Telecom Dictionary - Definitions of terms

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

V

Validation - 1. Tests to determine whether an implemented system fulfills its requirements. 2. The checking of data for correctness or for compliance with applicable standards, rules, and conventions. 3. [The] process of applying specialized security test and evaluation procedures, tools, and equipment needed to establish acceptance for joint usage of an AIS by one or more departments or agencies and their contractors. Note: This action will include, as necessary, final development, evaluation, and testing, preparatory to acceptance by senior security test and evaluation staff specialists. 4. In universal personal telecommunications, the process of verifying that a user or terminal is authorized to access UPT services.

Value-Added Carrier: - A company that sells the services of a value-added network.

Value-Added Network (VAN) - A network using the communication services of other commercial carriers, using hardware and software that permit enhanced telecommunication services to be offered.

VAN - Acronym for value-added network.

Variable Length Buffer - A buffer into which data may be entered at one rate and removed at another rate without changing the data sequence. Note: Most first-in first-out (FIFO) storage devices are variable-length buffers in that the input rate may be variable while the output rate is constant or the output rate may be variable while the input rate is constant. Various clocking and control systems are used to allow control of underflow or overflow conditions.

Variable Slope Delta Modulation - A type of delta modulation in which the size of the steps of the approximated signal is progressively increased or decreased as required to make the approximated signal closely match the input analog wave.Same as continuously variable slope delta modulation.

Variance - In statistics, in a population of samples, the mean of the squares of the differences between the respective samples and their mean, expressed mathematically as:where n is the number of samples, x _i is the value of sample i , is the mean of the samples, and ² is the variance. Note: The square root of the variance, , is the standard deviation.

Variant - 1. One of two or more code symbols which have the same plain text equivalent. 2. One of several plain text meanings that are represented by a single code group.

Variation Monitor - In ac power distribution, a device for sensing deviations of any measured variable, such as voltage, current, or frequency, and capable of initiating a programmed action, such as transfer to other power sources, when programmed limits of voltage, current, frequency, or time are exceeded, or providing an alarm, or both.

vars - Abbreviation for volt-amperes reactive. In alternating-current power transmission and distribution, the product of the rms voltage and amperage, i.e., the apparent power, multiplied by the sine of the phase angle between the voltage and the current. Note 1: Vars represents the power not consumed by a reactive load, i.e., when there is a phase difference between the applied voltage and the current. Note 2: Only effective power, i.e., the actual power delivered to or consumed by the load, is expressed in watts. Volt-amperes reactive is properly expressed only in volt-amperes, never watts. Note 3: To maximize transmission efficiency, vars must be minimized by balancing capacitive and inductive loads, or by the addition of an appropriate capacitive or inductive reactance to the load.

VC - Abbreviation for virtual circuit. A communications arrangement in which data from a source user may be passed to a destination user over various real circuit configurations during a single period of communication. (188) Note: Virtual circuits are generally set up on a per-call basis and are disconnected when the call is terminated; however, a permanent virtual circuit can be established as an option to provide a dedicated link between two facilities. Synonyms logical circuit, logical route.

V-Chip - The V-chip is a technology that enables parents to block television programming based on a program's rating. The ratings are encoded within the television signal. The V-chip reads the encoded rating information of each program and blocks shows according to the parents' blocking selections.

VCR: Abbreviation for Video Cassette Recorder.

VDU: Abbreviation for visual display unit. 1. Software or hardware that is used to scrutinize and to display, record, supervise, control, or verify the operations of a system. Note: Possible uses of monitors are to indicate significant departures from the norm, or to determine levels of utilization of particular functional units. 2. A device used for the real-time temporary display of computer output data. Note: Monitors usually use cathode-ray-tube or liquid-crystal technology. Synonyms video display terminal, video display unit, visual display unit.

Vector Processor - Synonym array processor. A processor capable of executing instructions in which the operands may be arrays rather than data elements.

