Initial EPS bearer on u-blox TOBY-L210

[Ulrich Mohr]

Hey,

When using ModemManager with a u-blox TOBY-L210 module (router mode) to 
connect to a LTE network, I face some problems.


What I see:
When the modem attaches to the network, it creates and activates a 
default EPS bearer (with CID=4). The APN resulting for this PDP Context 
depends on the SIM card and operator.
This default EPS bearer may be sufficient to get IP connectivity (seen 
on SIM Card from eseye.com, operator Vodefone.de, AT+CGDCONT? shows 
"eseye1.mnc058.mcc234.gprs") or can be a APN that does not route data 
(seen on SIM Card from Telekom, operator Telekom.de, AT+CGDCONT? shows 
"NODATA.MNC001.MCC262.GPRS").
When ModemManager tries to connect, it creates a new PDP context (since 
the existing one does not match) and activates it. This succeeds, but 
has no effect, since the modem internally does only route data from the 
PDP context activated first (see u-blox AT command manual, section 
"Networking"). This can be examined using the AT+UIPROUTE command.
In case of the Telekom SIM card, this leads to the ModemManager showing 
"connected", but no data can be transferred.


The solution requested by the u-blox manual is to use the AT+UCGDFLT 
command to configure the default eps bearer for LTE. I implemented this 
as first step in the 3gpp activation, but it has a major drawback: The 
the modem has to be switched to airplane mode (AT+CFUN=4) before issuing 
the command and back afterwards (AT+CFUN=1). This is detected by the 
ModemManager an leads to a cancellation of the 3gpp activation ( a 
second 3gpp activation later on will succeed if the AT+UCGDFLT is not 
issued a second time when the default EPS bearer already set).


In addition, this leads to other problems: ModemManager does not match 
the default PDP context, so it will create a new one (which has no 
effect as explained above). I altered the match (according to this 
thread: 
https://lists.freedesktop.org/archives/modemmanager-devel/2016-August/003362.html) 
which worked, but leads to a new problem: The default EPS context can 
not be disabled (this is documented), so disconnecting will no longer work.


Now the question is: How can I properly implement using the default 
bearer in ModemManager? Do you have any ideas? Or is this problem 
already beeing addressed and I do something wrong?

Any information welcome :-)

Thank you,


-- 
Best regards / Mit freundlichen Grüßen / Salutations distinguées

Ulrich Mohr

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