U-interface - 2-wire BRI interface. A twisted pair subscriber loop that connects the NT1 reference point to the ISDN network, as defined in the I.411 recommendation. This interface provides Basic Rate Access with an operating frequency of 160 kbps and an information rate144 kbps. Under U.S. regulations, this also marks the line of demarcation between customer-owned equipment and the public network.

UL - Underwriters Laboratory-U.S. Safety standards body.

u-Law - Quantization for voice PCM used in North America.

UNI - User-Network Interface. Customized device that provides entry for   equipment over ATM networks.

UNIX - An immensely powerful and complex operating system for computers for running data processing and telephone systems. UNIX provides multitasking or multi-user capabilities that allow multiple programs to run simultaneously and multiple users to operate a single computer.

Uplink - The station used to transmit signals for a satellite videoconference.

VAR - (Value-added reseller) is a company that takes an existing product, adds its own "value" usually in the form of a specific application for the product (for example, a special computer application), and resells it as a new product or "package". 

VBR - Variable Bit Rate.

VC - Virtual Channel.

VCS - Video Conferencing System.

VGA - Video Graphics Adapter. Standard PC video format.

Videocache - Device which stores video in digital form with formats such as MPEG-1, MPEG-2 and is accessible over all types of networks. Can store video content in customizable amounts with the able to have multiple users access to the same files.

Videocasting - Transmitting of digital video  to multiple users on the network at the same time, on one or more channels.

Videoconferencing - The real-time, usually two-way, transmission of digitized video images between two or more locations. Teleconferencing requires a wideband transmission facility. Transmitted images may be freeze-frame ( where television screen is repainted every few seconds to every 20 seconds ) or full motion. Bandwidth requirements for two-way videoconferencing range from 6 MHz for analog, full-motion, full-color, commercial grade TV to 56 kbps for digitally-encoded freeze-frame to 1.544 kbps for very good quality, full-color, full-motion TV.

Video On Demand ( VOD ) - The ability to access video  on the network. Can be used for  applications such as web documents,  video clips,  PowerPoint and Word documents. Users can simply "point and click" to access and begin a video.

Voice activated switching - In multiway videoconferencing, used so that all participating sites automatically see the site which is currently speaking. 

Voice over internet protocol - The transmission of voice signals over the IP based Internet.

Voice-tracking - camera automatically tracks the voice of the person speaking.

VSAT - Very Small Aperture Terminal. Terminal used as the interface to a satellite-based network.

V.24 - Same as RS-232

V.34 - Modem standard for 33.6 kbps operation.

V.35 - High-speed 15-pin data interface. CCITT standard for trunk interface between a network access device and a packet network that defines signaling for data rates greater than 19.2 kbps.