Mini Glossary for NII Users

amplitude modulation (AM): Modulation in which the amplitude of a carrier wave is varied in accordance with some characteristic of the modulating signal. (188) Note: Amplitude modulation implies the modulation of a coherent carrier wave by mixing it in a nonlinear device with the modulating signal to produce discrete upper and lower sidebands, which are the sum and difference frequencies of the carrier and signal. The envelope of the resultant modulated wave is an analog of the modulating signal. The instantaneous value of the resultant modulated wave is the vector sum of the corresponding instantaneous values of the carrier wave, upper sideband, and lower sideband. Recovery of the modulating signal may be by direct detection or by heterodyning.

analog data: Data represented by a physical quantity that is considered to be continuously variable and has a magnitude directly proportional to the data or to a suitable function of the data. (188)

audible: Synonym for audible ringing tone.

audible ringing tone: In telephony, a signal, usually consisting of an audio tone interrupted at a slow repetition rate, provided to a caller to indicate that the called-party instrument is being sent a ringing signal. (188) Note: The audible ringing tone may be generated by the called-party servicing switch or by the calling-party servicing switch, but it is not generated by the called telephone instrument. Synonyms audible, ringback tone.

audio frequency (AF): The band of frequencies (approximately 20 Hz to 20 kHz) that, when transmitted as acoustic waves, can be heard by the normal human ear. (188)

blue screening: Synonym chroma keying.

bridge: 1.

bridge lifter: A device that electrically or physically removes bridged telephone pairs. (188) Note: Relays, saturable inductors, and semiconductors are used as bridge lifters.

bridged ringing: The part of a signaling system in which ringers associated with a particular line are connected across that line.

blurring: In video systems, a global distortion, characterized by reduced sharpness of edges and limited spatial detail. [After T1]

cable TV (CATV): A television distribution method in which signals from distant stations are received, amplified, and then transmitted by (coaxial or fiber) cable or microwave links to users. Note 1: CATV originated in areas where good reception of direct broadcast TV was not possible. Now CATV also consists of a cable distribution system to large metropolitan areas in competition with direct broadcasting. Note 2: The abbreviation CATV originally meant "community antenna television. " However, CATV is now usually understood to mean cable TV.

channel: 1. A connection between initiating and terminating nodes of a circuit. (188) 2. A single path provided by a transmission medium via either (a) physical separation, such as by multipair cable or (b) electrical separation, such as by frequency- or time-division multiplexing. (188) 3. A path for conveying electrical or electromagnetic signals, usually distinguished from other parallel paths. (188) 4. Used in conjunction with a predetermined letter, number, or codeword to reference a specific radio frequency. [JCS1-DoD] (188) 5. The portion of a storage medium, such as a track or a band, that is accessible to a given reading or writing station or head. 6. In a communications system, the part that connects a data source to a data sink.

CD ROM: Abbreviation for compact disk read-only memory. An optical digital storage device, of high capacity, capable of being read from but not written to.

chroma keying: In television, nearly instantaneous switching between multiple video signals, based on the state, i.e. , phase, of the color (chroma) signal of one, to form a single composite video signal. Note 1: Chroma keying is used to create an overlay effect in the final picture, e.g. , to insert a false background, such as a weather map or scenic view, behind the principal subject being photographed. Note 2: The principal subject is photographed against a background having a single color or a relatively narrow range of colors, usually in the blue. When the phase of the chroma signal corresponds to the preprogrammed state or states associated with the background color, or range of colors, behind the principal subject, the signal from the alternate, i.e ., false, background is inserted in the composite signal and presented at the output. When the phase of the chroma signal deviates from that associated with the background color(s) behind the principal subject, video associated with the principal subject is presented at the output. Synonym color keying. Colloquial synonym blue screening. Contrast with chrominance signal, composite video.

chrominance signal: In color television, that signal or portion of the composite signal that bears the color information.

client: In networking, a software application that allows the user to access a service from a server computer, e.g., a server computer on the Internet.

client-server: Any hardware/software combination that generally adheres to a client-server architecture, regardless of the type of application.

client-server architecture: Any network-based software system that uses client software to request a specific service, and corresponding server software to provide the service from another computer on the network.

