Data and control channels for a modem in Linux?

I have a USB modem detected as a device device file (/dev/ttyACM0). Redirecting AT commands to the device file seems to work as the modem responds. Am going with the following assumption, the upper part represents a control channel. Through it one can dial up, setup the modem, cancel the connection... in general do things related to controlling the modem without actually handling the data. By data I mean everything that aren't controls (for example HTTP packets). **Where does the data flow happen? If it's through the same file, how is that multiplexed?** Also ideally, one would like the modem just to be an underlying "service" that provides a network interface with an IP, so one can abstract that out and use TCP/IP sockets. The sockets are implemented in the kernel. **How does this fit into the picture once the modem is exposed as /dev/ttyACM0 in user space**?