USB communications device class
Encyclopedia
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 interface implemented such as a custom control interface, data interface, audio, or mass storage
related interfaces.
The communications device class is primarily used for modem
s. 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 network
ing akin to a network card
, providing an interface 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.
This class is generally implemented in embedded system
s like mobile phone
s 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.
Universal Serial Bus
USB is an industry standard developed in the mid-1990s that defines the cables, connectors and protocols used in a bus for connection, communication and power supply between computers and electronic devices....
device class. It provides a single device class, but there may be more than one interface implemented such as a custom control interface, data interface, audio, or mass storage
USB mass storage device class
The USB mass storage device class, otherwise known as USB MSC or UMS, is a protocol that allows a Universal Serial Bus device to become accessible to a host computing device, to enable file transfers between the two...
related interfaces.
The communications device class is primarily used for modem
Modem
A modem is a device that modulates an analog carrier signal to encode digital information, and also demodulates such a carrier signal to decode the transmitted information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce the original digital data...
s. However it also supports ISDN and fax
Fax
Fax , sometimes called telecopying, is the telephonic transmission of scanned printed material , normally to a telephone number connected to a printer or other output device...
machines. It also supports plain telephony
Telephony
In telecommunications, telephony encompasses the general use of equipment to provide communication over distances, specifically by connecting telephones to each other....
applications for performing regular voice calls.
Additionally this device class supports computer network
Computer network
A computer network, often simply referred to as a network, is a collection of hardware components and computers interconnected by communication channels that allow sharing of resources and information....
ing akin to a network card
Network card
A network interface controller is a computer hardware component that connects a computer to a computer network....
, providing an interface for transmitting Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
or ATM
Asynchronous Transfer Mode
Asynchronous Transfer Mode is a standard switching technique designed to unify telecommunication and computer networks. It uses asynchronous time-division multiplexing, and it encodes data into small, fixed-sized cells. This differs from approaches such as the Internet Protocol or Ethernet that...
frames onto some physical media. Microsoft Windows
Microsoft Windows
Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...
versions prior to Windows Vista
Windows Vista
Windows Vista is an operating system released in several variations developed by Microsoft for use on personal computers, including home and business desktops, laptops, tablet PCs, and media center PCs...
do not support the networking parts of the USB CDC, instead promoting Microsoft's own derivative named Microsoft RNDIS
RNDIS
The Remote Network Driver Interface Specification is a Microsoft proprietary protocol used mostly on top of USB. It provides a virtual Ethernet link to most versions of the Windows operating system...
, a serialized version of the Microsoft NDIS
Network Driver Interface Specification
The Network Driver Interface Specification is an application programming interface for network interface cards . It was jointly developed by Microsoft and 3Com Corporation, and is mostly used in Microsoft Windows, but the open-source NDISwrapper and Project Evil driver wrapper projects allow...
(Network Driver Interface Specification). With a vendor-supplied INF file
INF file
An INF file or Setup Information file, is a plain text file used by Microsoft Windows for installation of software and drivers. INF files are most commonly used for installing device drivers for hardware components. Windows includes IExpress.exe for the creation of INF-based installations...
, Windows Vista supports USB CDC and USB WMCDC.
This class is generally implemented in embedded system
Embedded system
An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...
s like mobile phone
Mobile phone
A mobile phone is a device which can make and receive telephone calls over a radio link whilst moving around a wide geographic area. It does so by connecting to a cellular network provided by a mobile network operator...
s 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.
External links
- USB-IF's Approved Class Specification Documents
- Class definitions for Communication Devices 1.2 (.zip file format, size 2.61 MB)
- Class definitions for Communication Devices 1.1
- a good guide (linux-oriented) about USB host-to-host, CDC 'ethernet' class and RNDIS
- http://www.atmel.com/dyn/resources/prod_documents/doc4322.pdf App Note, Migrating from RS-232 to USB Bridge Specification. Explains the use of USB CDC (Communications Device Class) ACM (Abstract Control Model) to emulate serial ports over USB.