Telit LE910Cx, MBIM vs AT

[Ulrich Mohr]

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
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