a: Abbreviation for atto (10^-18). See metric system.

abandoned call: A call in which the call originator disconnects or cancels the call after a connection has been made, but before the call is established.

abandonment: Network replacement of a connect signal with an on-hook signal (network) prior to receiving a CI (customer installation) answer signal; abandonment is the only way to end an unanswered call attempt. [T1.405-1989]

abbreviated address: An address that has fewer characters than the full address, usually for special communications and other services or for certain users. Note: Examples of abbreviated addresses are (a) a four-digit telephone number for a user calling another user connected to the same switching exchange, and (b) message addresses that have only the addressee name and station code or number. [After Weik '96]

abbreviated address calling: Calling that enables a user to employ an address having fewer characters than the full address when initiating a call. Note: Communications network users may be allowed to designate a given number of abbreviated address codes. The allocation of the abbreviated address codes to a destination or group of destinations may be changed as required, by means of a suitable procedure. [After Weik '96]

abbreviated dialing: A telephone service feature that (a) permits the user to dial fewer digits to access a network than are required under the nominal numbering plan, and (b) is limited to a subscriber-selected set of frequently dialed numbers. Synonym speed dialing.

abort: 1. In a computer or data transmission system, to terminate, usually in a controlled manner, a processing activity because it is impossible or undesirable for the activity to proceed. 2. In data transmission, a function invoked by a sending station to cause the recipient to discard or ignore all bit sequences transmitted by the sender since the preceding flag sequence.

abrasive: Any of a number of hard materials, such as aluminum oxide, silicon carbide, and diamond, that are powdered and carefully graded according to particle size, and used to shape and/or finish optical elements, including the endfaces of optical fibers and connectors. Note: For finishing the endfaces of optical fiber connectors, abrasive particles are adhered to a substrate of plastic film, in a fashion after that of sandpaper. The film is in turn supported by a hard, flat plate. The connector is supported by a fixture that holds it securely in the proper position for finishing. The grinding motion may be performed manually or by a machine. [After FAA]

absolute address: In communications, computer, and data processing systems, an address that directly identifies a storage location without the use of an intermediate reference, e.g., a base address or a relative address. [After Weik '96]

absolute delay: 1. The time interval or phase difference between transmission and reception of a signal. 2. The total time between the instant a bit enters the network and the instant a corresponding bit exists the network.[T1.503-1989] [T1.507-1996]

absolute gain: 1. Of an antenna, for a given direction and polarization, the ratio of (a) the power that would be required at the input of an ideal isotropic radiator to (b) the power actually supplied to the given antenna, to produce the same radiation intensity in the far-field region. Note 1: If no direction is given, the absolute gain of an antenna corresponds to the direction of maximum effective radiated power. Note 2: Absolute gain is usually expressed in dB. Synonym isotropic gain. 2. Of a device, the ratio of (a) the signal level at the output of the device to (b) that of its input under a specified set of operating conditions. Note 1: Examples of absolute gain are no-load gain, full-load gain, and small-signal gain. Note 2: Absolute gain is usually expressed in dB.

absolute temperature: See thermodynamic temperature.

absorptance: The ratio of the luminous flux or absorbed radiant flux to the incident flux.

absorption: In the transmission of electrical, electromagnetic, or acoustic signals, the conversion of the transmitted energy into another form, usually thermal. [After 2196] Note 1: Absorption is one cause of signal attenuation. Note 2: The conversion takes place as a result of interaction between the incident energy and the material medium, at the molecular or atomic level.

absorption band: A spectral region in which the absorption coefficient reaches a relative maximum, by virtue of the physical properties of the matter in which the absorption process takes place. [FAA]

absorption coefficient: A measure of the attenuation caused by absorption of energy that results from its passage through a medium. [After 2196] Note 1: Absorption coefficients are usually expressed in units of reciprocal distance. Note 2: The sum of the absorption coefficient and the scattering coefficient is the attenuation coefficient.

absorption index: 1. A measure of the attenuation caused by absorption of energy per unit of distance that occurs in an electromagnetic wave of given wavelength propagating in a material medium of given refractive index. Note: The value of the absorption index K ' is given by the relation

where K is the absorption coefficient, is the wavelength in vacuum, and n is the refractive index of the absorptive material medium. [After 2196] 2. The functional relationship between the Sun angle--at any latitude and local time--and the ionospheric absorption.

absorption loss: That part of the transmission loss caused by the dissipation or conversion of electrical, electromagnetic, or acoustic energy into other forms of energy as a result of its interaction with a material medium.

absorption modulation: Amplitude modulation of the output of a radio transmitter by means of a variable-impedance circuit that is caused to absorb carrier power in accordance with the modulating wave.

absorption peak: 1. The wavelength or frequency at which a particular substance absorbs the most power (or, where the attenuation of the propagated signal is the greatest) whenever the substance is bombarded or irradiated with audio, electromagnetic, or light waves. Note: Whenever a material is thus bombarded, there is reflection, transmission through the material, and absorption within the material. In the case of air, O² has multiple absorption peaks. See figure. 2. In optical communications media, the specific wavelength at which a particular impurity absorbs the most power, i.e., causes a maximum attenuation of propagated lightwaves. Note: Absorption by these impurities at other wavelengths is less than that of the absorption peak. Glass quartz silica, and plastics used in optical fibers, slab dielectric waveguides, optical integrated circuits (OICs), and similar media, usually display absorption peaks. Impurities that cause absorption peaks include copper, iron, nickel, chromium, manganese, and hydroxyl ions. [From Weik]

absorptivity: Of a material propagation medium, absorptance per unit path length.

abstraction: 1. Broadly, the use of specialized software, such as an application programming interface (API), as a means of shielding software from device dependencies or the complexities of underlying software. Note: For instance, hardware abstraction enables programs to focus on a task, such as communications, instead of on individual differences between communications devices. 2. In object-oriented programming, the process of reducing an object to its essence so that only the necessary elements are represented. Abstraction defines an object in terms of its properties (attributes), behaviors (functionality), and interface (means of communicating with other objects). [MS]

abstract syntax: In open systems architecture, the specification of application-layer data or application-protocol control information by using notation rules that are independent of the encoding technique used to represent the information.

abstract syntax notation one (ASN.1): A standard, flexible method that (a) describes data structures for representing, encoding, transmitting, and decoding data, (b) provides a set of formal rules for describing the structure of objects independent of machine-specific encoding techniques, (c) is a formal network-management Transmission Control Protocol/Internet Protocol (TCP/IP) language that uses human-readable notation and a compact, encoded representation of the same information used in communications protocols, and (d) is a precise, formal notation that removes ambiguities.

ac: Abbreviation for alternating current.

accept: In data transmission, the condition assumed by a primary or secondary station upon correct receipt of a frame for processing.

acceptance: The condition that exists when a system or functional unit meets the specified performance and security requirements.

acceptance angle: In fiber optics, half the vertex angle of that cone within which optical power may be coupled into bound modes of an optical fiber. Note 1: The axis of the cone is collinear with the fiber axis, the vertex of the cone is on the fiber end-face, and the base of the cone faces the optical power source. Note 2: The acceptance angle is measured with respect to the fiber axis. Note 3: Rays entering an optical fiber at angles greater than the acceptance angle are coupled into unbound modes. [After 2196]

acceptance cone: In fiber optics, the cone within which optical power may be coupled into the bound modes of an optical fiber. Note: The acceptance cone is derived by rotating the acceptance angle about the fiber axis. [After 2196]

acceptance criterion: See acceptance test.

acceptance limit (AL): The bound on performance that is allowed at service turnup or interexchange carrier (IC) acceptance of a circuit or connection, or when corrective action is taken to restore a parameter after an immediate action limit (IAL) failure. Performance as measured by a parameter is satisfactory if the value of the parameter is equal to or better than the limit. [T1.208-1989]

acceptance pattern: 1. Of an antenna, for a given plane, a distribution plot of the off-axis power relative to the on-axis power as a function of angle or position. [After 2196] Note: The acceptance pattern is the equivalent of a horizontal or vertical antenna pattern. 2. Of an optical fiber or fiber bundle, a curve of total transmitted power plotted against the launch angle.

acceptance test: A test of a system or functional unit to ensure that contractual requirements are met. Note: An acceptance test may be performed at the factory or user premises by the user, vendor, or a third party.

acceptance testing: Operating and testing of a communication system, subsystem, or component, to ensure that the specified performance characteristics have been met.

acceptance trial: A trial carried out by nominated representatives of the eventual military users of the weapon or equipment to determine if the specified performance and characteristics have been met.

