 9:02 AM, Ulrich Mohr wrote:
>> I am facing this problem, too (see https://lists.freedesktop.org/archives/modemmanager-devel/2018-August/006633.html).
>>
>> I addressed that problem by changing the modem manager logic to set the default bearer using the UCGDFLT command whenever a the bearer changes. This seems to work reasonably (with the drawback as described in the thread linked above). I am still testing this, since I am not yet sure that this really does the job.
>>
>>
>> @Aleksander, concerning your planned implementation. You wrote in the thread above:
>>
>> This should be some API that is totally independent from the connection process I think.
>>
>> It is not clear to me how this will work: I don't get any connection not using the UCGDFLT command, so if a bearer is set, this needs to be result somehow in the UCGDFLT command. So how can this be independent from the connection process? If someone sets the APN, e.g. via the Network Manager, the UCGDFLT command must be issued, otherwise it will not work...
> Yes, I'm planning to add this as a new API, a new way to be able to manage the initial default LTE attach bearer. The user will be able to use this API when the module is in low-power mode only (e.g. CFUN=4) and the idea is that it will setup the bearer details as we currently do during connection (e.g. APN, user, pass...). I'm probably going to suggest that we automatically create a bearer object upon boot if the module allows to manage the initial default LTE attach bearer settings (e.g. u-blox and MBIM at least).
Sounds good. I think that automatically creating a bearer object is the 
right approach, because that reflects what the modem (at least the TOBY) 
acutally does.
> Upon connection from e.g. NM, MM will check whether the requested settings (APN, ip-type...) match the ones from the default LTE attach bearer, and if they do, MM will report that the modem is connected right away without trying to create and connect a different context. This is already more or less what the u-blox plugin does. And when disconnecting the connection, if it was the initial default LTE bearer, we'll just report disconnection without touching the actual bearer in the modem (as that would probably return an error). This procedure is IIRC what Microsoft suggests in the MBIM extensions that allow this kind of management, see:
> https://docs.microsoft.com/en-us/windows-hardware/drivers/network/mb-lte-attach-operations
>
Ok, that approach more or less matches the implementation I test at the 
moment, except that I set the default bearer upon first connection 
establishment with that APN (lacking other possibilities to do so).

That leads me directly to the one open point: How will the use case 
works when using Network Manger with your new API?
The user will enter the APN information to set up the connection 
information (in my case, that will be a device configuration UI). That 
will result in a new (or changed) Network manager connection 
configuration. Now, the Network manager will have to bring the modem to 
low power and set the default LTE bearer and then bring the modem back 
to full power. How will this be done?

BTW: Since I did an implementation that works for me, I could provide 
that source code somehow, if your are interested. I don't think it is 
perfect and will need changes to work the way you plan, but perhaps it 
might help nevertheless? If interested, let me know which works best for 
you how to provide the source code ...

-- 
Best regards / Mit freundlichen Grüßen / Salutations distinguées

Ulrich Mohr

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