Telit LE910Cx, MBIM vs AT

[Dan Williams]

On Thu, 2021-12-02 at 15:57 +0100, Ulrich Mohr wrote:
> 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?

In the end, it may just be a difference in how the firmware calculates
and reports the value.

>  * 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.

> So my question are:
>  * Is it normal / ok to see those inconsistencies?

Yes.

>  *  Where do these inconsistencies come from? 

The firmware implementation of AT (PPP) vs. MBIM is different.

>  * Where to start looking to fix those inconsistencies?

By asking the manufacturer to change the firmware, unfortunately.

>  * Can I safely ignore the SimPin2 lock?

Yes.

>  * 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.

Dan

> 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 .... :-)