Hi Gus, I am quite happy to share the reasons for CDC support for modems. I’ve already explained some of this before.
I am using the SIMCOM SIM7600E-PCIE which is a PCI Express Module which makes adding this to your board as easy as adding 3.3V and connecting USB. That modem also has GPS.
If we use the UART, we are stuck with only a single channel for AT commands which means we cannot support GPS, SMS and PPP at the same time. We also can’t query the signal strength etc. I have plans to use the SC20260D as a mobile and data logging platform so simultaneous GPS, PPP and SMS are critical.
There is a multi-plex option available over the UART port but this is going to need a driver developed and CDC support on the host would probably take as long to develop, possibly sooner as you already have this for the client port.
With CDC support, the modem appears as a number of USB Serial Ports and you can use them to control the modem via AT commands to setup PPP on one port, GPS on another and SMS and signal query on a 3rd. There are some additional ports for debug etc.
Now, I realise that your concerns may be security issues with the AT commands but I don’t see any difference with the fact that the PPP connection will be setup via AT commands if we use UART or USB-UART.
Hope this helps to make it clearer, the need for the CDC support on the host port?
PS. I assume if we add a USB HUB to the existing host port, we can expand the number of USB host ports and still get the same capability on each port?