USB communications device class

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USB communications device class (or "USB CDC") is a composite Universal Serial Bus device class. It provides a single device class, but there may be more than one interfaces implemented such as a custom control interface, data interface, audio and mass storage related interfaces etc.

The communications device class is primarily used for modems. However it also supports ISDN and Fax machines. It also supports plain telephony applications for performing regular voice calls.

Additionally this device class supports computer networking akin to a network card, providing an interfaces for transmitting Ethernet or ATM frames onto some physical media. Microsoft Windows versions, prior to Windows Vista do not support the networking parts of the USB CDC, instead promoting Microsoft's own derivative named Microsoft RNDIS, a serialized version of the Microsoft NDIS (Network Driver Interface Specification). With a vendor-supplied INF file, Windows Vista supports USB CDC and USB WMCDC ^[1].

This class is generally implemented in embedded systems like mobile phones to achieve more than one functionality from the device, so that a phone may be used as a modem, Fax or network port. The data interfaces are generally used to perform bulk data transfer.