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Re: [lwip-users] Help with PPP connection - modem disconnects for no obv

From: Freddie Chopin
Subject: Re: [lwip-users] Help with PPP connection - modem disconnects for no obvious reason...
Date: Wed, 27 Mar 2019 12:22:51 +0100
User-agent: Evolution 3.32.0

Hello again!

Last week I finally found a proper way to talk to this modem and today
I've tried to systematically investigate it deeper.

First of all - the modem works, I can connect with LTE 4G network, I
can connect to the internet with lwip's PPP, I can connect to MQTT test
broker (with full encryption and authentication of both parties) with
lwip's MQTT code. So far so good (;

It seems that for correct operation of the modem - which turned out to
be the one and only obstacle in making it all work smoothly - I need to
make sure the following conditions are all met.

1. Modem's preferneces of networks is set to 4G-3G-2G

2. Modem selects the network automatically (`AT+COPS=0`).

3. This seems to be most important part! There must be TWO PDP contexts
available - at CID 1 (exactly at 1!) I need to have the usual APN
configuration, at CID 16 (exactly at 16!) I need to have a context with
IP protocol and empty APN (it actually doesn't matter what's in the APN
here) - without that, the modem will NEVER connect to LTE 4G network.

+CGDCONT: 1,"IP","internet","",0,0
+CGDCONT: 16,"IP","","",0,0

4. There is no need to automatically activate any of the PDP contexts,
however CID 16 will be activated automatically whenever the modem is
connected to LTE 4G.

+CGACT: 1,0
+CGACT: 16,1

5. The modem should be manually attached to the "PS domain" with
`AT+CGATT=1`, otherwise it will work with 3G even if 4G is available.
Executing this command will automatically activate PDP context with CID
16 and connect to LTE 4G.

There may be some flaws here, but so far this procedure has worked
well. I'm not 100% sure that these are all required steps to actually
use a brand new, unconfigured modem, but we will see - let's hope that
this method will work too.


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