Velocity of Propagation - Of an electrical or electromagnetic signal, its speed in a physical medium such as a coaxial cable or optical fiber.

Verified Off-Hook - In telephone systems, a service provided by a unit that is inserted on each end of a transmission circuit for the purpose of verifying supervisory signals on the circuit. See automatic ringdown circuit.

Vertex Angle - In an optical fiber, the angle formed by the extreme bound meridional rays accepted by the fiber, or emerging from it, equal to twice the acceptance angle; the angle formed by the largest cone of light accepted by the fiber or emitted from it.

Vertical Redundancy Check (VRC) - Synonym transverse redundancy check (TRC) . In synchronized parallel bit streams, a redundancy check (a) that is based on the formation of a block check following preset rules, (b) in which the check-formation rule applied to blocks is also applied to characters, and (c) in which the check is made on parallel bit patterns. Note 1: When the TRC is based on a parity bit applied to each character and block, the TRC can only detect, with limited certainty, whether or not there is an error. It cannot correct the error. Detection cannot be guaranteed because an even number of errors in the same character or block will escape detection, regardless of whether odd or even parity is used. Note 2: Two-dimensional arrays of bits may be used to represent characters or blocks in synchronized parallel data streams. When VRC is combined with longitudinal redundancy checking (LRC), individual erroneous bits can be corrected.

Very High Frequency (VHF) - Frequencies from 30 MHz to 300 MHz. This is the part of the radio spectrum from 30 to 300 megahertz, which includes TV Channels 2-13, and the FM broadcast band.

Very Low Frequency (VLF) - Frequencies from 3 kHz to 30 kHz.

Vestigial Sideband (VSB) Transmission - Modified AM transmission in which one sideband, the carrier, and only a portion of the other sideband are transmitted.

VF - Abbreviation for voice frequency. Pertaining to those frequencies within that part of the audio range that is used for the transmission of speech. (188) Note 1: In telephony, the usable voice-frequency band ranges from approximately 300 Hz to 3400 Hz. Note 2: In telephony, the bandwidth allocated for a single voice-frequency transmission channel is usually 4 kHz, including guard bands. Synonym voice band.

VFCT - Abbreviation for voice frequency carrier telegraph. A method of multiplexing one or more dc telegraph channels onto a nominal 4-kHz voice frequency channel. Synonym voice frequency carrier telegraph.

VFCTG - Abbreviation for voice-frequency carrier telegraph. A method of multiplexing one or more dc telegraph channels onto a nominal 4-kHz voice frequency channel. Synonym voice frequency carrier telegraph.

VF patch bay - See voice frequency primary patch bay. A patching facility that provides the first appearance of local-user VF circuits in the technical control facility (TCF). Note: The VF primary patch bay provides patching, monitoring, and testing for all VF circuits. Signals will have various levels and signaling schemes depending on the user terminal equipment.

VFTG - Abbreviation for voice-frequency telegraph. A method of multiplexing one or more dc telegraph channels onto a nominal 4-kHz voice frequency channel. Synonym voice frequency carrier telegraph.

VGA - Abbreviation for Video Graphics Array. This is a display mode with a resolution of 640 x 480 pixels.

VHF - Abbreviation for very high frequency. Frequencies from 30 MHz to 300 MHz which includes TV Channels 2-13, and the FM broadcast band.

Via Net Loss (VNL) - Pertaining to circuit performance prediction and description that allows circuit parameters to be predetermined and the circuit to be designed to meet established criteria by analyzing actual, theoretical, and calculated losses.

Video - 1. An electrical signal containing timing (synchronization), luminance (intensity), and often chrominance (color) information that, when displayed on an appropriate device, gives a visual image or representation of the original image sequences. 2. Pertaining to the sections of a television system that carry television signals, either in unmodulated or modulated form. 3. Pertaining to the demodulated radar signal that is applied to a radar display device.4. Pertaining to the bandwidth or data rate necessary for the transmission of real-time television pictures. Note: In practice, the baseband bandwidth required for the transmission of NTSC television pictures (not including the audio carriers) is approximately 5 MHz.

