USB communications device class (or USB CDC class) is a composite

Universal Serial Bus Universal Serial Bus (USB) is an industry standard that establishes specifications for cables, connectors and protocols for connection, communication and power supply (interfacing) between computers, peripherals and other computers. A broad ...

device class. The communications device class is used for

computer network A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections are ...

ing devices akin to a

network card A network interface controller (NIC, also known as a network interface card, network adapter, LAN adapter or physical network interface, and by similar terms) is a computer hardware component that connects a computer to a computer network. Ear ...

, providing an interface for transmitting

Ethernet Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...

or ATM frames onto some physical media. It is also used for

modem A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by Modulation#Digital modulati ...

ISDN Integrated Services Digital Network (ISDN) is a set of communication standards for simultaneous digital transmission of voice, video, data, and other network services over the digitalised circuits of the public switched telephone network. Wo ...

fax Fax (short for facsimile), sometimes called telecopying or telefax (the latter short for telefacsimile), is the telephonic transmission of scanned printed material (both text and images), normally to a telephone number connected to a printer o ...

machines, and

telephony Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...

applications for performing regular voice calls.

Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for serv ...

versions prior to

Windows Vista Windows Vista is a major release of the Windows NT operating system developed by Microsoft. It was the direct successor to Windows XP, which was released five years before, at the time being the longest time span between successive releases of ...

do not work with the networking parts of the USB CDC, instead using Microsoft's own derivative named Microsoft RNDIS, a serialized version of the Microsoft NDIS (Network Driver Interface Specification). With a vendor-supplied

INF file Inf or INF may refer to: Politics * Intermediate-Range Nuclear Forces Treaty, a 1987 arms control treaty between the United States and the Soviet Union * International Naturist Federation, the global umbrella organisation representing official ...

, Windows Vista works with USB CDC and USB WMCDC devices. This class can be used for industrial equipment such as CNC machinery to allow upgrading from older

RS-232 In telecommunications, RS-232 or Recommended Standard 232 is a standard originally introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a ''DTE'' (''data terminal equipment'') such a ...

serial controllers and robotics, since they can keep software compatibility. The device attaches to an RS-232 communications line and the operating system on the USB side makes the USB device appear as a traditional RS-232 port. While chip manufacturers such as Prolific Technology,

FTDI Future Technology Devices International Limited, commonly known by its acronym FTDI, is a Scottish privately held semiconductor device company, specialising in Universal Serial Bus (USB) technology. It develops, manufactures, and supports devi ...

Microchip An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny M ...

, and

Atmel Atmel Corporation was a creator and manufacturer of semiconductors before being subsumed by Microchip Technology in 2016. Atmel was founded in 1984. The company focused on embedded systems built around microcontrollers. Its products included micr ...

manufacture USB chips and provide drivers that expose the chip as a virtual RS-232 device, the chips do not use USB CDC protocol and rather use their custom protocols, though there are some exceptions (PL2503PL2503 datasheet
/ref>). Devices of this class are also implemented in

embedded system An embedded system is a computer system—a combination of a computer processor, computer memory, and input/output peripheral devices—that has a dedicated function within a larger mechanical or electronic system. It is ''embedded'' as ...

s such as

mobile phone A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive calls over a radio frequency link whil ...

s so that a phone may be used as a modem, fax or

network port In computer networking, a port is a number assigned to uniquely identify a connection endpoint and to direct data to a specific service. At the software level, within an operating system, a port is a logical construct that identifies a specific ...

. The data interfaces are generally used to perform bulk data transfer.

References

External links

USB-IF's Approved Class Specification Documents

Class definitions for Communication Devices 1.2
(.zip file format, size 3.43 MB)
Class definitions for Communication Devices 1.1a good guide (linux-oriented) about USB host-to-host, CDC 'ethernet' class and RNDISArchived Version

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. Communications device class {{compu-hardware-stub