Glossary

Glossary - V

VAD

Value-added distributor.

VAN

Value-added network.

VAR

Value-added reseller.

VBR

Variable bit rate.

VCI

Virtual circuit identifier.

VCS

Virtual circuit switch.

VDSL

Very high data rate Digital Subscriber Line. This is one of the family of the DSL services. It offers csutomers a very high data rate over relatively short distances (can be up to 55Mbit/s over a line length of only 300 metres).

Video Conferencing

Video conferencing is usually a two-way, real-time, transmission of images and voice between at least two or more locations. Both voice and video (analog) signals are actually digitized by video codices. This happens before a transmission which can involve wide bandwidths. To conserve bandwidth, some of the systems employ a "freeze frame," which is where a television screen is just "repainted" every few seconds. Codecs for a higher-quality full motion video act with the attempt to minimize the bandwidth requirements and do so by taking advantage of the intervals occuring with relatively little motion (those that require smaller bandwidths), and also by trading off smooth motion and picture and tracking resolution.

Virtual Private Networks (VPN)

VPNs are services that use public network facilities that are augmented by network control points and also service management system facilities. Within this, traffic is routed through public networks constantly under computer control. This is done in a manner that makes VPN service indistinguishable from other dedicated facilities that are based on private networks. Customers have the ablility to define, control and change control network resources. This can be done with the same or even more flexibility as is afforded by the facilities based on private networks.

Voice mail

The facility to leave a voice message that can be accessed from many different locations.

Voice telephony service

This operates as a service that's available to the public enabling the commercial provision of the direct transport of real-time speech. This is done via the public switched network or alternatively through networks that any user can access the equipment connected to a the termination point of a network at its fixed location which leads to communication with another user of equipment that is connected to an alternative termination point.

VoIP (Voice over Internet Protocol)

This is the generic name for the transport of voice traffic by using Internet Protocol (IP) technology. VoIP traffic can be carried on private managed networks or the public Internet (please see Internet telephony) or even a combination of both. Some organisations may use the term 'IP telephony' as an alternative to 'VoIP'.

VPI

Virtual path identifier.

VRU

Voice response unit.

VSAT

Very small aperture terminal.

VTN - Virtual Text Network

This is a BT network product that is currently in development. It would enable textphone users to be able to make greater and more efficient use of there text relay services and also the network generally.