closed captioning: In broadcast and cable television, the insertion, into the blank lines between frames, of information that may be decoded and displayed on the screen as written words corresponding to those being spoken and transmitted via the conventional audio subcarrier. Note: Closed captioning, developed for the hearing-impaired, requires a special decoder, which may be external to, or built into, the television receiver. Closed captioning is mandated by the Americans with Disabilities Act of 1990.

closed circuit: 1. In radio and television transmission, pertaining to an arrangement in which programs are directly transmitted to specific users and not broadcast to the general public. (188) 2. In telecommunications, a circuit dedicated to specific users. (188) 3. A completed electrical circuit.

coax: See coaxial cable.

coaxial cable (coax): A cable consisting of a center conductor surrounded by an insulating material and a concentric outer conductor. (188) Note: Coaxial cable is used primarily for wideband, video, or rf applications.

codec: Acronym for coder-decoder . 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.

color errors: In video systems, distortion of hues in all or a portion of the received image. [After T1]

color keying: See chroma keying.

communications satellite: An orbiting vehicle that relays signals between (a) terrestrial communications stations, (b) a terrestrial communications station and another communications satellite, or (c) other communications satellites.

communications saturation: See saturation.

computer graphics: 1. Graphics implemented through the use of computers. 2. Methods and techniques for converting data to or from graphic displays via computers. [After FP] 3. The branch of science and technology concerned with methods and techniques for converting data to or from visual presentation using computers. [After FP]

concurrent operation: 1. Synonym multitasking. 2. In data link operations, the operation in which two or more data links are used during the same, usually short, time interval, while adhering to the protocols of each link without providing data forwarding among the links.

CPU: Abbreviation for central processing unit . 1. The portion of a computer that includes circuits controlling the interpretation and execution of instructions. (188) 2. The portion of a computer that executes programmed instructions, performs arithmetic and logical operations on data, and controls input/output functions. Synonym central processor. 3. Abbreviation for communications processor unit: The portion of a digital communications switch that executes programmed instructions, performs arithmetic and logical operations on signals, and controls input/output functions.

connectionless mode transmission: In a packet-switched network, transmission in which each packet is encoded with a header containing a destination address sufficient to permit the independent delivery of the packet without the aid of additional instructions. Note 1: A packet transmitted in a connectionless mode is frequently called a datagram. Note 2: In connectionless mode transmission of a packet, the service provider usually cannot guarantee there will be no loss, error insertion, misdelivery, duplication, or out-of-sequence delivery of the packet. However, the risk of these hazards' occurring may be reduced by providing a reliable transmission service at a higher protocol layer, such as the Transport Layer of the Open Systems Interconnection—Reference Model.

connectionless transmission: See connectionless-mode transmission.

connection-mode transmission: See connection-oriented mode transmission.

connection-oriented data transfer protocol: A data-transfer protocol in which a logical connection is established between end user terminals.

connection-oriented mode transmission: In a packet-switched network, a mode of transmission in which there is a complete information transfer transaction for each packet or group of packets, i.e., the information transfer phase is preceded by an access phase and followed by a disengagement phase. Note 1: During the information transfer phase of connection-oriented mode transmission, more than one packet may be transmitted. The header of each information packet contains a sequence number and an identifier field that associates the packet with the connection that was established during the access phase before the information transfer phase begins. Note 2: Connection-oriented mode transmission usually enables detection of lost, erroneous, duplicated, or out-of-sequence packets because a connection is established from end to end before transmission begins. Note 3: The CCITT X.25 protocols are widely used to implement connection-oriented mode transmission on packet-switched public data networks. The protocols are implemented at Layers 1, 2, and 3 of the Open Systems Interconnection—Reference Model.

crosstalk (XT): 1. Undesired capacitive, inductive, or conductive coupling from one circuit, part of a circuit, or channel, to another. 2. Any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. (188) Note: In telephony, crosstalk is usually distinguishable as speech or signaling tones.

crosstalk coupling: The ratio of the power in a disturbing circuit to the induced power in the disturbed circuit observed at specified points of the circuits under specified terminal conditions. Note: Crosstalk coupling is usually expressed in dB. (188) Synonym crosstalk coupling loss.