Video Codec - See codec. 1. An assembly consisting of an encoder and a decoder in one piece of equipment. (188) 2. A circuit that converts analog signals to digital code and vice versa. 3. An electronic device that converts analog signals, such as video and voice signals, into digital form and compresses them to conserve bandwidth on a transmission path. (188) Note: Codecs in this sense are used in this sense for video conferencing systems. Acronym for coder-decoder .

Videoconference: See video teleconference. 1. A teleconference that includes video communications. 2. Pertaining to a two-way electronic communications system that permits two or more persons in different locations to engage in the equivalent of face-to-face audio and video communications. Note: Video teleconferences may be conducted as if all of the participants were in the same room.

Video Display Terminal - Synonym visual display unit. See monitor. 1. Software or hardware that is used to scrutinize and to display, record, supervise, control, or verify the operations of a system. Note: Possible uses of monitors are to indicate significant departures from the norm, or to determine levels of utilization of particular functional units. 2. A device used for the real-time temporary display of computer output data. Note: Monitors usually use cathode-ray-tube or liquid-crystal technology. Synonym video display unit.

Video Display Unit - Synonym visual display terminal. See monitor. 1. Software or hardware that is used to scrutinize and to display, record, supervise, control, or verify the operations of a system. Note: Possible uses of monitors are to indicate significant departures from the norm, or to determine levels of utilization of particular functional units. 2. A device used for the real-time temporary display of computer output data. Note: Monitors usually use cathode-ray-tube or liquid-crystal technology. Synonym video display terminal,

Video Frame - In video display, one complete scanned image from a series of video images. Note: A video frame is usually composed of two interlaced fields.

Videophone - 1. A telephone that is coupled to an imaging device that enables the call receiver or the call originator, or both, to view one another as on television, if they so desire. 2. A military communications terminal that (a) has video teleconference capability, (b) is usually configured as a small desktop unit, designed for one operator, and (c) is a single, integrated unit.

Video Teleconference - 1. A teleconference that includes video communications. 2. Pertaining to a two-way electronic communications system that permits two or more persons in different locations to engage in the equivalent of face-to-face audio and video communications. Note: Video teleconferences may be conducted as if all of the participants were in the same room.

Video Teleconferencing Unit (VTU) - Equipment that performs video teleconference functions, such as coding and decoding of audio and video signals and multiplexing of video, audio, data, and control signals, and that usually does not include I/O devices, cryptographic devices, network interface equipment, network connections, or the communications network to which the unit is connected.

View - In satellite communications, the quality or degree of visibility of a satellite to a ground station; i.e., the degree to which the satellite is sufficiently above the horizon and clear of obstructions so that it is within a clear line of sight by an Earth terminal. Note: A pair of satellite Earth terminals has a satellite in mutual view when both have unobstructed line-of-sight contact with the satellite simultaneously.

Viewdata - A type of information-retrieval service in which a subscriber can (a) access a remote database via a common carrier channel, (b) request data, and (c) receive requested data on a video display over a separate channel. Note: The access, request, and reception are usually via common carrier broadcast channels. Contrast with teletext.

Violation - See AMI violation. A "mark" that has the same polarity as the previous "mark" in the transmission of alternate mark inversion (AMI) signals. Note: In some transmission protocols, AMI violations are deliberately introduced to facilitate synchronization or to signal a special event.

Virtual Call - A call, established over a network, that uses the capabilities of either a real or virtual circuit by sharing all or any part of the resources of the circuit for the duration of the call.

Virtual Call Capability - A service feature in which (a) a call set-up procedure and a call disengagement procedure determine the period of communication between two data terminal equipments (DTEs) in which user data are transferred by the network in the packet mode of operation, (b) end-to-end transfer control of packets within the network is required, (c) data may be delivered to the network by the call originator before the call access phase is completed, but the data are not delivered to the call receiver if the call attempt is unsuccessful, (d) the network delivers all the user data to the call receiver in the same sequence in which the data are received by the network, and (e) multi-access DTEs may have several virtual calls in progress at the same time. Synonym virtual call facility.