accepted interference: Interference at a higher level than that defined as permissible interference and which has been agreed upon between two or more administrations without prejudice to other administrations. [NTIA] [RR]

access: 1. The ability and means necessary to store data in, to retrieve data from, to communicate with, or to make use of any resource of a system. 2. To obtain the use of a resource. 3. (COMSEC) [The] capability and opportunity to gain detailed knowledge of or to alter information or material. [NIS] 4. (AIS) [The] ability and means to communicate with (i.e. , input to or receive output from), or otherwise make use of any information, resource, or component in an AIS. Note [for 3 and 4]: An individual does not have "access" if the proper authority or a physical, technical, or procedural measure prevents him/her from obtaining knowledge or having an opportunity to alter information, material, resources, or components. [NIS] 5. An assigned portion of system resources for one data stream of user communications or signaling. 6. [An] opportunity to make use of an information-system (IS) resource. [INFOSEC-99]

access arrangement: An arrangement to transport access traffic between an end office and an IC POT (interexchange carrier point of termination) and may be either direct routed, tandem routed, or a combination of direct and tandem routed. (In the case of direct routed only, the access arrangement and access connection are synonymous.) [T1.Rpt 11-1991]

access attempt: The process by which one or more users interact with a telecommunications system to enable initiation of user information transfer. Note: An access attempt begins with an issuance of an access request by an access originator. An access attempt ends either in successful access or in access failure.

access barred signal: In a communications system, a signal sent in the backward direction to indicate that a call will not be completed because of a call-originator or a call-receiver facility requirement. Note: An access barred signal may occur for many reasons, such as the failure of a closed user group validation check on an incoming calls-barred facility. [After Weik '96]

access category: A class to which a user, such as a person, program, process, or equipment, of a system may be assigned, based on the resources each user is authorized to use.

access channel: A designated part of the information transfer capability having specified characteristics, provided at the user-network interface. [T1.667-1999]

access charge: 1. A fee charged by a local exchange carrier for the use of its local exchange networks. [FCC] 2. A charge made by a local exchange carrier for use of its local exchange facilities for a purpose such as the origination or termination of traffic that is carried to or from a distant exchange by an interexchange carrier.

access code: 1. The preliminary digits that a user must dial to be connected to a particular outgoing trunk group or line. 2. A uniform code assigned by the telephone company to an individual customer in the form 101xx and 950-xx. [NECA/FCC-5]

access connection: In ISDN technology, a connection (using either the B-Channel or a logical link on the D-Channel) established between the user equipment and a packet-mode handier function, over which packet-mode calls (incoming and outgoing) are established. [After T1.615-1992]

access contention: In ISDN applications, synonym contention.

access control: 1. A service feature or technique used to permit or deny use of the components of a communication system. 2. A technique used to define or restrict the rights of individuals or application programs to obtain data from, or place data onto, a storage device. 3. The definition or restriction of the rights of individuals or application programs to obtain data from, or place data into, a storage device. 4. Limiting access to information system resources only to authorized users, programs, processes, or other systems. [INFOSEC-99] 5. That function performed by the resource controller that allocates system resources to satisfy user requests.

access control list: 1. In security, a list of entities, together with their access rights, that are authorized to access a resource. 2. [A] mechanism implementing discretionary and/or mandatory access control between subjects and objects. [INFOSEC-99]

access control mechanism: 1. In security, a hardware, software, or firmware feature, operating procedure, or management procedure that (a) permits authorized access to a system, such as a communications, computer, and data processing system, (b) prevents unauthorized access to the system, and (c) is considered to have failed when unauthorized access is permitted or when authorized access is prevented. [From Weik '96] 2. [A] Security safeguard designed to detect and deny unauthorized access and permit authorized access in an information system (IS). [INFOSEC-99]

access control message: A message that is a user request, a resource controller response, or a request/response between resource controllers.

access coupler: Deprecated term. See directional coupler.

access denial: 1. Access failure caused by the issuing of a system blocking signal by a communications system that does not have a call-originator camp-on feature. 2. Access failure caused by exceeding the maximum access time and nominal system access time fraction during an access attempt. Synonym system blocking.

access-denial time: The time between the start of an access attempt and access failure caused by access denial, i.e., system blocking. Note: Access denial times are measured only on access attempts that result in access denial.

access digit: In automatic direct outward telephone dialing, a digit, often a 1, or a 9, that (a) enables access to an outside facility, e.g. , a PBX or local exchange, and (b) is prefixed to the specific number being dialed. Note 1: Throughout the United States, an access digit, usually 1, must be prefixed to an area code before dialing the area code and the specific number to which a connection is desired. Note 2: The access digit 9 is often used to establish a connection between a PBX and a local exchange.

access failure: In a communications system, an unsuccessful access that results in termination of an access attempt in any manner other than initiation of user information transfer between the intended source and destination (sink) within the specified maximum access time. Note: Access failure can be the result of access denial, access outage, user blocking, or incorrect access.

access function: A set of processes in a network that provides for interaction between the user and a network. [T1.667-1999]

access group: A group of one or more stations having identical rights to use the available resources on a PBX, network or host computer.

accessibility: With reference to point code, the ability of a local MTP (message transfer part) to send a message to the MTP at the remote signaling point. [T1.110.2-1999]

access level: 1. In security, the level of authority required from an entity to access a protected resource. Note: An example of access level is the authority to access information at a particular security level. [2382-pt.8] 2. [The] hierarchical portion of the security level used to identify sensitivity of information-system (IS) data and the clearance or authorization of users. Access level, in conjunction with the nonhierarchical categories, forms the sensitivity label of an object. [INFOSEC-99]

access line: 1. A transmission path between end user terminal equipment and a switching center. 2. A channel between an end user's network interface and local end office. [T1.506-1989] Synonym [loosely] loop. 3. A communications facility extending from a customer's premises to a serving central office comprising a subscriber line and, if necessary, a trunk facility, e.g., a WATS access line, TWX access line. [47 CFR Pt.36-A]

access list: 1. In information systems (IS), a compilation of users, programs, or processes and the access levels and types to which each is authorized. [INFOSEC-99] 2. In COMSEC, a roster of persons authorized admittance to a controlled area. [INFOSEC-99]

access loop: See attendant access loop.

access node: In switching systems, the point where user traffic enters and exits a communications network. Note: Access node operations may include various operations, such as protocol conversion and code conversion.

access originator: The functional entity responsible for initiating a particular access attempt. Note: An access attempt can be initiated by a source user, a destination user, or the telecommunications system.

access path: The steps required to obtain the use of a system or device. Note: Examples of access paths are (a) the operations required of a database management system to obtain access to a database and (b) the sequence of steps required to reach a file. [From Weik '96]

access period: In security, a segment of time, generally expressed in days or weeks, during which specified access rights prevail. [INFOSEC-99]

access permission: All of a subject's access rights with respect to some object. [2382-pt.8]

access phase: In an information-transfer transaction, the phase during which an access attempt is made. Note: The access phase is the first phase of an information-transfer transaction.

access point: 1. A point where connections may be made for testing or using particular communications circuits. 2. In telephony, a junction point in outside plant consisting of a splice at a junction between a branch feeder cable and distribution cables.

access profile: 1. In security, a profile that associates each user with a list of protected objects that the user may access. 2. [An itemization that] associates each user with a list of protected objects the user may access. [INFOSEC-99]

access provider: 1. An organization that provides users with access to a computer network. [2385-35] 2. A party responsible for traffic originating and terminating in jurisdictional areas defined by regulatory agencies. [T1.246-1998]

access request: 1. A control message issued by an access originator for the purpose of initiating an access attempt. 2. A signal sent to a network for the purpose of initiating the establishment of a network connection. In this definition, a signal may take the form of a message. [T1.507-1996]

access right: Permission for a subject to access a particular object for a specific type of operation. Note: An example of an access right is the permission for a process to read a file but not write to it. [2382-pt.8]

access service area: A geographic area established for the provisioning and administration of telecommunications service. An access service area encompasses one or more exchanges, which are grouped in access service areas to serve common social, economic, and other purposes. Contrast with LATA. [After T1.104-1988]

access system: In communications, computer, and data processing systems, a program that (a) allows an operator to call up different parts of the program package and (b) usually allows functions to be selected from menus in the same way as other commands. Note: An example of an access system is the program supplied with a common spreadsheet program that (a) allows the computer to shift between the spreadsheet program and a graph-printing facility and (b) provides access to various support functions. [From Weik '96]

access tandem: 1. A telephone company or centralized equal access provider switching system that provides a concentration and distribution function for originating or terminating traffic between end offices and a customer-designated premises. [NECA/FCC-5] 2. An exchange carrier switching system that provides a traffic concentration and distribution function for inter-LATA traffic originating/terminating within a LATA. [T1.506-1989]

access time: 1. In a telecommunication system, the elapsed time between the start of an access attempt and successful access. Note: Access time values are measured only on access attempts that result in successful access. 2. In a computer, the time interval between the instant at which an instruction control unit initiates a call for data and the instant at which delivery of the data is completed. 3. The time interval between the instant at which storage of data is requested and the instant at which storage is started. 4. In magnetic disk devices, the time for the access arm to reach the desired track and the delay for the rotation of the disk to bring the required sector under the read-write mechanism.

access type: 1. In computer security, a type of operation specified by an access right. Note: Examples of access type are "read," "write," "execute," "append," "modify," "delete," and "create." [2382-pt.8] 2. [The] privilege to perform action on an object. Read, write, execute, append modify, delete, and create are examples of access types. [INFOSEC-99]

access unit: See medium access unit (MAU).

access unit interface: Synonym attachment unit interface.

accountability: 1. The property that ensures that the actions of an individual or an institution may be traced uniquely to that individual or institution. [After X9.57] 2. In information systems (IS), the process of tracing IS activities to a responsible source. [INFOSEC-99] 3. In COMSEC, the principle that an individual is entrusted to safeguard and control equipment, keying material, and information and is answerable to proper authority for the loss or misuse of that equipment or information. [INFOSEC-99]

accounting management: In network management, a set of functions that (a) enables network service use to be measured and the costs for such use to be determined and (b) includes all the resources consumed, the facilities used to collect accounting data, the facilities used to set billing parameters for the services used by customers, maintenance of the data bases used for billing purposes, and the preparation of resource usage and billing reports. [After ANSI T1.210]

accreditation: 1. In computer security, the authorization and approval--granted by a designated authority to a data processing system, computer network, organization, or individual--to process sensitive information or data. [2382-pt.8] 2. Formal declaration by a designated approving authority that an information system (IS) is approved to operate in a particular security mode using a prescribed set of safeguards at an acceptable level of risk. [INFOSEC-99]

accrediting authority: Synonym designated approving authority.

accumulator: 1. A register in which one operand can be stored and subsequently replaced by the result of an arithmetic or logic operation. 2. A storage register. 3. A storage battery.

accuracy: The degree of conformity of a measured or calculated value to its actual or specified value.