crosstalk coupling loss: Synonym crosstalk coupling.

dedicated circuit: A circuit designated for exclusive use by specified users. Note: DoD normally considers a dedicated circuit to be between two users only. (188)

dedicated service: In a communications system, a specified set of functions provided to designated users. (188) Note: Dedicated service is usually specified in a communications format, such as voice, digital data, facsimile, or video.

database: 1. A set of data that is required for a specific purpose or is fundamental to a system, project, enterprise, or business. (188) Note: A database may consist of one or more data banks and be geographically distributed among several repositories. 2. A formally structured collection of data. Note: In automated information systems, the database is manipulated using a database management system.

database engineering: The discipline involving (a) the conception, modeling, and creation, i.e., programming, of a database, (b) data analysis and administration of the database, and (c) database documentation.

database management system (DBMS): A software system that facilitates (a) the creation and maintenance of a database or databases, and (b) the execution of computer programs using the database or databases. (188)

data bus: A bus used to transfer data within or to and from a processing unit or storage device.

diagnostic program: 1. A program used to investigate the cause or the nature of conditions or problems within specified elements of a system. (188) 2. A computer program that detects, locates, or identifies a fault in equipment, an error in input data, or an error in a computer program. (188)

dial-up: 1. A service feature in which a user initiates service on a previously arranged trunk or transfers, without human intervention, from an active trunk to a standby trunk. (188) 2. A service feature that allows a computer terminal to use telephone systems to initiate and effect communications with other computers.

direction of scanning: In a facsimile transmitting apparatus, the scanning of the plane (developed in the case of a drum transmitter) of the message surface along lines running from right to left commencing at the top so that scanning commences at the top right-hand corner of the surface and finishes at the bottom left-hand corner; this is equivalent to scanning over a right-hand helix on a drum. Note 1: The orientation of the message on the scanning plane will depend upon its dimensions and is of no consequence. At the receiving apparatus, scanning takes place from right to left and top to bottom (in the above sense) for "positive" reception and from left to right and top to bottom (in the above sense) for "negative" reception. (188) Note 2: This is the CCITT Recommendation for phototelegraphic equipment.

display device: An output unit that gives a visual representation of data.

dithering: A technique for varying raster color or intensity by illuminating and blanking pixels in patterns. Dithering is used to create a wide variety of patterns for use as backgrounds, fills, and shadings, as well as for creating halftones, and for correcting aliasing. Simulating a gray scale by using a group of pixels, each capable of displaying only black and white.

frame grabber: A device that can seize and record a single frame of video information out of a sequence of many frames.

frame pitch: The distance, time, or number of bits between corresponding points, i.e., significant instants, in two consecutive frames. [From Weik '89]

frame rate: The number of frames transmitted or received per unit time. (188) Note 1: The frame rate is usually expressed in frames per second. Note 2: In television transmission, the frame rate must be distinguished from the field rate, which in the NTSC and other systems, is twice the frame rate. synonym frame frequency.

frame relay: An interface protocol for statistically multiplexed for packet-switched data communications in which (a) variable-sized packets (frames) are used that completely enclose the user packets they transport, and (b) transmission rates are usually between 56 kb/s and 1.544 Mb/s (the T-1 rate). Note 1: In frame relay, (a) there is neither flow-control nor an error-correction capability, (b) there is infor- mation-content independence, (c) there is a correspondence only to the ISO Open systems Interconnection—Reference Model Layers 1 and 2, (d) variable-sized user packets are enclosed in larger packets (frames) that add addressing and verification information, (e) frames may vary in length up to a design limit, usually 1 kilobyte or more, (f) one frame relay packet transports one user packet, (g) implementation of fast-packet technology is used for connection-oriented frame relay services, and (h) there is a capability to handle time-delay insensitive traffic, such as LAN interworking and image transfer. Note: Frame relay is referred to as the local management interface (LMI) standard and is specified in ANSI T1.617 .

frame slip: In the reception of framed data, the loss of synchronization between a received frame and the receiver clock, causing a frame misalignment event, and resulting in the loss of the data contained in the received frame. (188) Note: A frame slip should not be confused with a dropped frame where synchronization is not lost, e.g. , in the case of buffer overflow.

