ACM Serial Emulation is an extension to the ACM. ACM Serial Emulation not only uses AT commands to control the modem, it also emulates some of the operations that software might perform on UART registers controlling a serial modem. USB modems that support ACM access behave much like serial modems and are generally usable in both Windows and Linux.
For more information on ACM and other models than most people would desire, see:
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