Hey,
I am using ModemManager with u-blox TOBY-L210 for a while now with older
versions of ModemManager (1.7.x). I did some patches and home-brew code
to make that combination work.
I now want to switch to newer versions of MM (1.18), benefit from the
good work on MM, and do it "the right way" this time, so I can
contribute to MM :-)
I tested the new version without modifications and found two "problems":
* The default bearer interface is not implemented for ublox modems
* The device does only sometimes get a LTE connection, sometimes it
doesn't.
While the reason for the first problem is quite obvious ;-), I did some
analysis on the second one and found the reason why it does not work:
Whenever there is a PDP context matching the APN, besides the context
with CID 4 -- which is used by the modem to automatically create a
connected bearer when it can connect using the default EPS bearer -- MM
selects that PDP context and tries to establish a connection using it.
But since there is already a connected context with CID 4, connecting a
secondary context to the same APN may fail (depending on the SIM card,
due to MNO restrictions I guess, see
https://www.u-blox.com/sites/default/files/LTE-initial-default-bearer_AppNote_%28UBX-20015573%29.pdf).
Now the question is why there is a second PDP context when trying to
connect. The answer is that this context has been created by MM when it
tries to connect while the default bearer is not connected: CID 4 is
unused when the initial bearer is not connected, so MM creates a second
one that gets in the way when we later on try to connect a connection
and the initial bearer is there again
Therefore I did some modification in MM to make the Toby work:
* I implemented the the initial EPS bearer setting interface
(functions set_initial_eps_bearer_settings*,
load_initial_eps_bearer_settings*) similar to the icera plugin. I
used the at+ucgdflt command for reading and writing the initial EPS
bearer setting (this command is available on Toby-L2 and MPCI-L2 only).
* I implemented the profile list interface (functions list_profiles*)
to prefer CID 4 when MM tries to reuse a PDP context for connecting
I got it up and running. But I want to push that changes into MM, so
there are some open questions regarding the implementation I want to
discuss.
* The Toby L210 does not allow to read back the username to use for
the initial bearer. Therefore, comparing the username after writing
the initial bearer fails. I omitted comparing the username for now
when comparing the previous EPS bearer settings with the new ones.
This change is not in the ublox plugin, so I doubt this is a good
idea. How to handle this properly?
* The ucgdflt command is not available for other ublox modems, so the
implementation I did only applies to Toby-L2 and MPCI-L2. This
information must be gathered somehow. I see two possibilities: using
the at+ucfgdlt=? to test the availability of the command (when would
I do that?), or introduce a flag similar to the ubandsel flag inside
the band configuration struct. What would be the method to prefer?
* Is implementing the profile list interface the right approach? The
fact that MM selects an already activated PDP context (cid 4), tries
to deactivate it (which fails, but is ignored), tries to activate it
again (which fails, but is ignored as well) and then uses it
"smells" and indicates to me that this might not be the right
solution. A better approach might be that MM accepts using an
already activated PDP right away without trying to disconnect and
connect again, but this would mean that there are massive changes
required in MM to support that. What is your opinion on that?
I found other small issues during work (build warnings, const issues
etc), but I guess this email is long enough already, so I postpone that
.... ;-)
I really look forward for any answers and ideas.
I hope I managed to describe the problem and my solution so that one can
understand it, otherwise just ask. I can also provide the code with
changes I did....)
--
Best regards / Mit freundlichen Grüßen / Salutations distinguées
Ulrich Mohr
SEMEX-EngCon GmbH
Carl-Merz-Strass 26
76275 Ettlingen
email: u.m...@semex-engcon.com
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Commercial register: Mannheim, HRB 718881
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