frame synchronization: Of a received stream of framed data, the process by which incoming frame alignment signals, i.e. , distinctive bit sequences, are identified, i.e. , distinguished from, data bits, permitting the data bits within the frame to be extracted for decoding or retransmission. Note: The usual practice is to insert, in a dedicated time slot within the frame, a noninformation bit that is used for the actual synchronization of the incoming data with the receiver.

frame synchronization pattern: In digital communications, a prescribed recurring pattern of bits transmitted to enable the receiver to achieve frame synchronization. (188)

framing: 1. In time-division multiplexing reception, synonym frame synchronization . (188) 2. In video reception, the process of adjusting the timing of the receiver to coincide with the received video synchronization pulse. 3. In facsimile, the adjustment of the facsimile picture to a desired position in the direction of line progression. (188)

framing bit: 1. A bit used for frame synchronization. 2. In a bit stream, a bit used in determining the beginning or end of a frame. Note 1: The framing bit occurs at a specific position in the frame. (188) Note 2: In a bit stream, framing bits are noninformation bits. Note 3: Framing in a digital signal is usually repetitive.

framing signal: See frame-alignment signal, framing bit.

freeze frame: A frame of visual information that is selected from a set of motion video frames, and is held in a buffer. (188) Contrast with still video.

freeze frame television: Television in which fixed ("still") images are transmitted sequentially at a rate far too slow to be perceived as continuous motion by human vision. Note: Transmission of an image is usually performed periodically by a processing unit that contains memory in which data representing the image are stored. For an image of specified quality, e.g., resolution and color fidelity, freeze-frame television has a lower bandwidth requirement than that of full-motion television.

FTP: Abbreviation for File Transfer Protocol. The Transmission Control Protocol/Internet Protocol (TCP/IP) protocol that is (a) a standard high-level protocol for transferring files from one computer to another, (b) usually implemented as an application level program, and (c) uses the Telnet and TCP protocols. Note: In conjunction with the proper local software, FTP allows computers connected to the Internet to exchange files, regardless of the computer platform.

fully connected network: See network topology.

fully connected topology: See network topology.

fully intermateable connectors: Connectors from one source that mate with complementary components from other sources without mechanical damage and with transmission properties maintained within specified limits. (188)

functional component (FC): In intelligent networks, an elemental call-processing component that directs internal network resources to perform specific actions, such as collecting dialed digits. Note: An FC is unique to the intelligent-network-IN/2 architecture.

functional profile: A standardization document that characterizes the requirements of a standard or group of standards, and specifies how the options and ambiguities in the standard(s) should be interpreted or implemented to (a) provide a particular information technology function, (b) provide for the development of uniform, recognized tests, and (c) promote interoperability among different network elements and terminal equipment that implement a specific profile.

FX: Abbreviation for fixed service, foreign exchange service.

full-motion operation: In television, a video frame update rate that provides the appearance of full motion without flicker or smear problems. (188) Note: Picture motion appears to be full at greater than 16 fps (frames per second). European television operates at 25 fps and North American television at 30 fps.

jitter: Abrupt and unwanted variations of one or more signal characteristics, such as the interval between successive pulses, the amplitude of successive cycles, or the frequency or phase of successive cycles. (188) Note 1: Jitter must be specified in qualitative terms ( e.g., amplitude, phase, pulse width or pulse position) and in quantitative terms ( e.g., average, RMS, or peak-to-peak). Note 2: The low-frequency cutoff for jitter is usually specified at 1 Hz. Contrast with drift, wander.

monitor jack: A jack used to access communications circuits to observe signal conditions without interrupting the services. (188)

monitor key: A key used to access communications circuits to observe signal conditions without interrupting the services. (188)

Mosaic: A portable World Wide Web browser that provides a graphical user interface to hypertext-based information.

mosquito noise: In a video display, distortion sometimes seen around the edges of moving objects, and characterized by moving artifacts around edges and/or by blotchy noise patterns superimposed over the objects, resembling a mosquito flying around a person's head and shoulders.

