The Communication Device Class (CDC) supports a wide range of devices that can perform telecommunications and networking functions. Examples for communications equipment are:

Telecommunications devices, such as analog phones and modems, ISDN terminal adapters, digital phones, as well as COM-port devices
Networking devices, such as ADSL and cable modems, as well as Ethernet adapters and hubs

A detailed description about CDC is provided by the USB Implementers Forum (USB-IF).

CDC Class Features

A communications device has three basic tasks:

Device management (controlling an configuring a specific device and notifying the USB Host of certain events)
Call management (establishing and terminating telephone calls or other connections)
Data transmission (sending and receiving application data)

The CDC implementation in the USB Component features:

Emulation of a Virtual COM-port using the ACM (Abstract Control Model) subclass of CDC.
Emulation of network connectivity using the RDNIS protocol using the ACM (Abstract Control Model) subclass of CDC. This enables network connections over USB between a Windows host PC and an embedded device, as well as USB Device RNDIS to Ethernet Bridge applications.
Emulation of an Ethernet adapter using the NCM (Network Control Model) subclass of CDC (only available for USB Device). Using CDC (NCM), you can create Ethernet-over-USB (for Linux hosts) applications on Linux-based host systems.

Note

The following endpoint configurations are supported:

One interrupt IN endpoint for notifications to the USB Host
One bulk IN and one bulk OUT endpoint for data transfer

Control Transfers

The documents available at Communications Device Class describe the nine available request types for the Abstract Control Model (ACM). All requests specified for the Network Control Model (NCM) by the documents in ECM120.pdf and NCM10.pdf are supported. are not supported.

USB Descriptors

The following descriptors are required in an USB CDC ACM Device:

Standard Device Descriptor
Standard Configuration Descriptor
Interface Association Descriptor
CDC Header Functional Descriptor
CDC Union Functional Descriptor
Call Management Functional Descriptor
Abstract Control Management Functional Descriptor
Standard Interface Descriptor for the CDC Class communication interface
Standard Endpoint Descriptor for Interrupt IN endpoint
Standard Interface Descriptor for the CDC Class data interface
Standard Endpoint Descriptors for Bulk IN and Bulk OUT endpoints

The following descriptors are required in an USB CDC NCM Device:

Standard Device Descriptor
Standard Configuration Descriptor
Interface Association Descriptor
CDC Header Functional Descriptor
CDC Union Functional Descriptor
CDC Ethernet Networking Functional Descriptor
NCM Functional Descriptor
Standard Interface Descriptor for the CDC Class communication interface
Standard Endpoint Descriptor for Interrupt IN endpoint
Standard Interface Descriptor for the CDC Class data interface
Standard Endpoint Descriptors for Bulk IN and Bulk OUT endpoints

The necessary descriptors are automatically generated by the USB Middleware Component. The page USB Descriptors provides more information on the topic.

Note

Further information on how to use CDC is given in the Reference section: