V.11
An ITU-T standard that describes electrical characteristics for balanced double-current interchange circuits for general use with integrated circuit equipment.

V.21
An ITU-T standard that describes 300bps modems for use in the PSTN.

V.24
ITU-T definitions for interchange circuits between data terminal equipment and data circuit-terminating equipment.

V.25
An ITU-T standard that describes automatic calling and/or answering equipment on the general switched network.

V.29
An ITU-T standard that describes 9,600bps modems for use in the PSTN.

V.35
An ITU-T standard that describes data transmission at 56Kbps that uses balanced transmission methods through a 34-pin physical interface.

VAD (voice activity detection)
A technique that reduces the amount of information needed to re-create the voice at the destination end by removing silent periods and redundant information found in human speech; this also helps with compression.

validation
An attempt to find errors by executing a program in a given environment.

value-added carrier
A company that sells services of a value-added network. It can be a PTT, or a subsidiary, or an independent company.

value-added service
A communications facility that uses communications common carrier networks for transmission and that provides enhanced extra data features with separate equipment; such extra features, including store-and-forward message switching, terminal interfacing, and host interfacing, are common.

VAN (value-added network) services
Telecommunication services provided over public or private networks which, in some way, add value to the basic carriage, usually through the applications of computerized intelligence, for instance, reservation systems, bulletin boards, information services.

VAR (value-added reseller)
A provider that deals with distribution and sales.

VBR (variable bit rate)
An ATM service class for network traffic that is typically from bursty data transfer applications, such as client/server and LAN to LAN interconnection. VBR offers guaranteed service delivery. VBR-RT (real-time) is designed for real-time voice and videoconferencing applications, and VBR-NRT (nonreal-time) is for mission-critical data applications.

VC (virtual circuit)
A logical connection established through a packet network, over which packets are routed, mimicking to some extent the behavior of a dedicated physical connection.

VC (virtual container)
A data structure designed for the transport and switching of sub-STM-0 network services such as CEPT-1. All network services below E-3 are mapped into VCs, and VCs are multiplexed into the SPE of an STM-1.

VCI (virtual channel identifier)
In ATM, the part of a cell header that identifies the channel associated with the cell.

VDSL (Very-High-Bit-Rate DSL)
A transmission medium that provides a maximum span of about 5,000 feet (1,500 meters) over a single twisted-pair cable. Over this distance, you can get up to a rate of 13Mbps downstream, and if you shorten the distance to 1,000 feet (300 meters), you can get up to 52Mbps downstream, which would be enough capacity to facilitate tomorrow’s digital TVs. VDSL gives you 1.5Mbps to 2.3Mbps upstream.

VDT (video dial tone)
A U.S. term defining the capability of a network access provider to offer video access and carriage directly to or from subscribers.

vertical blanking interval
Unused lines in each field of a TV signal. Some of these lines may be used for captions and specialized signal and cable service.

VF (voice frequency)
Any frequency within the part of the audio frequency range that is essential for the transmission of speech of commercial quality; that is, 250Hz to 3,400Hz. Also called telephone frequency.

VHF (very high frequency)
The portion of the electromagnetic spectrum with frequencies between about 30MHz and 300MHz. Operating band for radio and television channels.

video signal
A signal comprising frequencies normally required to transmit moving image information.

videoconferencing
A two-way communication between two or more parties that involves the exchange of images as well as voice. The images may or may not be in full motion.

videotelephony
Interactive communication between two-parties involving the transmission and reception of images, as well as voice. The degree to which the images are in full motion depends upon the level of data compression used, as well as the available bandwidth.

videotext
A public or private telecommunication service that offers interactive browsing of a menu of textual and graphical information. The most widely used public videotex service is France Telecom’s service, offered via Minitel terminals.

virtual channel
In ATM, the specific conversation path over which the cells from a given conversation flow from one ATM switch to another.

virtual circuit
A series of logical, rather than physical, connections between sending and receiving devices. With a virtual circuit, two hosts can communicate as though they have a dedicated connection, although the packets may be taking very different routes to arrive at their destination.