motion compensation: Interframe coding that (a) is used to compress motion of video images and (b) uses an algorithm to examine a sequence of image frames to measure the difference from frame to frame in order to send motion vector information. (188)

motion response degradation: The deterioration of motion video quality, resulting in a loss of perceived spatiotemporal resolution. [After T1]

motion video: In video systems, temporally varying visual imagery intended to communicate or to convey movement or change.

mouse: A hand-held computer input device that generates signals that increment, i.e., slew, the position of a cursor on a video display. Note: A mouse is placed on a flat surface and moved manually in the direction in which it is desired to move the cursor. A mouse has momentary switches ("buttons") that may be finger-operated to trigger an event after the cursor is positioned correctly.

multimedia: Pertaining to the processing and integrated presentation of information in more than one form, e.g., video, voice, music, or data. Contrast with multiple media.

multitasking: The concurrent performance or interleaved execution of two or more tasks. [After FP] Synonym concurrent operation.

object persistence: In a video display, distortion wherein the entirity of some object (or objects) that appeared in a previous frame (and that should no longer appear) remain in the current frame and in subsequent frames as a faded image or as an outline.

object retention: In a video display, distortion in which a fragment of an object that appeared in a previous frame (and should no longer appear) remains in the current and subsequent video frames.

PAL-M: A modified version of the phase-alternation-by-line (PAL) television signal standard (525 lines, 50 Hz, 220 V primary power), used in Brazil.

panning: 1. On the viewing screen of a display device, e.g., a computer monitor, horizontal shifting of the entire displayed image. Note: The panning direction is at a right angle with respect to the scrolling direction. 2. In video technology, the use of a camera to scan a subject horizontally. 3. In antenna systems, successively changing the azimuth of a beam of radio-frequency energy over the elements of a given horizontal region, or the corresponding process in reception.

picture black: In TV and facsimile, pertaining to the signal or signal level that corresponds to the darkest part, i.e. , the spot with the lowest luminance or reflectivity, of the object being scanned.

picture element: See pel, pixel.

picture frequency: In analog facsimile systems, a baseband frequency generated by scanning an object. (188) Note: Picture frequencies do not include frequencies that are present in a modulated carrier.

picture white: In TV and facsimile, pertaining to the signal level that corresponds to the brightest part, i.e. , the spot with the highest luminance or reflectivity, of the object being scanned.

radio telephony: The transmission of speech by means of modulated radio waves. [JP1]

raster: A predetermined pattern of scanning lines within a display space. Note: An example of a raster is the pattern followed by an electron beam scanning the screen of a television camera or receiver. (188)

raster count: The total number of raster scanning lines within a display space. [From Weik '89]

raster density: In display systems, the number of scanning lines per unit distance perpendicular to the scanning direction. [From Weik '89]

raster scanning: Scanning in which the motion of the scanning spot follows a raster. (188)

read head: A magnetic head capable of reading only. [After FP]

reading: The acquisition or interpretation of data from a storage device, from a data medium, or from another source. [After FP]

read-only memory (ROM): A memory in which data, under normal conditions, can only be read. [After FP] Synonym nonerasable storage.

read-only storage: A storage device in which the contents cannot be modified, except by a particular user, or when operating under particular conditions, for example, a storage device in which writing is prevented by a lockout. (188) [After FP] Synonym fixed storage.

read/write opening: Synonym read/write slot.

read/write slot: An opening in the jacket of a diskette to allow access to the read/write heads. Synonym read/write opening.

real time: 1. The actual time during which a physical process occurs. (188) 2. Pertaining to the performance of a computation during the actual time that the related physical process occurs, in order that results of the computation can be used in guiding the physical process.

record: 1. A set of data treated as a unit. (188) 2. To write data on a medium, such as magnetic tape, magnetic disk, or optical disk.

record medium: 1. The physical medium on which information is stored in recoverable form. (188) 2. In facsimile transmission, the physical medium on which the recorder forms an image of the object, i.e., creates the recorded copy. (188) Note: The record medium and the record sheet may be identical. Synonym record sheet.

record sheet: Synonym record medium.