ACD: Abbreviation for automatic call distributor.

ac-dc ringing: Telephone ringing that makes use of both ac and dc voltages and currents. Note: An alternating current may be used to operate a ringer and direct current to aid the relay action that stops the ringing when the called telephone is answered.

achromat: A usually two-element lens that is corrected to bring two specified or distinct wavelengths to a common focal point. Note 1: The term "achromatic" literally means "without color." This is not strictly true, however. Early lenses consisted of only a single element, and therefore could bring only a single wavelength to a given focal point; i.e. , they suffered from what is termed "chromatic aberration." The invention of lenses with two elements meant that two distinct wavelengths could be brought to a common focus. This represented a vast improvement over the single-element lens; hence the designation "achromat(ic)." Note 2: The residual chromatic aberration manifested in the image produced by an achromat (and other multi-element lenses) is usually referred to as the "secondary spectrum." Synonyms achromatic doublet, achromatic lens.

achromatic doublet: Synonym achromat.

achromatic lens: Synonym achromat.

ACK: Abbreviation for acknowledge character.

acknowledge character (ACK): A transmission control character transmitted by the receiving station as an affirmative response to the sending station. Note: An acknowledge character may also be used as an accuracy control character.

acknowledgement: 1. A response sent by a receiver to indicate successful receipt of a transmission. Note: An example of an acknowledgement is a protocol data unit, or element thereof, between peer entities, to indicate the status of data units that have been successfully received. 2. A message from the addressee informing the originator that the originator's communication has been received and understood. [After JP1]

acknowledgement delay period: Synonym [loosely] sliding window.

A-condition: In a start-stop teletypewriter system, the significant condition of the signal element that immediately precedes a character signal or block signal and prepares the receiving equipment for the reception of the code elements.

acoustic coupler: 1. An interface device for coupling electrical signals by acoustical means--usually into and out of a telephone instrument. 2. A terminal device used to link data terminals and radio sets with the telephone network. Note: The link is achieved through acoustic (sound) signals rather than through direct electrical connection.

acoustic delay line: A device that introduces a delay in the propagation of an electrical signal by (a) employing a transducer to convert the signal into an acoustic wave, (b) propagating the acoustic wave through a medium such as a column of mercury or a carbon or ferrite rod, and (c) by means of another transducer, converting the acoustic wave back to an electrical signal. Note: An acoustic delay line may be used for temporary storage of information, e.g. , a digital data stream.

acoustic noise: 1. An undesired audible disturbance in the audio frequency range. 2. Any undesired acoustic wave or signal, or undesired component of a desired acoustic signal, whether or not audible to the human ear; e.g., interference accompanying a sonar echo.

acoustics: The branch of science and technology that is devoted to the production, transmission, control, processing, transformation, reception, and effects of sound, longitudinal waves, particularly as vibration, pressure, or elastic waves and shock phenomena in material media. [After Weik '96]

acoustic wave: A longitudinal wave that (a) consists of a sequence of pressure pulses or elastic displacements of the material, whether gas, liquid, or solid, in which the wave propagates, (b) in gases, consists of a sequence of compressions (dense gas) and rarefactions (less dense gas) that travel through the gas, (c) in liquids, consists of a sequence of combined elastic deformation and compression waves that travel though the liquid, and (d) in solids, consists of a sequence of elastic compression and expansion waves that travel though the solid. Note 1: The speed of an acoustic wave in a material medium is determined by the temperature, pressure, and elastic properties of the medium. In air, acoustic waves propagate at 332 m/s (1087 ft/s) at 0°C, at sea level. In air, sound-wave speed increases approximately 0.6 m/s (2 ft/s) for each kelvin above 0°C. Note 2: Acoustic waves audible to the normal human ear are termed sound waves . [From Weik '89]

acousto-optic effect: A variation of the refractive index of a material caused by interaction with acoustic energy in the form of a wave or pulse. Note: The acousto-optic effect is used in devices that modulate or deflect light.

acousto-optics: The discipline devoted to the interactions between acoustic waves and light waves in a material medium. [After 2196] Note: Acoustic waves can be made to modulate, deflect, and/or focus light waves by causing a variation in the refractive index of the medium.

acquisition: 1. In satellite communications, the process of locking tracking equipment on a signal from a communications satellite. 2. The process of achieving synchronization. 3. In servo systems, the process of entering the boundary conditions that will allow the loop to capture the signal and achieve lock-on.

acquisition and tracking orderwire: See ATOW.

acquisition time: 1. In a communications system, the time interval required to attain synchronism. 2. In satellite control communications, the time interval required for locking tracking equipment on a signal from a communications satellite.

ACR: Abbreviation for alternate carrier routing.

active device: A device that requires a source of energy for its operation and has an output that is a function of present and past input signals. Note: Examples of active devices include controlled power supplies, transistors, LEDs, amplifiers, and transmitters.

active key state: A condition of readiness for a key to be used to secure information from the originator, and to process received secure information. [After X9.17]

active laser medium: Within a laser, the material that emits coherent radiation or exhibits gain as the result of electronic or molecular transitions to a lower energy state or states, from a higher energy state or states to which it had been previously stimulated. Note: Examples of active laser media include certain crystals, gases, glasses, liquids, and semiconductors. Synonym laser medium.

active satellite: 1. A satellite carrying a station intended to transmit or retransmit radio communication signals. [NTIA] [RR] Note: An active satellite may perform signal processing functions such as amplification, regeneration, frequency translation, and link switching, to make the signals suitable for retransmission. 2. An Earth satellite carrying a station intended to transmit or re-transmit radiocommunication signals. [47CFR]

active sensor: 1. A detection device that requires input energy from a source other than that which is being sensed. Note: An example of an active sensor is a photoconductive cell. 2. In surveillance, a detection device that emits energy capable of being detected by itself. Note: An example of an active sensor is a measuring instrument that generates a signal, transmits it to a target, and receives a reflected signal from the target. Information concerning the target is obtained by comparison of the received signal with the transmitted signal. 3. A measuring instrument in the Earth exploration-satellite service or in the space research service by means of which information is obtained by transmission and reception of radio waves. [NTIA] [RR]

active signaling link: A signaling link that has successfully completed the initial alignment procedures and carries (or is ready to carry) signaling traffic. [T1.226-1992]

active star: See star coupler, multiport repeater.

active threat: Any threat of a deliberate unauthorized change to the state of a data processing system. Note: For example, an active threat that would result in modification of messages, insertion of spurious messages, masquerade, or denial of service. [2382-pt.8]

active video frame identification: The process of comparing each output video frame with its preceding frame(s) in sequence and quantifying the extent of correspondence between each pair; when there is limited correspondence between a pair of frames (such that the differences measured are distinguishable from the measurement noise), and the corresponding input sequence of frames possesses distinguishable differences, then the current frame is categorized as an active frame. [T1.801.04-1997]

activity factor: For a communications channel during a specified time interval, such as the busy hour, the percentage of time that a signal is present in the channel in either direction.

ACU: Abbreviation for automatic calling unit.

