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

[Sylvain Rochet]

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.


> 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.


Sylvain

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