recovery: In a database management system, the procedures and capabilities available for reconstruction of the contents of a database to a state that prevailed before the detection of processing errors and before the occurrence of a hardware or software failure that resulted in the destruction of some or all of the stored data.

recovery procedure: 1. The actions necessary to restore an automated information system's data files and computational capability after a system failure. 2. In data communications, a process whereby a data station attempts to resolve conflicting or erroneous conditions arising during the transfer of data.

reference black level: [In television,] The level corresponding to the specified maximum excursion of the luminance signal in the black direction. [47CFR]

reframing time: The time interval between the instant at which a valid frame-alignment signal is available at the receiving data terminal equipment and the instant at which frame alignment is established. Note: The reframing time includes the time required for replicated verification of the validity of the frame-alignment signal. Synonym frame-alignment recovery time.

refresh: To reproduce, repeatedly, a display image on a display surface, so that the image remains visible. [After FP]

regeneration: 1. In a regenerative repeater, the process by which digital signals are amplified, reshaped, retimed, and retransmitted. (188) 2. In a storage or display device, the restoration of stored or displayed data that have deteriorated. (188) Note: For example, conventional cathode-ray tube displays must be continually regenerated for the data to remain displayed. 3. In computer graphics, the sequence of events needed to generate a display image from its representation in storage. [After FP]

snow: In video display systems, noise that (a) is uniformly distributed on the display surface, such as that of a television or radar screen, (b) has the appearance of a uniform distribution of fixed or moving spots, mottling, or speckling, and (c) is usually caused by random noise on an intensity-modulated signal in a display device, such as a cathode-ray tube.

source program: 1. A computer program written in a source language. Note: An example of a source program is a program that serves as the input to an assembler, compiler, or translator. 2. A computer program that must be assembled, compiled, or translated before it can be executed by a computer. [From Weik '89]

source quench: A congestion-control technique in which a computer experiencing data traffic congestion sends a message back to the source of the messages or packets causing the congestion, requesting that the source stop transmitting.

source user: The user providing the information to be transferred to a destination user during a particular information transfer transaction. Synonym information source.

spatial application: An application requiring high spatial resolution, possibly at the expense of reduced temporal positioning accuracy, i.e., increased jerkiness. Note: Examples of spatial applications include the requirement to display small characters and to resolve fine detail in still video, or in motion video that contains very limited motion. [After T1]

spatial edge noise: In a video display, that form of edge busyness that is characterized by spatially varying distortion that occurs in close proximity to the edges of objects.

special grade access line: In the Defense Switched Network, an access line specially conditioned, usually by providing amplitude and delay equalization, to give it characteristics suitable for handling special services, such as reducing data signaling rates (DSR) to a rate between 600 b/s and 2400 b/s. (188)

special purpose computer: A computer that is designed to operate on a restricted class of problems. [After FP]

speech synthesizer: A device that is capable of accepting digital or analog data and developing intelligible speech sounds that correspond to the input data, without resorting to recorded sounds or without simply being a speech scrambler operating in reverse. [From Weik '89]

speed calling: A service feature that enables a switch or station to store certain telephone numbers and dial them automatically when a short (1-, 2-, or 3-digit) code is entered. (188) Contrast with repertory dialer, speed dialing.

speed dialing: 1. Synonym abbreviated dialing . 2. Dialing at a speed greater than the normal ten pulses per second. (188)

split screen: On a display device, display space that has been divided into two or more areas, so that each area can display different portions of the same file or portions of different files. Note 1: The split screen excludes the data lying between the portions of the file or files being displayed and includes the desired data in the two or more windows afforded by the split screen. Note 2: Examples of split screens are screens in which different portions of a spreadsheet, database, graph, or picture that are too far apart in storage to be viewed or displayed simultaneously as a single image, are viewed adjacently on a single screen. [From Weik '89]

splitter: See directional coupler .

spooling: The use of auxiliary storage as buffer storage to reduce processing delays when transferring data between peripheral equipment and the processors of a computer. [After FP] Note: The term is derived from the expression "simultaneous peripheral operation on line."

spot beam: In satellite communications systems, a narrow beam from a satellite station antenna that illuminates, with high irradiance, a limited area of the Earth by using beam (directive) antennas rather than Earth-coverage antennas.