A-D: Abbreviation for analog-to-digital. See analog transmission.

adaptive compression: In computer science, a software-oriented compression process based on continuous analysis of the data stream, and depending on the type and content of the data and the storage medium, corresponding compensation of the compression algorithm.

adaptive prediction: In ADPCM coding, a time-varying process that computes an estimate of the input signal from the quantized difference signal. [T1.310-1991]

adaptive quantization: In ADPCM (adaptive differential pulse code modulation) coding, a process in which the quantizer step size varies as a function of the quantized input signal's variance. [T1.310-1991]

Ada®: The official, high-level computer language of DOD for embedded-computer, real-time applications as defined in MIL-STD-1815. Note: Ada® is a registered trademark of the U.S. Government (Ada Joint Program Office).

adaptive channel allocation: In communications system traffic flow control, channel allocation in which information-handling capacities of channels are not predetermined but are assigned on demand. Note: Adaptive channel allocation is usually accomplished by means of a multiplexing scheme.

adaptive communications: Any communications system, or portion thereof, that automatically uses feedback information obtained from the system itself or from the signals carried by the system to modify dynamically one or more of the system operational parameters to improve system performance or to resist degradation. Note: The modification of a system parameter may be discrete, as in hard-switched diversity reception, or may be continuous, as in a predetection combining algorithm.

adaptive differential pulse-code modulation (ADPCM): Differential pulse-code modulation in which the prediction algorithm is adjusted in accordance with specific characteristics of the input signal.

adaptive equalization: Equalization (a) that is automatically accomplished while traffic is being transmitted and (b) in which signal characteristics are dynamically adjusted to compensate for changing transmission path characteristics.

adaptive predictive coding (APC): Narrowband analog-to-digital conversion that uses a one-level or multilevel sampling system in which the value of the signal at each sampling instant is predicted according to a linear function of the past values of the quantized signals. Note: APC is related to linear predictive coding (LPC) in that both use adaptive predictors. However, APC uses fewer prediction coefficients, thus requiring a higher sampling rate than LPC.

adaptive radio: A radio that (a) monitors its own performance, (b) monitors the path quality through sounding or polling, (c) varies operating characteristics, such as frequency, power, or data rate, and (d) uses closed-loop action to optimize its performance by automatically selecting frequencies or channels.

adaptive routing: Routing that is automatically adjusted to compensate for network changes such as traffic patterns, channel availability, or equipment failures. Note: The experience used for adaptation comes from the traffic being carried.

adaptive system: A system that has a means of monitoring its own performance, a means of varying its own parameters, and uses closed-loop action to improve its performance or to optimize traffic.

ADC: Abbreviation for analog-to-digital converter, analog-to-digital conversion.

ADCCP: Abbreviation for Advanced Data Communication Control Procedures.

ad clicks: Synonym click-through.

add/drop multiplexing: A multiplexing function offered in connection with SONET that allows lower level signals to be added or dropped from a high-speed optical carrier channel in a wire center. The connection to the add/drop multiplexer is via a channel to a central office port at a specific digital speed (i.e., DS3, DS1, etc. ). [NECA/FCC-5]

added bit: A bit delivered to the intended destination user in addition to intended user information bits and delivered overhead bits. Synonym extra bit.

added block: Any block, or other delimited bit group, delivered to the intended destination user in addition to intended user information bits and delivered overhead bits. Synonym extra block.

adder: 1. A device whose output data are a representation of the sum of the numbers represented by its input data. Note: An adder may be serial or parallel, digital or analog. 2. A device whose output data are a representation of the sum of the quantities represented by its input data. Note: An adder can add things other than representations of numbers. It can add voltages, etc. Analog adders are not limited to summing representations of numbers. An adder may operate on digital or analog data.

adder-subtracter: A device that acts as an adder or subtracter depending upon the control signal received; the adder-subtracter may be constructed so as to yield a sum and a difference at the same time. Note: An arithmetic adder-subtracter yields arithmetic sums and differences, whereas a logical adder-subtracter yields logical sums and differences.

additive white gaussian noise (AWGN): Synonym white noise.

add mode: In addition and subtraction operations, a mode in which the decimal marker is placed at a predetermined location with respect to the last digit entered.

add-on conference: A service feature that allows an additional party to be added to an established call without attendant assistance. Note: A common implementation provides a progressive method that allows a call originator or a call receiver to add at least one additional party.

address: 1. In communications, the coded representation of the source or destination of a message. 2. In data processing, a character or group of characters that identifies a register, a particular part of storage, or some other data source or destination. 3. To assign to a device or item of data a label to identify its location. 4. The part of a selection signal that indicates the destination of a call. 5. To refer to a device or data item by its address.

addressability: 1. In computer graphics, the capability of a display surface or storage device to accommodate a specified number of uniquely identifiable points. 2. In micrographics, the capability of a specified field frame to contain a specific number of uniquely identifiable points. Note: The addressability is usually specified as the number of identifiable horizontal points by the number of identifiable vertical points, such as 3000 by 4000.

addressable point: In computer graphics, any point of a device that can be addressed.

address field: The portion of a message that contains the source-user address and the destination-user addresses. Note: In a communications network, the address field is usually contained within the message header portion of the message. A message usually consists of the message header, the user data, and a trailer.

address-indicating group (AIG): A station or address designator, used to represent a set of specific and frequently recurring combinations of action or information addresses. Note: The identity of the message originator may also be included in the AIG. An address group is assigned to each AIG for use as an address designator.

address message: A message sent in the forward direction that contains (a) address information, (b) the signaling information required to route and connect a call to the called line, (c) service-class information, (d) information relating to user and network facilities, and (e) call-originator identity or call-receiver identity.

address message sequencing: In common-channel signaling, a procedure for ensuring that address messages are processed in the correct order when the order in which they are received is incorrect.

address part: A part of an instruction that usually contains only an address or part of an address.

address pattern: A prescribed structure of data used to represent the destination(s) of a block, message, packet, or other formalized data structure.

address resolution protocol (ARP): A Transmission Control Protocol/Internet Protocol (TCP/IP) protocol that dynamically binds a Network-Layer IP address to a Data-Link-Layer physical hardware address, e.g., Ethernet address.

address separator: A character that separates the different addresses in a selection signal.

address signaling: A process used to convey address information; two address signaling methods, dial pulse (DP) and dual tone multifrequency (DTMF), are used in telephone systems. [After T1.401-1988]

ADH: Abbreviation for automatic data handling.

adiabatic computer circuit: A circuit that avoids loss or gain of heat. For example, a computer could clear a "1" in a register by subtracting the "1" instead of erasing it, thereby expending less heat.

adjacent-channel interference: Extraneous power from a signal in an adjacent channel. Note 1: Adjacent channel interference may be caused by inadequate filtering, such as incomplete filtering of unwanted modulation products in frequency modulation (FM) systems, improper tuning, or poor frequency control, in either the reference channel or the interfering channel, or both. Note 2: Adjacent-channel interference is distinguished from crosstalk.

adjacent signaling points: Two signaling points that are directly interconnected by one or more signaling links. [T1.226-1992]

adjunct service point (ASP): An intelligent-network feature that resides at the intelligent peripheral equipment and responds to service logic interpreter requests for service processing.

administration: 1. Any governmental department or service responsible for discharging the obligations undertaken in the convention of the International Telecommunication Union and the Regulations. [RR] 2. Internal management of units. [JP1] 3. The management and execution of all military matters not included in strategy and tactics. [JP1] 4. In international telecommunications for a given country, the government agency assigned responsibility for the implementation of telecommunications standards, regulations, recommendations, practices, and procedures. 5. In network management, network support functions that ensure that (a) services are performed, (b) the network is used efficiently, and (c) prescribed service-quality objectives are met.

administrative management complex (AMC): In network management, a complex that is controlled by a network provider, and is responsible for and performs network management functions such as network maintenance. [After T1.218-1991]

ADP: Abbreviation for automatic data processing.

ADPCM: Abbreviation for adaptive differential pulse-code modulation.

ADPE: Abbreviation for automatic data processing equipment.

ADP system: Synonym computer system.

Advanced Data Communication Control Procedures (ADCCP): A bit-oriented Data-Link-Layer protocol used to provide point-to-point and point-to-multipoint transmission of data frames that contain error-control information. Note: ADCCP closely resembles high-level data link control (HDLC) and synchronous data link control (SDLC).

advanced intelligent network (AIN): 1. A telecommunications network architecture that uses databases to facilitate call processing, call routing, and network management, allowing carriers to change the routing of both inbound and outbound calls from moment to moment. [FCC-5] 2. A proposed intelligent-network (IN) architecture that includes both IN/1+ and IN/2 concepts.

advanced television (ATV): A family of television systems that is intended to be improvements over current commercial-quality television. Note: The ATV family includes improved-definition television (IDTV), extended-definition television (EDTV), and high-definition television (HDTV).