VISP (virtual Internet service provider)
A provider that offers outsourced Internet service, running as a branded ISP. It is a turnkey ISP product aimed at affinity groups and mass marketers that want to add Internet access to their other products and services.

VLAN (virtual local area network)
A network of computers that behave as if they are connected to the same wire even though they may actually be physically located on different segments of a LAN. VLANs are configured through software rather than hardware, which makes them extremely flexible. A big advantage of VLANs is that when a computer is physically moved to another location, it can stay on the same VLAN without any hardware reconfiguration.

VLF (very low frequency)
Frequencies below 30KHz.

VoATM (Voice over ATM)
A technology for transporting integrated digital voice, video, and data over ATM networks.

vocoders (voice coder/decoder)
A device used for compression of voice traffic. High-bit-rate vocoders are used by PCS, wireless local loops, and wireless office telecommunication systems applications. Low-bit-rate vocoders are used in cellular systems that deal with vehicular traffic, where there are large cells and a need to facilitate a large number of conversations.

VOD (video-on-demand)
A service whereby a subscriber can order and (almost) immediately view films and other entertainment from a television set. In “true” video-on-demand, the film can be paused, rewound, or fast-forwarded. In “near” video-on-demand these functions are not possible.

VoDSL (Voice over DSL)
A technology for delivering voice transmissions over DSL lines. VoDSL enables service providers to deliver high-speed data access and up to 16 telephone lines over 1 DSL line.

VoFR (Voice over Frame Relay)
A technology for transporting integrated digital voice, video, and data over Frame Relay networks.

voice digitization
Conversion of an analog voice into digital symbols for storage or transmission.

voice grade
A telecommunications link with a bandwidth (about 4KHz) appropriate to an audio telephone line.

voice-grade channel
A channel with a frequency range of 4KHz. Also referred to as a telephone channel.

voice recognition
A technology that allows spoken words in the form of human voice to provide input to a computer.

voice synthesis
Computer-generated sounds that simulate the human voice.

VoIP (Voice over IP)
A technology for transporting integrated digital voice, video, and data over IP networks.

VoIP gateway
A device that bridges the traditional circuit-switched PSTN and the packet-switched Internet.

volatile storage
Memory that loses its contents when electrical power is removed.

VP (virtual path)
A generic term for a collection of virtual channels that have the same endpoint.

VPI (virtual path identifier)
In ATM, the portion of a cell header that identifies the virtual path to which the cell belongs. Virtual paths are defined to permit groups of virtual channels to be manipulated as if they were a single channel.

VPN (virtual private network)
A software-defined network offered by telephone carriers for voice and data communications among multiple sites. The VPN provides the appearance of a private line network, except that it makes use of the public switched network rather than physically dedicated leased lines. In customer-based VPNs, carriers install gateways, routers, and other VPN equipment on the customer premises. This is preferred when customers want to have control over all aspects of security. In network-based VPNs, the carrier houses all the necessary equipment at a point-of-presence (POP) near the customer’s location. Customers that want to take advantage of the carrier’s VPN economies of scale prefer this type of VPN.

VPN gateway
A device that enables VPNs to set up and maintain secure tunnels through the Internet.

VR (virtual reality)
A computer-based application that provides a human-computer interface such that the computer and its devices create a sensory environment called the virtual world. The sensory environment is dynamically controlled by actions of the individual so that the environment appears real.

VRC (vertical redundancy check)
An error-checking method that uses a parity bit for each character.

VSAT (very-small-aperture terminal)
An earth station with a small antenna, usually 0.6 meters or less.

VT (virtual tributary)
A data structure designed for the transport and switching of sub-STS-1 network services such as DS-1, DS-1C, and DS-2. All network services below DS-3 are mapped into VTs, and VTs are multiplexed into the SPE of an STS-1.