Virtual Call Facility - Synonym virtual call capability. A service feature in which (a) a call set-up procedure and a call disengagement procedure determine the period of communication between two data terminal equipments (DTEs) in which user data are transferred by the network in the packet mode of operation, (b) end-to-end transfer control of packets within the network is required, (c) data may be delivered to the network by the call originator before the call access phase is completed, but the data are not delivered to the call receiver if the call attempt is unsuccessful, (d) the network delivers all the user data to the call receiver in the same sequence in which the data are received by the network, and (e) multi-access DTEs may have several virtual calls in progress at the same time. Synonym virtual call capability.

Virtual Carrier Frequency - In radio or carrier systems in which no carrier is transmitted, e.g., single sideband or double sideband with suppressed carrier, the location in the frequency spectrum that the carrier would occupy if it were present.

Virtual Channel - In telecommunications, a channel designation which differs from the actual channel or frequency on which the signal travels. The term is most often applied to television, where Digital Television (DTV) channels are in-band adjacent to analog ones.

A station branded as Channel 8, for example, might actually use channel 32 for its ATSC or DVB transmission, but a Virtual Channel Map allows viewers to tune in the station on channel 8 on a digital set. The virtual channel thus enables viewers to tune in the station by choosing the same channel number as they would have previously.

Virtual Circuit (VC) - A communications arrangement in which data from a source user may be passed to a destination user over various real circuit configurations during a single period of communication. Note: Virtual circuits are generally set up on a per-call basis and are disconnected when the call is terminated; however, a permanent virtual circuit can be established as an option to provide a dedicated link between two facilities. Synonyms logical circuit, logical route.

Virtual Circuit Capability - A network-provided service feature in which a user is provided with a virtual circuit. Note: Virtual circuit capability is not necessarily limited to packet mode transmission. For example, an analog signal may be converted to a digital signal and then be routed over the network via any available route.

Virtual Connection - A logical connection that is made to a virtual circuit.

Virtual Height - The apparent height of an ionized layer, as determined from the time interval between the transmitted signal and the ionospheric echo at vertical incidence.

Virtual Host - A Web site that includes a customized Domain Name and unique IP address to your home page, giving the appearance that you have your own server directly connected to the Internet.

Virtual Instruction - Synonym teletraining. Training that (a) in which usually live instruction is conveyed in real time via telecommunications facilities, (b) that may be accomplished on a point-to-point basis or on a point-to-multipoint basis, and (c) may assume many forms, such as a teleseminar, a teleconference, or an electronic classroom, usually including both audio and video. Synonyms distance learning, distance training, electronic classroom, virtual instruction.

Virtual Memory - In computer systems, the memory as it appears to, i.e., as it is available to, the operating programs running in the central processing unit (CPU). Note: The virtual memory may be smaller, equal to, or larger than the real memory present in the system.

Virtual Network - A network that provides virtual circuits and that is established by using the facilities of a real network

Virtual Office - A work environment in which employees work cooperatively from different locations using a computer network.

Virtual Path - See virtual circuit. A communications arrangement in which data from a source user may be passed to a destination user over various real circuit configurations during a single period of communication. Note: Virtual circuits are generally set up on a per-call basis and are disconnected when the call is terminated; however, a permanent virtual circuit can be established as an option to provide a dedicated link between two facilities. Synonyms logical circuit, logical route.

Virtual phone number - A feature of VoIP that allows you to attach additional phone numbers with different area codes to your basic VoIP service. This feature allows people to phone you without incurring long-distance charges from the same or adjacent nontoll area codes. All outgoing calls, however, are billed as if coming from your main phone number. Virtual phone numbers typically each cost a few extra dollars per month.