AECS: Abbreviation for Aeronautical Emergency Communications System. See Aeronautical Emergency Communications System Plan.

aerial cable: A communications cable designed for installation on, or suspension from, a pole or other overhead structure.

aerial insert: In a direct-buried or underground cable run, a cable rise to a point above ground, followed by an overhead run, e.g. , on poles, followed by a drop back into the ground. Note: An aerial insert is used in places where it is not possible or practical to remain underground, such as might be encountered in crossing a deep ditch, canal, river, or subway line.

aeronautical advisory station: An aeronautical station used for advisory and civil defense communications primarily with private aircraft stations. Synonym UNICOM station. [NTIA]

aeronautical broadcast station: An aeronautical station which makes scheduled broadcasts of meteorological information and notices to airmen. (In certain instances, an aeronautical broadcast station may be placed on board a ship.) [NTIA]

aeronautical Earth station: An Earth station in the fixed-satellite service, or, in some cases, in the aeronautical mobile-satellite service, located at a specified fixed point on land to provide a feeder link for the aeronautical mobile-satellite service. [NTIA] [RR]

Aeronautical Emergency Communications System (AECS) Plan: The AECS Plan provides for the operation of aeronautical communications stations, on a voluntary, organized basis, to provide the President and the Federal Government, as well as heads of state and local governments, or their designated representatives, and the aeronautical industry with an expeditious means of communications during an emergency situation. [47CFR]

aeronautical fixed service: A radiocommunication service between specified fixed points provided primarily for the safety of air navigation and for the regular, efficient and economical operation of air transport. [NTIA] [RR]

aeronautical fixed station: A station in the aeronautical fixed service. [NTIA][RR]

aeronautical mobile (OR) [off-route] service: An aeronautical mobile service intended for communications, including those relating to flight coordination, primarily outside national or international civil air routes. [NTIA] [RR]

aeronautical mobile (R) [route] service: An aeronautical mobile service reserved for communications relating to safety and regularity of flight, primarily along national or international civil air routes. [NTIA] [RR]

aeronautical mobile-satellite service: A mobile-satellite service in which mobile Earth stations are located on board aircraft; survival craft stations and emergency position-indicating radiobeacon stations may also participate in this service. [NTIA] [RR]

aeronautical mobile-satellite (OR) [off-route] service: An aeronautical mobile-satellite service intended for communications, including those relating to flight coordination, primarily outside national and international civil air routes. [NTIA] [RR]

aeronautical mobile-satellite (R) [route] service: An aeronautical mobile-satellite service reserved for communications relating to safety and regularity of flight, primarily along national or international civil air routes. [NTIA] [RR]

aeronautical mobile service: A mobile service between aeronautical stations and aircraft stations, or between aircraft stations, in which survival craft stations may participate; emergency position-indicating radiobeacon stations may also participate in this service on designated distress and emergency frequencies. [NTIA]

aeronautical multicom service: A mobile service not open to public correspondence, used to provide communications essential to conduct activities being performed by or directed from private aircraft. [NTIA]

aeronautical radionavigation-satellite service: A radionavigation-satellite service in which Earth stations are located on board aircraft. [NTIA] [RR]

aeronautical radionavigation service: A radionavigation service intended for the benefit and for the safe operation of aircraft. [NTIA] [RR]

aeronautical station: A land station in the aeronautical mobile service. In certain instances, an aeronautical station may be located, for example, on board ship or on a platform at sea. [NTIA] [RR]

AF: Abbreviation for audio frequency.

AFNOR: Acronym for Association Francaise de Normalisation. France's national standards-setting organization.

AGC: Abbreviation for automatic gain control.

agent: Synonym droid.

aggregation: In security, the acquisition of sensitive information by collecting and correlating information of lesser sensitivity. [2382-pt.8]

aggregator: Any entity that, in the ordinary course of its operations, makes telephones available to the public or to transient users of its premises, for interstate telephone calls using a provider of operator services. [NECA/FCC-5]

AI: Abbreviation for artificial intelligence.

AIM: Abbreviation for amplitude intensity modulation. See intensity modulation.

AIN: Abbreviation for advanced intelligent network.

AIOD: Abbreviation for automatic identified outward dialing.

AIOD leads: Terminal equipment leads used solely to transmit automatic identified outward dialing (AIOD) data from a PBX to the public switched telephone network or to switched service networks (e.g., EPSCS), so that a vendor can provide a detailed monthly bill identifying long-distance usage by individual PBX stations, tie trunks, or the attendant. . . . [47CFR]

airborne radio relay: 1. Airborne equipment used to relay radio transmission from selected originating transmitters. [JP1] 2. A technique employing aircraft fitted with radio relay stations for the purpose of increasing the range, flexibility, or physical security of communications systems.

air-conditioning: The simultaneous controlling of the characteristics of air, such as temperature, humidity, cleanliness, motion, and pollutant concentration, in a space to meet the requirements of the occupants, a process, or equipment. Synonym environmental control.

aircraft Earth station: A mobile Earth station in the aeronautical mobile-satellite service located on board an aircraft. [NTIA] [RR]

aircraft emergency frequency: An international aeronautical emergency frequency, such as 121.5 MHz (civil) and 243.0 MHz (military), for aircraft stations and stations concerned with safety and regulation of flight along national or international civil air routes and maritime mobile service stations authorized to communicate for safety purposes.

aircraft station: A mobile station in the aeronautical mobile service, other than a survival craft station, located on board an aircraft. [RR]

airdrome control station: An aeronautical station providing communication between an airdrome control tower and aircraft. [NTIA] Synonym airport control station.

air-floating head: Synonym floating head.

air-ground radiotelephone service: A radio service in which common carriers are authorized to offer and provide radio telecommunications service for hire to subscribers in aircraft. [47CFR]

air-ground worldwide communications system: A worldwide military network of ground stations that (a) provides two-way communications links between aircraft and ground stations for navigation and control, including air route traffic control and (b) may also provide support for special functions, such as for civil aircraft providing assistance to military missions and for meeting communications requirements for aircraft flying distinguished visitors.

air portable: Denotes materiel that is suitable for transport by an aircraft loaded internally or externally, with no more than minor dismantling and reassembling within the capabilities of user units. This term must be qualified to show the extent of air portability. [JP1]

airport control station: Synonym airdrome control station.

air sounding: Measuring atmospheric phenomena or determining atmospheric conditions, especially by means of apparatus carried by balloons, rockets, or satellites.

air-spaced coaxial cable: A coaxial cable in which air is the primary dielectric (insulator) between the inner and outer conductors. Note: Proper separation between the inner and outer conductors is maintained by a continuous helical insulator or a series of insulating washers spaced at regular intervals.

air terminal: In grounding systems, the lightning rod or conductor placed on or above a building, structure, or external conductors for the purpose of intercepting lightning.

AIS: Abbreviation for alarm indication signal, automated information system.

alarm: An alerting indication to a condition that may have immediate or potential negative impact on the state of the monitored network element (NE). [T1.226-1992]

alarm center: A location that receives local and remote alarms. Note: An alarm center is usually in a technical control facility.

alarm event: An instantaneous occurrence that changes at least one of the attributes of the alarm status of an object. This status change may be persistent or temporary, thus allowing for surveillance, monitoring, and performance measurement functionality, etc. Alarm events may or may not generate alarm reports; they may trigger other events or may be triggered by one or more other events. [T1.215-1994]

alarm indication signal (AIS): 1. A signal transmitted in lieu of the normal signal to maintain transmission continuity and to indicate to the receiving equipment that there is a transmission interruption located either at the equipment originating the AIS signal or upstream of that equipment. [T1.231-1997] 2. A signal transmitted in lieu of the normal signal to maintain transmission continuity and indicate to the receiving terminal that there is a transmission fault located either at the transmitting terminal or upstream of the transmitting terminal. [T1.Rpt25-1993]

alarm indicator: A device that responds to a signal from an alarm sensor. Note: Examples of alarm indicators include bells, lamps, horns, gongs, and buzzers.

alarm sensor: 1. In communications systems, any device that (a) can sense an abnormal condition within the system and provide a signal indicating the presence or nature of the abnormality to either a local or remote alarm indicator, and (b) may detect events ranging from a simple contact opening or closure to a time-phased automatic shutdown and restart cycle. 2. In a physical security system, an approved device used to indicate a change in the physical environment of a facility or a part thereof. Note: Alarm sensors may also be redundant or chained, such as when one alarm sensor is used to protect the housing, cabling, or power protected by another alarm sensor.

alarm status: A set of attributes that describes the alarms currently defined for an object, for example, perceived severity, alarm state, etc. The alarm status of an object is a subset of the global status of that object. [T1.215-1990]

alarm surveillance: A set of functions that enables the monitoring or interrogation (or both) of the telecommunications network concerning alarm-related events or conditions. [T1.215-1994]

a-law: See a-law algorithm.

a-law algorithm: A standard compression algorithm, used in digital communications systems of the European digital hierarchy, to optimize, i.e., modify, the dynamic range of an analog signal for digitizing. Note: The wide dynamic range of speech does not lend itself well to efficient linear digital encoding. A-law encoding effectively reduces the dynamic range of the signal, thereby increasing the coding efficiency and resulting in a signal-to-distortion ratio that is superior to that obtained by linear encoding for a given number of bits.

