How do smart phones use AT commands and data connection(s)? gsm mux? multiple uarts?

Question

I am involved in a project where we have some kind of IoT device. An nxp processor with an LTE modem on a PCB. The software running on it connects to the modem over a single uart interface. It will initialize the modem through AT commands, and finally made a data call to the provider (PPP). Then, it uses lwIP (light weight IP) to open some mqtt subscriptions, and allow user code to make http get/post requests to our servers.

Every 15 minutes we want to retrieve signal strength from the modem and report this back to the server. What I do now, is put the modem back in command mode, retrieve the signal strength info, go back to data mode, and resume normal operation.

The round trip from data mode, to commando mode, and back to data mode takes several seconds (4-5 ish). This is annoying, because during that time we are not receptive for commands.

I've read about gsm mux 07.10. By following some defined protocol it allows to create virtual serial ports, over one physical uart. That sounds nice, although I realize this will go at the cost of performance (bytes will be added to each frame we send to either command mode / data mode).

The gsm mux 07.10 spec dates from 1999. I am far from an expert in mobile solutions. I was wondering: is muxing still the way to go? How does a typical smart phone deals with this for example? Do they include modems with more than one uart to have parallel access to AT commands and a live internet connection? Or do they in fact still rely on gsm mux?

If somebody would be so kind to give some insights. Also on potential C libraries that are available that implement gsm mux 07.10? It seems that TinyGSM implements it (although I can't seem to find where), and I also can find the linux kernel driver that implements gsm mux 07.10. But that driver is written on top the tty interfaces in linux, so that would mean I would have to reverse engineer the kernel driver and strip out the tty stuff and replace it with my own uart implementation.

Answer 1

First of all, the spec numbering is the old GSM specification numbering, so those old specs will never be updated, the new specifications with new numbering scheme will. I do not remember when the switch was made, but I do remember someone at work giving a presentation on 07.10 probably around 1998/1999, so probably a few years after that or around that time (and definitely before 2009).

The newer spec numbering scheme uses three digits for the first part. So for instance the old AT command spec 07.07 is now 27.007, and the current 07.10 multiplex specification is 27.010.

---

The following is what I remember of 07.10.

The motivations for developing 07.10 was to exactly support the kind of scenario that you describe. Remember back in the mid 90's, if mobile phones had a serial interface then that was RS-232 though each manufacturer's proprietary connector at the bottom of the phone. One single serial interface.

However, in order to use 07.10 mux in serial communication you needed to install some specific serial drivers in Windows with support for 07.10 (and I think maybe there was some reliability issue with them?), and for that reason 07.10 never took of and became anything more than an rarely used solution.

Also by the end of the 90's additional serial interfaces like Bluetooth and IrDA became available on many phones, and later USB as well, which both added additional physical interfaces as well as natively multiplexing within each protocol.

So the need for multiplexing over physical RS-232 became less of an issue, and whatever little popularity 07.10 ever had dwindled down to virtual nothing.

Fast forward a couple of decades and suddenly someone asks about it on stackoverflow. Good on you :) As far as I can tell I cannot see any fundamental problems with using it for the purpose you present.

Modern smart phones that support AT commands will most likely have a code base for the AT command parsing with roots in the 90's, which most likely include the AT+CMUX command. Of course manufacturers today have zero explicit wish for supporting it, but when it is already present it will just come along with the collection of all other legacy AT commands that they support.

So if the modem supports AT+CMUX you should be good to go. I have no experience or recommendation with regards to client protocol libraries.