spot size: 1. The size of the electron spot on the face of a cathode ray tube. Note: The spot size is larger than the diameter of the electron beam because of the spillover of electrons into adjacent areas of the screen near the spot. The spot size is a function of the ability of the tube to focus the electron beam, as well as of the electron gun aperture. [From Weik '89] 2. In facsimile systems, the diameter of the scanning spot or the recording spot. [From Weik '89] 3. In single-mode optical fibers, the effective core diameter.

spot speed: In facsimile systems, the speed of the scanning or recording spot along the available line. (188) Note: The spot speed is usually measured on the object or on the recorded copy.

stagger: In facsimile systems, periodic error in the position of the recorded spot along the recorded line. (188)

star network: See network topology.

still image: Nonmoving visual information, i.e., fixed images, such as graphs, drawings, and pictures. (188)

still video: Video imagery that is not intended to convey the appearance of movement. Contrast with freeze frame, freeze-frame television.

stop-record signal: In facsimile systems, a signal used for stopping the process of converting the electrical signal to an image on the record medium. (188)

stop signal: 1. In start-stop transmission, a signal at the end of a character that prepares the receiving device for the reception of a subsequent character. A stop signal is usually limited to one signal element having any duration equal to or greater than a specified minimum value. [After FP] (188) 2. A signal to a receiving mechanism to wait for the next signal. [After FP]

storage: 1. The retention of data in any form, usually for the purpose of orderly retrieval and documentation. [JP1] 2. A device consisting of electronic, electrostatic, electrical, hardware or other elements into which data may be entered, and from which data may be obtained, as desired. [JP1]

storage cell: 1. An addressable storage unit. [After FP] 2. The smallest subdivision of storage into which a unit of data can be entered, stored, and retrieved. [After FP] Synonym storage element.

storage element: Synonym storage cell.

storage register: See register.

store-and-forward (S-F): Pertaining to communications systems in which messages are received at intermediate routing points and recorded i.e., stored, and then transmitted, i.e., forwarded, to the next routing point or to the ultimate recipient. (188)

store-and-forward switching center: A message switching center in which a message is accepted from the originating user, i.e., sender, when it is offered, held in a physical storage, and forwarded to the destination user, i.e., receiver, in accordance with the priority placed upon the message by the originating user and the availability of an outgoing channel. (188)

stored-program computer: A computer that (a) is controlled by internally stored instructions, (b) can synthesize and store instructions, and (c) can subsequently execute those instructions. [After FP]

string: A sequence of data elements, such as bits or characters, considered as a whole.

stroke: A straight line or arc that is used as a segment of a graphic character. [After FP]

stroke edge: In character recognition, the line of discontinuity between a side of a stroke and the background, obtained by averaging, over the length of the stroke, the irregularities resulting from the printing and detecting processes. [After FP]

stroke speed: In facsimile systems, the rate at which a fixed line perpendicular to the direction of scanning is crossed in one direction by a scanning or recording spot. (188) Note 1: Stroke speed is usually expressed as a number of strokes per minute. When the system scans in both directions, the stroke speed is twice this number. Note 2: In most conventional mechanical systems, the stroke speed is equivalent to drum speed.

stroke width: In character recognition, the distance between the two edges of a stroke, measured perpendicular to the stroke centerline. [After FP]

swim: Slow, graceful, undesired movements of display elements, groups, or images about their mean position on a display surface, such as that of a monitor. Note 1: Swim can be followed by the human eye, whereas jitter usually appears as a blur. Note 2: Jitter, swim, wander, and drift have increasing periods of variation in that order.

video codec: See codec.

videoconference: See video teleconference.

video display terminal: Synonym visual display unit. See monitor.

video display unit: Synonym visual display unit. See monitor.

video frame: See frame (def. #6).

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. [From Weik '89] 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. (188) [From Weik '89]

video teleconference: 1. A teleconference that includes video communications. (188) 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 are in the same room. (188)

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. (188)

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. (188) 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.

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 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. [After FP]

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. Note: A virus is usually manifested by a destructive or disruptive effect on the executable program that it affects.

voice band: Synonym voice frequency.