ALE: Abbreviation for automatic link establishment.

alerting signal: A signal used by the network to indicate the presence of an incoming call. It consists of a ringing signal periodically interrupted by silent (no ringing signal) intervals. [T1.401-1988]

algorithmic language: An artificial language established for expressing a given class of algorithms.

algorithm identifier: In encryption, a unique identifier or recognizer for a given encryption or hash algorithm, together with any required parameters. [After X9.31-1]

alias: 1. See aliasing. 2. In networking, one of a set of domain names of an Internet resource. [2382-pt.35] 3. Synonyms personal number, UPT number.

aliasing: In any technology or process involving (a) sampling a signal, e.g. , an electrical signal or (a series of images of) a moving subject; (b) processing, storing, or transmitting representations of the samples; and (c) replicating the original signal from the representations: the production of artifacts as a result of sampling at intervals too great to permit faithful replication of the original signal. Note 1: A common example of aliasing in video or motion picture technology is the apparent slowing, freezing, or even reversing of direction of, the motion of spokes in the wheels on a moving vehicle. Note 2: In the sampling and replication of electrical signals, if the sampling interval is too great, high-frequency components may, for example, be replicated as low-frequency components, distorting the replicated signal. Note 3: Faithful reproduction of a sampled electrical signal requires a theoretical sampling interval not greater than one-half the inverse of the highest Fourier frequency component in the sampled signal (Nyquist interval). In practice, to achieve a given level of fidelity in the replicated signal, the sampling interval must usually be somewhat shorter than the theoretical (Nyquist) interval, because the samples are usually processed, stored, or transmitted in digital form, and quantization errors in the digitizing process will also result in distortion of the (waveform of the) replicated signal. Note 4: The effects of aliasing in the replicated signal may be avoided by filtering the original signal to remove frequency components that are higher than those desired in the replicated signal. Note 5 (from SMPTE): Video images are sampled in two or three dimensions, and computer graphics are sampled in three. In a composite (complex) spectral channel, any crosstalk of the information cannot be eliminated by filtering, and will produce aliasing, for example as cross-color and/or cross-luminance. In a rectilinearly sampled system, lines not parallel to a sampling axis will appear stepped unless anti-aliasing processing algorithms have been applied.

alias point code: 1. A Signaling System No. 7 (SS7) address that is shared by more than one system supporting the same GTT (global title translation) function (or other SS7 function). It allows other systems to address the function instead of maintaining the status of multiple point codes. [T1.711-1999] 2. A point code that may be assigned to more than one signaling point, each of which is also assigned a "real" point code. Note: Each of the signaling points having the same "alias" point code is capable of providing a predefined functionality (capability) in the network. As an example for management of global translation capability, an STP (signaling transition point) and its mate can be assigned the same "alias" point code. This provides the capability such that if a message that requires global title translation is routed based on "alias" point code, then either STP answering to the "alias" point code can provide the global title translation functionality. Synonym capability code. [T1.226-1992]

aligned bundle: A bundle of optical fibers in which the relative spatial coordinates of each fiber are the same at the two ends of the bundle. Note: Such a bundle may be used for the transmission of images. Synonym coherent bundle.

alignment error rate monitoring: A procedure by which the error rate of a signaling link is measured during the initial alignment process. [After T1.110.2-1992] [After T1.226-1992]

alignment jitter: The short-term variations between the optimum sampling instants of a digital signal and the sampling clock derived from it. [T1.Rpt17-1993]

Allan variance: One half of the time average over the sum of the squares of the differences between successive readings of the frequency deviation sampled over the sampling period. Note: The Allan variance is conventionally expressed by _y²(). The samples are taken with no dead-time between them. Synonym two-sample variance.

allcall: In adaptive high-frequency (HF) radio automatic link establishment (ALE), a general broadcast that does not request responses and does not designate any specific addresses. Note: This essential function is required for emergencies ("HELP"), sounding-type data exchanges, and propagation and connectivity tracking. [After FED-STD-1045A]

all-glass fiber: Synonym all-silica fiber.

allocation (of a frequency band): 1. Entry in the Table of Frequency Allocations of a given frequency band for the purpose of its use by one or more (terrestrial or space) radiocommunication services or the radio astronomy service under specified conditions. This term shall also be applied to the frequency band concerned. [NTIA] [RR] 2. The process of designating radio-frequency bands for use by specific radio services.

allotment (of a radio frequency or radio frequency channel): Entry of a designated frequency channel in an agreed plan, adopted by a component Conference, for use by one or more administrations for a (terrestrial or space) radiocommunication service in one or more identified countries or geographical areas and under specified conditions. [NTIA]

all-silica fiber: An optical fiber composed of a silica-based core and cladding. Note: The presence of a protective polymer overcoat does not disqualify a fiber as an all-silica fiber, nor does the presence of a tight buffer. [FAA] Synonym all-glass fiber.

all trunks busy (ATB): An equipment condition in which all trunks (paths) in a given trunk group are busy. Note: All-trunks-busy registers do not indicate subsequent attempts to reach trunk groups.

alphabet: 1. An ordered set of all the letters used in a language, including letters with diacritical signs where appropriate, but not including punctuation marks. 2. An ordered set of all the symbols used in a language, including punctuation marks, numeric digits, nonprinting control characters, and other symbols. Note: Examples of alphabets include the Roman alphabet, the Greek alphabet, the Morse Code, and the 128 characters of the American Standard Code for Information Interchange (ASCII) [IA No. 5].

alphabetic character set: A character set that contains letters and may contain control characters, special characters, and the space character, but not digits.

alphabetic code: A code according to which data are represented through the use of an alphabetic character set.

alphabetic string: 1. A string consisting solely of letters from the same alphabet. 2. A character string consisting solely of letters and associated special characters from the same alphabet.

alphabetic word: 1. A word consisting solely of letters from the same alphabet. 2. A word that consists of letters and associated special characters, but not digits.

alphabet translation: Deprecated synonym for alphabet transliteration. See alphabet transliteration.

alphabet transliteration: The substitution of the characters of one alphabet for the corresponding characters of a different alphabet, usually accomplished on a character-by-character basis. Note 1: An example of alphabet transliteration is the substitution of the Roman letters a, b, and p for the Greek letters , , and , respectively. Note 2: Alphabet transliteration is reversible. Note 3: Alphabet transliteration often becomes necessary in telecommunications systems because of the different alphabets and codes used worldwide. Note 4: In alphabet transliteration, no consideration is given to the meaning of the characters or their combinations.

alphanumeric: 1. Pertaining to a character set that contains letters, digits, and sometimes other characters, such as punctuation marks. 2. Pertaining to a set of unique bit patterns that are used to represent letters of an alphabet, decimal digits, punctuation marks, and other special signs and symbols used in grammar, business, and science, such as those displayed on conventional typewriter keyboards.

alphanumeric character set: A character set that contains both letters and digits, special characters, and the space character.

alphanumeric code: 1. A code derived from an alphanumeric character set. 2. A code that, when used, results in a code set that consists of alphanumeric characters.

alphanumeric data: Data represented by letters, digits, and sometimes by special characters and the space character.

alpha profile: See power-law index profile.

altazimuth mount: A mounting, e.g. , for a directional antenna, in which slewing takes place in (a) the plane tangent to the surface of the Earth or other frame of reference and (b) elevation about, i.e., above or below, that plane. Synonym x-y mount.

alteration: In encryption, the process of changing one or more message elements in a message as a means of committing a fraud. [After X9.19]

alternate access provider: A company that enters a market area as a competitive service provider (CAP) to provide exchange service in competition with one or more existing exchange carrier(s) in the same market area. [After T1.251-1996]

alternate carrier routing (ACR): A special advanced intelligent network (AIN) feature supporting Government Emergency Telecommunications Service (GETS) calls. The feature automatically routes GETS calls to selected carriers to provide improved call completion probability during times of network congestion or damage during disasters or crises.

alternate mark inversion (AMI) signal: A pseudoternary signal, representing binary digits, in which (a) successive "marks" are of alternately positive and negative polarity and the absolute values of their amplitudes are normally equal and (b) "spaces" are of zero amplitude. Synonym bipolar signal.

alternate party: In multilevel precedence and preemption, the call receiver, i.e. , the destination user, to which a precedence call will be diverted. Note 1: Diversion will occur when the response timer expires, when the call receiver is busy on a call of equal or higher precedence, or when the call receiver is busy with access resources that are non-preemptable. Note 2: Alternate party diversion is an optional terminating feature that is subscribed to by the call receiver. Thus, the alternate party is specified by the call receiver at the time of subscription.

alternate route: A second or subsequent choice path between two points. [T1.Rpt24-1993]

alternate routing: 1. The routing of a call or message over a substitute route when a primary route is unavailable for immediate use. 2. In signaling, the process of allocating substitute routes for a given signaling traffic stream in case of failure(s) affecting the signaling links or routes involved in the normal routing of that signaling traffic stream. [T1.226-1992] 3. In signaling, the routing of a given signaling traffic flow in case of failures affecting the signaling link(s), or route(s), involved in the normal routing of that signaling traffic flow. [T1.110-1999] Synonym alternative routing.

alternating mode: Synonym half-duplex (HDX) operation.

alternative routing: 1. In signaling, the process of allocating substitute routes for a given signaling traffic stream in case of failure(s) affecting the signaling links or routes involved in the normal routing of that signaling traffic stream. [T1.226-1992] 2. In signaling, the routing of a given signaling traffic flow in case of failures affecting the signaling link(s), or route(s), involved in the normal routing of that signaling traffic flow. [T1.110-1999] Synonym alternate routing.

altitude of the apogee or of the perigee: The altitude of the apogee or perigee above a specified reference surface serving to represent the surface of the Earth. [NTIA] [RR] Note: In technical usage, the definite article is not used with the term apogee or perigee alone. A body orbiting the Earth is said simply to be "at apogee" or "at perigee." It may, however, properly be said to be "at the point of apogee" or "at the point of perigee."

ALU: Abbreviation for arithmetic and logic unit.

AM: Abbreviation for amplitude modulation.