Virtual Reality - An interactive, computer-generated simulated environment with which users can interact using specialized peripherals such as data gloves and head-mounted computer-graphic displays.

Virtual Storage - The storage space that may be regarded as addressable main storage by the user of a computer system in which virtual addresses are mapped into real addresses. Note: The size of virtual storage is limited by the addressing scheme of the computer system and by the amount of auxiliary storage available, and not by the actual number of main storage locations.

Virus - 1. An unwanted program which places itself into other programs, which are shared among computer systems, and replicates itself. Note: A virus is usually manifested by a destructive or disruptive effect on the executable program that it affects. 2. Self-replicating, malicious program segment that attaches itself to an application program or other executable system component and leaves no obvious signs of its presence.

Visual Display Unit (VDU) - See monitor. 1. Software or hardware that is used to scrutinize and to display, record, supervise, control, or verify the operations of a system. Note: Possible uses of monitors are to indicate significant departures from the norm, or to determine levels of utilization of particular functional units. 2. A device used for the real-time temporary display of computer output data. Note: Monitors usually use cathode-ray-tube or liquid-crystal technology.

Visual Telephone Services - A group of audiovisual services including videophone as defined in ITU-T Recommendation F.721 and videoconferencing (defined in ITU-T H-Series Recommendations).

Virtual Terminal (VT) - In open systems, an application service that (a) allows host terminals on a multi-user network to interact with other hosts regardless of terminal type and characteristics, (b) allows remote log-on by local-area-network managers for the purpose of management, (c) allows users to access information from another host processor for transaction processing, and (d) serves as a backup facility.

Virus - An unwanted program which places itself into other programs which are shared among computer systems, and replicates itself. Note: A virus is usually manifested by a destructive or disruptive effect on the executable program that it affects.

Visible Spectrum - The region of the electromagnetic spectrum that can be perceived by human vision, approximately the wavelength range of 0.4 m to 0.7 m.

Visual Display Unit (VDU) - See monitor. 1. Software or hardware that is used to scrutinize and to display, record, supervise, control, or verify the operations of a system. Note: Possible uses of monitors are to indicate significant departures from the norm, or to determine levels of utilization of particular functional units. 2. A device used for the real-time temporary display of computer output data. Note: Monitors usually use cathode-ray-tube or liquid-crystal technology. Synonyms video display terminal, video display unit, monitor.

Vitreous Silica - Glass consisting of almost pure silicon dioxide (SiO₂). Synonym fused silica . Silicon dioxide (SiO₂). Note 1: Silica may occur in crystalline or amorphous form, and occurs naturally in impure forms such as quartz and sand. Note 2: Silica is the basic material of which the most common communication-grade optical fibers are presently made.

VLF - Abbreviation for very low frequency. Frequencies from 3 kHz to 30 kHz.

VNL - Abbreviation for via net loss. Pertaining to circuit performance prediction and description that allows circuit parameters to be predetermined and the circuit to be designed to meet established criteria by analyzing actual, theoretical, and calculated losses.

Voice Frequency (VF) - Pertaining to those frequencies within that part of the audio range that is used for the transmission of speech. Note 1: In telephony, the usable voice-frequency band ranges from approximately 300 Hz to 3400 Hz. Note 2: In telephony, the bandwidth allocated for a single voice-frequency transmission channel is usually 4 kHz, including guard bands. Synonym voice band.

V Number - In an optical fiber, a dimensionless quantity, V , given by

where a is the core radius, is the wavelength in vacuum, n ₁ is the maximum refractive index of the core, and n ₂ is the refractive index of the homogeneous cladding. Note 1: In multimode operation of an optical fiber having a power-law refractive index profile, the approximate number of bound modes, i.e., the mode volume, is given by

where V is the normalized frequency greater than 5 and g is the profile parameter. Note 2: For a step index fiber, the mode volume is given by V ²/2. For single-mode operation, V < 2.405. Synonym normalized frequency (V ).

