Telit LE910Cx, MBIM vs AT

[Ulrich Mohr]

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%)
  * While PPP does not show any sim lock, MBIM shows a SimPin2 lock
    (although we are connected).

So my question are:

  * Is it normal / ok to see those inconsistencies?
  * Where do these inconsistencies come from?
  * Where to start looking to fix those inconsistencies?
  * Can I safely ignore the SimPin2 lock?
  * What limitations do I get when I use PPP instead MBIM?

I have no experience with MBIM at all, so perhaps I got something wrong 
completely?

BTW: I also tried RNDIS, but that is ignored by MM completely. Instead, 
the AT ports are used for PPP connection. I assume there is no support 
for RNDIS within the telit plugin?

Thank you in advanced, any help appreciated .... :-)

-- 
Best regards / Mit freundlichen Grüßen / Salutations distinguées

Ulrich Mohr

SEMEX-EngCon GmbH
Carl-Merz-Strass 26
76275 Ettlingen
Phone: +49 (0) 7243 5143596
email:u.mohr at semex-engcon.com
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