First of all, thank you for the instant answer :-)
See comments below.
Hi there,
I got a Telit LE910C1-EU attached to an embedded device, using
ModemManager 1.19.0 (main branch, 2 commits behind) and libmbim
1.27.3 (main branch, 1 commit behind). Linux Kernel is v5.12.
I was able connect successfully to an LTE network using both PPP (USB
profile 0x1201, 3 AT ports) and MBIM (USB profile 0x1252, MBIM + 2AT
ports). Both configurations seem to work -- I was able to ping a
server using those interfaces.
But there are some inconsistencies that I see:
* While PPP shows a signal quality of 85%, MBIM only shows between 9
and 19 % signal quality. (When I use a AT+CSQ command in MBIM mode,
the result is 20 which is in the middle of the range, so I that
should be more than 20%)
AT+CSQ and MBIM are different ways of measuring, so it depends on what
the numbers you get are. Are you able to run mbimcli's "query-signal-
state" option on the device? What do you see?
What is the raw value from CSQ?
* CSQ gives a value of 20, which maps to -73 dBm according to the
documenation.
* The MBIM command returns a value of 3 for the rssi, which maps to
-117 dBm according to the MBIM spec (which matches the 9% I see).
* I also checked the CIND command, that gives an rssi value of 3 as
well, but here the range is 0 to 5, so indicating -68dBm.
In the end, it may just be a difference in how the firmware calculates
and reports the value.
To me, that looks a little bit like its a bug in the MBIM implementation
in the modem: The returned the rssi in the wrong scaling when using MBIM
(Just guessing....)
* While PPP does not show any sim lock, MBIM shows a SimPin2 lock
(although we are connected).
This is normal, the PIN2 lock controls stuff that isn't necessary for
normal operation.
That is good, so I can ignore it. Is that PIN2 lock something specific
to MBIM? Or is it just again that the implementation differs from PPP to
MBIM?
[...]
* What limitations do I get when I use PPP instead MBIM?
Mostly speed. PPP will be pretty bandwidth limited and you won't be
able to achieve anywhere near full LTE speeds without MBIM. MBIM is
also typically more reliable than AT+PPP since it talks to the modem
more directly and the control path is simpler.
Ok, that matches my expectations.
Thank you!
Dan
Uli
