Re: SARA-R4 unreliable LTE-M connection problem


Am Montag, 5. Juli 2021, 11:14:38 CEST schrieb Aleksander Morgado:
Connection can be established, I see data on the modems RX line, but
nothing on the TX line, which means the host sends data to the modem, but
gets back nothing.

I tried again. According to scope baudrate on RX of the modem module seems to
be 115200 in connected state.

Can this be a problem with the baudrate? I mean it's a classical serial
UART, so I would assume baudrates of host and modem must match. In AT
command mode the baudrate is 115200 and ModemManager can successfuly send
AT commands and receives reponses. Is it possible NetworkManager and/or
pppd change this somehow?

Have you tried to set the ID_MM_TTY_FLOW_CONTROL udev tag in the TTY
port so that flow control settings are applied by ModemManager before
the connection is started? E.g. ID_MM_TTY_FLOW_CONTROL set to
"rts-cts" or to "xon-xoff"

Also, if using 115200, have you set ID_MM_TTY_BAUDRATE to 115200 as
well as a udev tag in the TTY port? The default in MM is 57600 really.

Yes, all this is set and I compared with the AT command lists for both modems,
that 115200 with rts-cts are the default settings:

root@unit:~ cat /usr/lib/udev/rules.d/78-mm-whitelist-platform-modem.rules
ACTION!="add|change|move", GOTO="mm_whitelist_platform_modem_end"

That looks good to me.

In all ModemManager debug output I did neither see command AT\Q nor AT+IPR, so
I assume ModemManager does not change this setting?

MM won't tell the modem what settings it should use; but MM should
know what settings the modem has configured by default. E.g. if you
set baudrate to 115200 in the udev tag, we're telling MM that the
baudrate in the modem is 115200. For the flow control, we do use
AT+IFC though.

I'm afraid I did not fully understand yet how baudrates with those LTE modems
are supposed to work in connected state, when pppd has taken over.

Can the modem change that baudrate and how would the host follow then?

Or is it the other way round, can pppd change the baudrate and does the modem
adapt itself?

Or does the baudrate remain constant at all time?

The baudrate would be constant all the time, both in data mode and in
command mode.

Then if 115200 is the default, that would limit actual throughput, even if the
modem could send/receive with a higher rate, right?

Yes, which is why if possible you should not use a TTY with pppd if
you're looking for LTE data rates; all new modes providing LTE level
datarate would also be integrated in the system exposing a network
interface, which doesn't have the limitations of the TTY.

I think you should try to run AT commands manually (through minicom)
and launch pppd manually as well, and try to debug that further.


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