Vocoder - Abbreviation for voice-coder. A device that usually consists of a speech analyzer, which converts analog speech waveforms into narrowband digital signals, and a speech synthesizer, which converts the digital signals into artificial speech sounds. Note 1: For COMSEC purposes, a vocoder may be used in conjunction with a key generator and a modulator-demodulator to transmit digitally encrypted speech signals over narrowband voice communications channels. These devices are used to reduce the bandwidth requirements for transmitting digitized speech signals. Note 2: Some analog vocoders move incoming signals from one portion of the spectrum to another portion.

Vodas - Acronym for voice-operated device anti-sing. A device used to prevent overall voice-frequency singing in a two-way telephone circuit by ensuring that transmission can occur in only one direction at any given instant.

Vogad - Acronym for voice-operated gain-adjusting device. A device that has a substantially constant output amplitude over a wide range of input amplitudes.

Voice Band - Synonym voice frequency. Pertaining to those frequencies within that part of the audio range that is used for the transmission of speech. (188) Note 1: In telephony, the usable voice-frequency band ranges from approximately 300 Hz to 3400 Hz. Note 2: In telephony, the bandwidth allocated for a single voice-frequency transmission channel is usually 4 kHz, including guard bands. Synonym voice frequency..

Voice Coder - See vocoder. A device that usually consists of a speech analyzer, which converts analog speech waveforms into narrowband digital signals, and a speech synthesizer, which converts the digital signals into artificial speech sounds. Note 1: For COMSEC purposes, a vocoder may be used in conjunction with a key generator and a modulator-demodulator to transmit digitally encrypted speech signals over narrowband voice communications channels. These devices are used to reduce the bandwidth requirements for transmitting digitized speech signals. Note 2: Some analog vocoders move incoming signals from one portion of the spectrum to another portion.

Voice-Data Signal - See quasi-analog signal. A digital signal that has been converted to a form suitable for transmission over a specified analog channel. Note: The specification of the analog channel should include frequency range, bandwidth, signal-to-noise ratio, and envelope delay distortion. When quasi-analog form of signaling is used to convey message traffic over dial-up telephone systems, it is often referred to as voice-data. A modem may be used for the conversion process.

Voice Fax Switch - A device that allows one or more phone lines to be shared for multiple phone connected devices. A common application would allow voice (phones and answering machine) and a fax machine and/or fax modem to operate transparently on a single phone line thus eliminating the monthly costs associated with a dedicated phone line. Applications include any variation of voice, a fax machine, fax/modem and/or modem(s).

Incomming calls are automatically routed to the proper device(s) with no human intervention. If the call comes in and it is a fax call, the call is automatically routed to the fax machine or fax.modem. If the call is a modem call, it is routed to the correct modem. Voice calls are routes to phone(s) and/or answering machine. May require a security access code (SAC) in the dialing string to automatically switch to the modem.

Voice Frequency (VF) - Pertaining to those frequencies within that part of the audio range that is used for the transmission of speech. (188) Note 1: In telephony, the usable voice-frequency band ranges from approximately 300 Hz to 3400 Hz. Note 2: In telephony, the bandwidth allocated for a single voice-frequency transmission channel is usually 4 kHz, including guard bands. Synonym voice band.

Voice Frequency Carrier Telegraph (VFCT) - Synonym voice-frequency telegraph. A method of multiplexing one or more dc telegraph channels onto a nominal 4-kHz voice frequency channel.

Voice-Frequency (VF) Channel - A channel capable of carrying analog and quasi-analog signals.

Voice Frequency (VF) Primary Patch Bay - A patching facility that provides the first appearance of local-user VF circuits in the technical control facility (TCF). Note: The VF primary patch bay provides patching, monitoring, and testing for all VF circuits. Signals will have various levels and signaling schemes depending on the user terminal equipment.

Voice-Frequency Telegraph (VFTG): A method of multiplexing one or more dc telegraph channels onto a nominal 4-kHz voice frequency channel. Synonym voice frequency carrier telegraph.

