USB CDC

Overview

There are three classes that make up the definition for communication devices: the Communication Device Class, Communication Interface Class and the Data Interface Class. The Communication Device Class is a device level definition and is used by the host to properly identify a communication device that may present several different types of interfaces. The Communication Interface Class defines a general-purpose mechanism that can be used to enable all types of communication services on the Universal Serial Bus (USB). The Data Interface Class defines a generalpurpose mechanism to enable bulk or isochronous transfer on the USB when the data does not meet the requirements for any other class. Several types of communication devices can benefit from the USB. This specification provides models for telecommunication devices, such as telephones, analog modems, ISDN devices and networking devices. It describes:

Specifications for:
- Communication Device Class
- Communication Interface Class
- Data Interface Class
Framework for building a communication device:
- Assembling the relevant USB logical structures into configurations.
- Communication Class interface and its usage.
- Data Class interface and its usage.
- Usage of additional class types or vendor specific interfaces.
Implementation examples of communication devices, such as a basic telephone, and an analog modem.

Spec

Given the broad nature of communication equipment, this specification does not attempt to dictate how all communication equipment should use the USB. Rather, it defines an architecture that is capable of supporting any communication device. The current release of the specification focuses on supporting connectivity to telecommunication services (devices that have traditionally terminated an analog or digital telephone line), and medium speed networking services (“Always Connected” LAN/WAN media types). The specification currently outlines the following types of devices: x Telecommunications devices: analog modems, ISDN terminal adapters, digital telephones, and analog telephones x Networking devices: ADSL modems, cable modems, 10BASE-T Ethernet adapters/hubs, and “Ethernet” cross-over cables. This specification does not attempt to redefine existing standards for connection and control of communication services. The Communication Class defines mechanisms for a device and host to identify which existing protocols to use. Where possible, existing data formats are used and the transport of these formats are merely enabled by the USB through the definition of the appropriate descriptors, interfaces, and requests. More specifically, this specification describes a framework of USB interfaces, data structures, and requests under which a wide variety of communication devices can be defined and implemented.

Implementation

This section describes the functional characteristics of the Communication Device Class, Communication Interface Class and Data Interface Class, including:

Device organization:
- Endpoint requirements.
- Constructing interfaces from endpoints.
- Constructing configurations from a variety of interfaces, some of which are defined by other class specifications.
- Identifying groups of interfaces within configurations that make functional units and assigning a master interface for each union.
Device operation

Although this specification defines both the Communication Interface Class and Data Interface Class, they are two different classes. All communication devices shall have an interface using the Communication Class to manage the device and optionally specify themselves as communication devices by using the Communication Device Class code. Additionally, the device has some number of other interfaces used for actual data transmission. The Data Interface Class identifies data transmission interfaces when the data does not match the structure or usage model for any other type of class, such as Audio.
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. Thecommunications 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 does not support the networking parts of the USB CDC , instead promoting its own derivative named Microsoft RNDIS, a serialized version of the Microsoft NDIS (Network Driver Interface Specification). 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.

Resources

Microsoft-Communications Device Class
Msdn-Support for the Wireless Mobile Communication Device Class (WMCDC)
USB CDC-answer the need of telecommunication services (devices that have traditionally terminated an analog or digital telephone line), and medium speed networking services ("Always Connected" LAN/WAN media types) and is ideal for devices such as, Mobile Phones, Analog/ADSL/Cable Modems, analog/digital telephones Wireless Terminals, Ethernet Adaptors as well as Legacy and other network devices.

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