Re: [lwip-users] lwip with 4G modem

[Giuseppe Modugno]

Il 12/10/2021 19:17, Sylvain Rochet ha scritto:
> Hi Giuseppe,
>
> On Tue, Oct 12, 2021 at 09:26:00AM +0200, Giuseppe Modugno wrote:
> > I have an embedded project where lwip is used in a microcontroller
> > (Cortex-M3 by LPC). The network interface is a standard Ethernet. The MCU
> > features a MMI interface with an external Ethernet PHY.
> >
> > It works well, I'm able to open a MQTTs connection with a broker, enable NTP
> > request, download some file from HTTP server.
> >
> > Now I'd like to use this project as a base for a new project, where the
> > network interface will be replaced by a 4G modem that is controlled by
> > standard AT commands.
> >
> > My question is: what should I change in my project to replace Ethernet with
> > 4G modem? I think I have to use ppp over a serial line to open a 4G data
> > connection and it seems ppp is supported by lwip distribution.
>
> Yes, you need to use PPPoS(erial).
>
> You have to setup your modem to enter PPP data mode then read/write
> HDLC/PPP bytestreams from/to the modem and lwIP.

If I understand well, there's a standard AT command to define a PPP-type 
PDP context for cellular modems (defined in ETS TS 127 007):

  AT+CGDCONT=<cid>,[<PDP_type>]....

where:

  <PDP_type> is a string that could be "PPP",...

So I think I must send the command:

  AT+CGDCONT=1,"PPP",...
  AT+CGATT=1
  AT+CGACT=1,1

to activate PDP context in PPP mode. However the specifications says, 
regarding AT+CGACT:

   After the command has completed, the MT remains in V.250
   command state.

So the question is: how to switch to V.250 online mode? The only command 
that should switch to V.250 online mode is +CGDATA, but I couldn't 
understand how to use, because it seems it replaces +CGATT and +CGACT. I 
think it should be:

  AT+CGDCONT=1,"PPP",...
  AT+CGDATA="PPP",1

There's another method to activate data connection, this is named "modem 
compatibility mode". You can use "old" ATD command to establish a data 
connection, receiveng CONNECT as the answer and entering V.250 online 
data state:

  ATD*99#;


Reading the specifications I noted there is another PDP_type, that is 
"IP". So I'm wondering if PPP is mandatary or can be avoided completely 
by activating a IP-based PDP type.

PPP is a little complex, so it would be nice if it can be avoided at all.



> > Another question (I don't know if it is OT), will I be able to make voice
> > calls at the same time the data connection is open and active?
>
> This is not related to lwIP, check your modem documentation ;-)
>
>
> > The serial channel for ppp/data and AT commands is single, so how can
> > I multiplex them? Should I use CMUX?
>
> CMUX is the proper way to do so.
>
>
> > Last question. If I have 4G modem and Ethernet PHY, is lwip able to manage
> > two network interface at the same time?
>
> Yes! Beware of routing though, you will probably have to set the default
> interface to the currently "working" interface and have to re-establish
> all currently active TCP/UDP sessions on routing change.
Ok, thanks for suggesting.