Voice Grade - In the public regulated telecommunications services, a service grade that is described in part 68, Title 47 of the Code of Federal Regulations [CFR] . Note: Voice-grade service does not imply any specific signaling or supervisory scheme.

Voice Mail - a specific application of an interactive voice response system. In its simplest form it mimics the functions of an answering machine, but uses a centralised system rather than equipment at the individual telephone. However it can be far more sophisticated, with the ability to forward messages to another voice mailbox and send simultaneous messages to multiple voice mailboxes, add voice notes to a message, store messages for future delivery, make calls to a telephone or paging service when a message is received, transfer callers to another phone for personal assistance and play different message greetings to different callers. Voicemail messages are stored in media used by computers to store other forms of data (hard drives, optical drives, and tape).

Many voicemail systems also offer an automated attendant facility, allowing callers answered by the system to dial a selected person's mail box or telephone.

Voicemail systems are found associated with many office telephone systems or (PBX). They may also be associated with public telephone lines as network services. Mobile phones generally have voicemail as a standard network feature.

The Phone Company Voice Mail feature answers calls and takes messages when you are on the phone, on the internet, sending a fax, or not at home. This feature also works when the electrical power is out. You can access your voicemail box through a local access number. You enter your PIN and follow the appropriate prompts to use the service. There is a maximum capacity for messages, minutes per message, and message retention length, as outlined in marketing information. Call Forwarding Busy/No Answer must be ordered but will not be billed when Voice Mail is ordered.

Voice Mail Plus - A phone company feature that includes the same functionality as Voice Mail except it provides additional mailboxes and recording time. Call Forwarding Busy/No Answer must be ordered but not billed when Voice Mail is ordered.

Voice-Operated Device Anti-Sing - See vodas. A device used to prevent overall voice-frequency singing in a two-way telephone circuit by ensuring that transmission can occur in only one direction at any given instant.

Voice-Operated Gain-Adjusting Device - See vogad. A device that has a substantially constant output amplitude over a wide range of input amplitudes.

Voice Operated Relay Circuit - Synonym vox. An acoustoelectric transducer and a keying relay connected so that the keying relay is actuated when sound, or voice, energy above a certain threshold is sensed by the transducer. Note: A vox is used to eliminate the need for push-to-talk operation of a transmitter by using voice energy to turn on the transmitter.

Voice Operated Transmit - Synonym vox. An acoustoelectric transducer and a keying relay connected so that the keying relay is actuated when sound, or voice, energy above a certain threshold is sensed by the transducer. Note: A vox is used to eliminate the need for push-to-talk operation of a transmitter by using voice energy to turn on the transmitter.

Voice-Plus Circuit - Synonym composited circuit. A circuit that can be used simultaneously either for telephony and dc telegraphy or for telephony and signaling. Note: Separation of the two may be accomplished by frequency discrimination.

VoIP – (Voice over Internet Protocol) – A technology used to transmit voice over a data network using Internet Protocol, such as the Internet or intranet networks. VoIP is the technology behind Internet phones. VoIP works by digitizing voice signals and sending them as packets through the same networking channels as your data.

VoIP Telephony - VoIP is a technology that is used to send voice information in the form of digital data packets, over Internet Protocol, as opposed to using traditional telephone lines. VoIP calls can be placed either from an ordinary telephone (broadband phone), or from a computer, using PC-to-Phone software.

VoIP Telephony has proven to be a considerably cheaper and more efficient way to make phone calls, and has become one of the fastest growing trends in telecommunications.

Volatile Storage - A storage device in which the contents are lost when power is removed.

Volatility - See data volatility. Pertaining to the rate of change in the values of stored data over a period of time.

Voltage Standing Wave Ratio (VSWR) - In a transmission line, the ratio of maximum to minimum voltage in a standing wave pattern. Note: The VSWR is a measure of impedance mismatch between the transmission line and its load. The higher the VSWR, the greater the mismatch. The minimum VSWR, i.e., that which corresponds to a perfect impedance match, is unity.