AMA: Abbreviation for automatic message accounting.

amateur-satellite service: A radiocommunication service using space stations on Earth satellites for the same purposes as those of the amateur service. [NTIA] [RR]

amateur service: A radiocommunication service for the purpose of self-training, intercommunication and technical investigation carried out by amateurs, that is, by duly authorized persons interested in radio technique solely with a personal aim and without pecuniary interest. [NTIA] [RR]

amateur station: A station in the amateur service. [NTIA] [RR]

ambient noise level: The level of acoustic noise existing at a given location, such as in a room, in a compartment, or at a place out of doors. Note 1: Ambient noise level is measured with a sound level meter. Note 2: Ambient noise level is usually measured in dB above a reference pressure level of 0.00002 Pa, i.e., 20 Pa (micropascals) in SI units. A pascal is a newton per square meter. Note 3: In the centimeter-gram-second system of units, the reference level for measuring ambient noise level is 0.0002 dyn/cm². Synonym room noise level.

ambient temperature: The temperature of air or other media in a designated area, particularly the area surrounding equipment.

AME: Abbreviation for amplitude modulation equivalent, automatic message exchange. See compatible sideband transmission.

American National Standards Institute (ANSI): The U.S. standards organization that establishes procedures for the development and coordination of voluntary American National Standards.

American Standard Code for Information Interchange (ASCII): See ASCII.

AMI: Abbreviation for alternate mark inversion. See alternate mark inversion signal.

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.

amplifier: 1. An electronic component that boosts the voltage or power level of a signal that is a linear replica of the input signal, but with greater power or voltage level, and sometimes with an impedance transformation. The output may also be a nonlinear analog function of the input signal, as in a signal compression device. 2. See fiber amplifier, optical repeater.

amplitude compression: 1. See signal compression. 2. In video production, the imposition of a nonlinear transfer function on (i.e. , the nonlinear processing of, to reduce the dynamic range of) signal amplitude values (e.g. , as in gamma correction). [After SMPTE]

amplitude distortion: Distortion occurring in a system, subsystem, or device when the output amplitude is not a linear function of the input amplitude under specified conditions. Note: Amplitude distortion is measured with the system operating under steady-state conditions with a sinusoidal input signal. When other frequencies are present, the term "amplitude" refers to that of the fundamental only.

amplitude equalizer: A corrective network that is designed to modify the amplitude characteristics of a circuit or system over a desired frequency range. Note: Such devices may be fixed, manually adjustable, or automatic.

amplitude hit: See hit.

amplitude intensity modulation (AIM): Deprecated term. See intensity modulation.

amplitude keying: Keying in which the amplitude of a signal is varied among the members of a set of discrete values.

amplitude modulation (AM): Modulation in which the amplitude of a carrier wave is varied in accordance with some characteristic of the modulating signal. 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.

amplitude modulation equivalent (AME): Synonym compatible sideband transmission.

amplitude-vs.-frequency distortion: Distortion in a transmission system caused by nonuniform attenuation, or gain, in the system with respect to frequency under specified operating conditions. Synonym frequency distortion.

AMPS: Abbreviation for automatic message processing system.

AMTS: Abbreviation for automated maritime telecommunications system.

analog component: Synonym CAV.

analog computer: A device that performs operations on data that are represented within the device by continuous variables having a physical resemblance to the quantities being represented. Note: The earliest analog computers were constructed with purely mechanical components, such as levers, cogs, cams, discs, and gears. These components represented the quantities being manipulated or the operator-inserted values. Modern analog computers usually employ electrical parameters, such as voltages, resistances, or currents to represent the quantities being manipulated.

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.

analog decoding: The portion of the digital-to-analog conversion process that generates an analog signal value from the digital signal that resulted from analog encoding. Note: Further action is required to integrate these samples to obtain a continuous approximation of the original signal, because analog decoding does not smooth the signal.

analog encoding: The portion of the analog-to-digital conversion process that samples an analog signal and creates a digital signal that represents the value of the sample. Note: Multiple samples are needed to digitize a waveform over a time interval.

analog facsimile equipment: Facsimile equipment in which (a) analog techniques are used to encode the image detected by the scanner and (b) the output is an analog signal. Note: Examples of analog facsimile equipment are CCITT Group 1 and CCITT Group 2 facsimile equipment.

analog signal: 1. A signal that has a continuous nature rather than a pulsed or discrete nature. Note: Electrical or physical analogies, such as continuously varying voltages, frequencies, or phases, may be used as analog signals. 2. A nominally continuous electrical signal that varies in some direct correlation with another signal impressed on a transducer. Note: For example, an analog signal may vary in frequency, phase, or amplitude in response to changes in physical phenomena, such as sound, light, heat, position, or pressure.

analog signaling data link: The data link that provides an interface to signaling terminals and is made up of voice-frequency analog transmission channels and modems. [T1.110.2-1992]

analog switch: Switching equipment designed, designated, or used to connect circuits between users for real-time transmission of analog signals.

analog-to-digital (A-D) coder: Synonym analog-to-digital converter (ADC).

analog-to-digital converter (ADC): A device that converts an analog signal to a digital signal that represents equivalent information. Synonyms analog-to-digital (A-D) coder, analog-to-digital (A-D) encoder.

analog-to-digital (A-D) encoder: Synonym analog-to-digital converter (ADC).

analog transmission: Transmission of a continuously varying signal as opposed to transmission of a discretely varying signal.

analytical attack: An attempt to break a code or to find a key using analytical methods. Note: Examples of an analytical attack are: a statistical analysis of patterns; a search for flaws in an encryption algorithm. [2382-pt.8] Synonym cryptanalytical attack.

angle modulation: Modulation in which the phase or frequency of a sinusoidal carrier is varied. Note: Phase and frequency modulation are particular forms of angle modulation.

angle of deviation: In optics, the net angular deflection experienced by a light ray after one or more refractions or reflections.

angle of incidence: The angle between an incident ray and the normal to a reflecting or refracting surface.

angstrom (): A unit of length equal to 10^-10 m. Note 1: The angstrom is not an SI (International System) unit, and it is not accepted for government use (Fed. Std. 376B). Note 2: The angstrom is, and historically has been, used in the fields of optics, spectroscopy, and microscopy.

angular misalignment loss: Power loss caused by the deviation from optimum angular alignment of the axes of source to waveguide, waveguide to waveguide, or waveguide to detector. Note 1: The waveguide may be dielectric (an optical fiber) or metallic. Note 2: Angular misalignment loss does not include lateral offset loss and longitudinal offset loss.

ANI: Abbreviation for automatic number identification.

anisochronous: Pertaining to transmission in which the time interval separating any two significant instants in sequential signals is not necessarily related to the time interval separating any other two significant instants. Note: Isochronous and anisochronous are characteristics, while synchronous and asynchronous are relationships.

anisochronous transmission: See asynchronous transmission.

anisotropic: Pertaining to a material whose electrical or optical properties vary with (a) the direction of propagation of a traveling wave or with (b) different polarizations of a traveling wave. Note 1: Anisotropy is exhibited by non-cubic crystals, which have different refractive indices for lightwaves propagating in different directions or with different polarizations. Note 2: Anisotropy may be induced in certain materials under mechanical strain.

anomalous propagation (AP): Abnormal propagation caused by fluctuations in the properties (such as density and refractive index) of the propagation medium. Note: AP may result in the reception of signals well beyond the distances usually expected.

anonymous call: A 7- or 10-digit call to the directory number of the public service answering point (PSAP) (where applicable) causing the E911 system (emergency 9-1-1 system) to send to the PSAP a multifrequency (MF) pulse train devoid of the caller's emergency service identification. [T1.414-1998]

anonymous FTP: The name of a file-transfer protocol that allows a user on one host to access and transfer files to and from another host over a network. [After 2382-pt.35]

ANS: Abbreviation for American National Standard.

ANSI: Abbreviation for American National Standards Institute.