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 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.

volatile storage: A storage device in which the contents are lost when power is removed. [After FP]

volume unit (vu): 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.

vu: Abbreviation for volume unit.

warm boot: Synonym warm restart .

warm restart: 1. A sequence of operations that is performed to reset a previously running system, after an unintentional shutdown. Synonym warm start. 2. In computer operations, the restarting of equipment, after a sudden shutdown, that allows reuse of previously retained initialized input data, retained programs, and retained output queues. Note 1: A warm restart may be needed after a program failure. Note 2: A warm start or restart cannot occur if initial data, programs, and files are not retained after closedown. Synonyms hot boot, warm boot. [From Weik '89]

warm start: Synonym warm restart.

WATS: Acronym for Wide Area Telephone Service.

white facsimile transmission: 1. In an amplitude-modulated facsimile system, transmission in which the maximum transmitted power corresponds to the minimum density, i.e., the white area, of the object. (188) 2. In a frequency-modulated facsimile system, transmission in which the lowest transmitted frequency corresponds to the minimum density i.e., the white area, of the object. (188)

Wide Area Telephone Service (WATS): A toll service offering for customer dial-type telecommunications between a given customer [user] station and stations within specified geographic rate areas employing a single access line between the customer [user] location and the serving central office. Each access line may be arranged for either outward (OUT-WATS) or inward (IN-WATS) service, or both. [47CFR] Note: The offering is for fixed-rate inter- and intra-LATA services measured by zones and hours.

wideband: 1. The property of any communications facility, equipment, channel, or system in which the range of frequencies used for transmission is greater than 0.1 % of the midband frequency. (188) Note: "Wideband" has many meanings depending upon application. "Wideband" is often used to distinguish it from "narrowband," where both terms are subjectively defined relative to the implied context. 2. In communications security systems, a bandwidth exceeding that of a nominal 4-kHz telephone channel. (188) [From Weik '89] 3. The property of a circuit that has a bandwidth wider than normal for the type of circuit, frequency of operation, or type of modulation. 4. In telephony, the property of a circuit that has a bandwidth greater than 4 kHz. (188) 5. Pertaining to a signal that occupies a broad frequency spectrum. Synonym broadband. [From Weik '89]

wideband channel: A communication channel of a bandwidth equivalent to twelve or more voice-grade channels. [47CFR]

wideband modem: 1. A modem whose modulated output signal can have an essential frequency spectrum that is broader than that which can be wholly contained within, and faithfully transmitted through, a voice channel with a nominal 4-kHz bandwidth. (188) 2. A modem whose bandwidth capability is greater than that of a narrowband modem.

window: 1. In fiber optics, a band of wavelengths at which an optical fiber is sufficiently transparent for practical use in communications applications. [After FAA] Synonyms spectral window, transmission window. See first window, second window, third window. 2. In imagery, a portion of a display surface in which display images pertaining to a particular application can be presented. Note: Different applications can be displayed simultaneously in different windows. 3. A period during which an event can occur, can be expected to occur, or is allowed to occur.

windowing: Sectioning of a video display area into two or more separate regions for the purpose of displaying images from different sources. (188) Note: In windowing, one window could display data, another motion video from a remote site, and another, graphics.

World Wide Web (WWW): An international, virtual-network-based information service composed of Internet host computers that provide on-line information in a specific hypertext format. Note 1: WWW servers provide hypertext metalanguage (HTML) formatted documents using the hypertext transfer protocol (HTTP). Note 2: Information on the WWW is accessed with a hypertext browser such as Mosaic, Viola, or Lynx. Note 3: No hierarchy exists in the WWW, and the same information may be found by many different approaches.

write: To make a permanent or transient recording of data in a storage device or on a data medium. [After FP]

write cycle time: The minimum time interval between the starts of successive write cycles of a storage device that has separate reading and writing cycles. [After FP]

write head: A magnetic head capable of writing only. [After FP]

write protection label: See write-protect tab.

write-protect tab: A movable or removable tab, label, or other device, the presence or absence of which on the casing of a recording medium prevents writing on the medium. Note: An example of a write-protect tab is the sliding tab on a 3 ½ -inch magnetic diskette of the type used in conjunction with desktop computers.