Does anybody know the technique how to get a USB modem working for both Data and SMS?
After the AVR attaches and enumerates the device, there is a number of endpoints available. Those endpoints belong to different interfaces.
If you use Windows HyperTerminal to send AT commands to the modem, it lets you send these commands and receive responses even when your modem is connected to the internet, i.e. it somehow differentiates between which pipes to use for data input, for command output and for getting responses to commands.
How to do the same for AVR? If you choose, say, 1 for DATA-IN, 2 for DATA-OUT, 3 for NOTIFICATIONS, then whatever DATA you send to the OUT pipe, in my understanding, will be sent to your peer on the other side, not the modem, and whatever response to a command you may be getting will mingle with the DATA from your peer.