Volt-Amperes Reactive (vars) - In alternating-current power transmission and distribution, the product of the rms voltage and amperage, i.e., the apparent power, multiplied by the sine of the phase angle between the voltage and the current. Note 1: Vars represents the power not consumed by a reactive load, i.e., when there is a phase difference between the applied voltage and the current. Note 2: Only effective power, i.e., the actual power delivered to or consumed by the load, is expressed in watts. Volt-amperes reactive is properly expressed only in volt-amperes, never watts. Note 3: To maximize transmission efficiency, vars must be minimized by balancing capacitive and inductive loads, or by the addition of an appropriate capacitive or inductive reactance to the load.

Volume - A portion of data, with its physical storage medium, that can be handled conveniently as a unit. Note: An example of a volume is a "floppy" diskette.

Volume Unit (VU) - A unit of measurement of the power of an audio-frequency signal, as measured by a vu meter. Note 1: The vu meter is built and used in accordance with American National Standard C16.5-1942. Note 2: When using the vu meter to measure sine wave test tone power, 0 vu equals 0 dBm.

VOX - An acoustoelectric transducer and a keying relay connected so that the keying relay is actuated when sound, or voice, energy above a certain threshold is sensed by the transducer. Note: A vox is used to eliminate the need for push-to-talk operation of a transmitter by using voice energy to turn on the transmitter. Synonyms voice operated relay circuit , voice operated transmit.

V Reference Point - The interface point in an ISDN environment between the line termination and the exchange termination.

VSB - Abbreviation for vestigial sideband. See vestigial sideband transmission. Modified AM transmission in which one sideband, the carrier, and only a portion of the other sideband are transmitted.

V-series Recommendations - Sets of telecommunications protocols and interfaces defined by CCITT (now ITU-T) Recommendations. Note: Some of the more common V.-series Recommendations are:

• V.21: A CCITT Recommendation for modem communications over standard commercially available lines at 300 b/s. This protocol is generally not used in the United States.

• V.22bis: A CCITT Recommendation for modem communications over standard commercially available voice-grade channels at 2,400 b/s and below.

• V.32: A CCITT Recommendation for modem communications over standard commercially available voice-grade channels at 9.6 kb/s and below.

• V.32bis: A CCITT Recommendation for modem communication over standard commercially available voice-grade channels at 14.4 kb/s and below.

• V.34: An ITU-T Recommendation for modem communication over standard commercially available voice-grade channels at 28.8 kb/s and below.

• V.42: A CCITT Recommendation for error correction on modem communications.

• V.42bis: A CCITT Recommendation for data compression on a modem circuit.

• V.FAST: A new CCITT Recommendation for high-speed modems currently under development.

VSWR - Abbreviation for voltage standing wave ratio. In a transmission line, the ratio of maximum to minimum voltage in a standing wave pattern. Note: The VSWR is a measure of impedance mismatch between the transmission line and its load. The higher the VSWR, the greater the mismatch. The minimum VSWR, i.e., that which corresponds to a perfect impedance match, is unity.

VU - Abbreviation for volume unit. A unit of measurement of the power of an audio-frequency signal, as measured by a vu meter. (188) Note 1: The vu meter is built and used in accordance with American National Standard C16.5-1942. Note 2: When using the vu meter to measure sine wave test tone power, 0 vu equals 0 dBm.

Vulnerability - 1. In communications security, see susceptibility. 2. The extent to which a system will degrade when subjected to a specified set of environmental conditions. 3. In communications interference, see susceptibility threshold. 4. In cryptology, the point of weakness within the encrypted system where threats can occur, such as equipment, facilities, hardware, software, operating systems and personnel. 5. [A] weakness in an information system (IS), [in] system security procedures, [in] internal controls, or [in] implementation that could be exploited.

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