ANSI/EIA/TIA-568: A U.S. industry standard that specifies a generic telecommunications cabling system, which will support a multiproduct, multivendor environment, for commercial buildings. Note 1: The standard specifies performance characteristics for unshielded twisted pair telecommunications cabling, including categories allowing data communications up to 100 Mb/s. These categories are designated 3, 4, and 5. Categories 1 and 2 have not been defined. Note 2: The standard has been adopted as FIPS PUB 174.

answer back: A signal sent by receiving equipment to the sending station to indicate that the receiver is ready to accept transmission.

answer indicator: A signal indicating acceptance of the call by the addressed user. [T1.507-1996]

answer signal: 1. A supervisory signal returned from the called telephone to the originating switch when the call receiver answers. Note 1: The answer signal stops the ringback signal from being returned to the caller. Note 2: The answer signal is returned by means of a closed loop. 2. An off-hook signal transmitted towards the network when the called party answers. 3. An off-hook signal transmitted towards the network to indicate when the customer installation (CI) equipment has changed to the communication state. [T1.405-1989]

antenna: 1. Any structure or device used to collect or radiate electromagnetic waves. 2. A device that converts radio frequency electrical energy to radiated electromagnetic energy and vice versa; in a transmitting station, the device from which radio waves are emitted. [47CFR]

antenna aperture: See aperture.

antenna array: An assembly of antenna elements with dimensions, spacing, and illumination sequence such that the fields for the individual elements combine to produce a maximum intensity in a particular direction and minimum field intensities in other directions.

antenna blind cone: The volume of space, usually approximately conical with its vertex at the antenna, that cannot be scanned by an antenna because of limitations of the antenna radiation pattern and mount. Note: An example of an antenna blind cone is that of an air route surveillance radar (ARSR). The horizontal radiation pattern of an ARSR antenna is very narrow. The vertical radiation pattern is fan-shaped, reaching approximately 70° of elevation above the horizontal plane. As the antenna is rotated about a vertical axis, it can illuminate targets only if they are 70° or less from the horizontal plane. Above that elevation, they are in the antenna blind cone. Synonym cone of silence.

antenna coupler: A device used to match the impedance of a transmitter and/or receiver to an antenna to provide maximum power transfer.

antenna dissipative loss: A power loss resulting from changes in the measurable impedance of a practical antenna from a value theoretically calculated for a perfect antenna.

antenna effective area: The functionally equivalent area from which an antenna directed toward the source of the received signal gathers or absorbs the energy of an incident electromagnetic wave. Note 1: Antenna effective area is usually expressed in square meters. Note 2: In the case of parabolic and horn-parabolic antennas, the antenna effective area is about 0.35 to 0.55 of the geometric area of the antenna aperture.

antenna efficiency: The ratio of the total radiated power to the total input power. Note: The total radiated power is the total input power less antenna dissipative losses.

antenna electrical beam tilt: The shaping of the radiation pattern in the vertical plane of a transmitting antenna by electrical means so that maximum radiation occurs at an angle below the horizontal plane. [47CFR]

antenna gain: The ratio of the power required at the input of a loss-free reference antenna to the power supplied to the input of the given antenna to produce, in a given direction, the same field strength at the same distance. Note 1: Antenna gain is usually expressed in dB. Note 2: Unless otherwise specified, the gain refers to the direction of maximum radiation. The gain may be considered for a specified polarization. Depending on the choice of the reference antenna, a distinction is made between:

• absolute or isotropic gain (G_i), when the reference antenna is an isotropic antenna isolated in space;

• gain relative to a half-wave dipole (G_d) when the reference antenna is a half-wave dipole isolated in space and with an equatorial plane that contains the given direction;

• gain relative to a short vertical antenna (G_r), when the reference antenna is a linear conductor, much shorter than one quarter of the wavelength, normal to the surface of a perfectly conducting plane which contains the given direction. [RR] Synonyms gain of an antenna, power gain of an antenna.

antenna gain-to-noise-temperature (G/T): In the characterization of antenna performance, a figure of merit, where G is the antenna gain in decibels at the receive frequency, and T is the equivalent noise temperature of the receiving system in kelvins.

antenna height above average terrain: The antenna height above the average terrain elevations from 3.2 to 16 kilometers (2 to 10 miles) from the antenna for the eight directions spaced evenly for each 45° of azimuth starting with true north. Note: In general, a different antenna height above average terrain will be determined in each direction from the antenna. The average of these eight heights is the antenna height above average terrain. In some cases, such as seashore, fewer than eight directions may be used.

antenna lobe: A three-dimensional section of the radiation pattern of a directional antenna, bounded by one or more cones of nulls or by regions of diminished irradiance.

antenna matching: The process of adjusting impedance so that the input impedance of an antenna equals or approximates the characteristic impedance of its transmission line over a specified range of frequencies. Note: The impedance of either the transmission line, or the antenna, or both, may be adjusted to effect the match.

antenna noise temperature: The temperature of a hypothetical resistor at the input of an ideal noise-free receiver that would generate the same output noise power per unit bandwidth as that at the antenna output at a specified frequency. Note 1: The antenna noise temperature depends on antenna coupling to all noise sources in its environment as well as on noise generated within the antenna. Note 2: The antenna noise temperature is a measure of noise whose value is equal to the actual temperature of a passive device.

anti-clockwise polarized wave: Synonym left-hand polarized wave.

anti-interference: Pertaining to equipment, processes, or techniques used to reduce the effect of natural and man-made noise on radio communications.

anti-jam: Measures ensuring that intended transmitted information can be received despite deliberate jamming attempts. [INFOSEC-99]

anti-node: A point in a standing wave at which the amplitude is a maximum.

antireflection coating: A thin, dielectric or metallic film, or several such films, applied to an optical surface to reduce its reflectance and thereby increase its transmittance. [After 2196] Note: For minimum reflection of a normal incident wave of a single wavelength, the antireflection coating may consist of a single layer and must have (a) a refractive index equal to the square root of the refractive indices of the materials bounding the coating, and (b) a thickness equal to one-quarter the wavelength in question (i.e. , the wavelength within the material of which the coating consists). For minimum reflection of multiple wavelengths, additional layers must be added.

anti-spoof: 1. Measures taken to prevent an unauthorized person/entity from impersonating an authorized person/entity to gain access to a data system. [DoJ] 2. Measures preventing an opponent's participation in an information system (IS). [INFOSEC-99]

anti-virus program: A computer program designed to detect computer-file viruses and possibly to suggest or take corrective action. Synonym vaccine program. [After 2382-pt.8]

anycall: In adaptive high-frequency (HF) radio automatic link establishment, a broadcast in which (a) the called stations are unspecified, (b) stations receiving the call stop scanning, and (c) each station automatically responds in pseudorandom time slots.

AP: Abbreviation for anomalous propagation.

APC: Abbreviation for adaptive predictive coding.

APD: Abbreviation for avalanche photodiode. Note: apd and a.p.d. are also used.

aperiodic antenna: An antenna designed to have an approximately constant input impedance over a wide range of frequencies. Note: Examples of aperiodic antennas include terminated rhombic antennas and wave antennas. Synonym nonresonant antenna.

aperture: 1. In a directional antenna, the portion of a plane surface very near the antenna normal to the direction of maximum radiant intensity, through which the major part of the radiation passes. 2. In an acoustic device that launches a sound wave, the passageway, determined by the size of a hole in the inelastic material and the wavelength. [After 2196]

aperture correction: 1. In a scanned image system, electrical compensation for the distortion introduced by the limiting size of a scanning aperture. [IEEE 100; After SMPTE] 2. In television technology, restoration of the depth of modulation to the higher (i.e., higher Fourier) frequency components of the video signal, with the objective of achieving a subjective improvement in image quality. [After SMPTE] Note: Aperture correction is required to compensate for the properties of the camera lens, optical beam-splitting installation, and camera tube, all of which contribute to a reduced signal at higher spatial frequencies. Problems requiring aperture correction arise in a scanning system when the frequency response falls off as the effective wavelength of the detail to be resolved in the image approaches the dimension of the scanning aperture and becomes zero when that effective wavelength equals the dimension of the scanning aperture.

aperture distortion: In facsimile, the distortion of the recorded image caused by the shape and finite size of the scanning and recording apertures. Note: The distortion may occur in one or more attributes of the recorded image, such as in resolution, density, or shape.

aperture illumination: 1. The field distribution, in amplitude and phase, over the antenna physical aperture. 2. The phase and amplitude of the element feed voltages or the distribution of the currents in an array of elements.

aperture-to-medium coupling loss: The difference between the theoretical gain of a very large antenna, such as the antennas in beyond-the-horizon microwave links, and the gain that can be realized in operation. Note 1: Aperture-to-medium coupling loss is related to the ratio of the scatter angle to the antenna beamwidth. Note 2: The "very large antennas" are referred to in wavelengths; thus, this loss can apply to line-of-sight systems also.

apogee: In an orbit of a satellite orbiting the Earth, the point that is farthest from the gravitational center of the Earth.

apogee altitude: See altitude of the apogee or of the perigee.

app: Abbreviation for application, application software.

apparent power: In alternating-current power transmission and distribution, the product of the rms voltage and amperage. Note 1: When the applied voltage and the current are in phase with one another, the apparent power is equal to the effective power, i.e., the real power delivered to or consumed by the load. If the current lags or leads the applied voltage, the apparent power is greater than the effective power. Note 2: Only effective power, i.e., the real power delivered to or consumed by the load, is expressed in watts. Apparent power is properly expressed only in volt-amperes, never watts. See diagram under effective power.

applet: A small, self-contained computer program that usually performs a task as part of, or under the control of, a larger software application. For example, most modern World Wide Web browsers are capable of making use of applets written in the Java programming language to perform simple tasks such as display animations or more complex tasks such as spreadsheet and database operations.

application: Software that performs a specific task or function, such as word-processing, creation of spreadsheets, generation of graphics, facilitating electronic mail, etc. Synonym application software.

application association: A cooperative relationship between two application entities, formed by their exchange of application protocol control information through their use of presentation services. [T1.215-1990]

application entity (AE): The system-independent application activities that are made available as application services to the application agent, e.g., a set of application service elements that together perform all or part of the communication aspects of an application process. [T1.667-1999]

Application Layer: See Open Systems Interconnection--Reference Model.

application platform: Synonym platform.

application program: See application.

application program interface (API): A formalized set of software calls and routines that can be referenced by an application program in order to access supporting network services.

application protocol: A set of rules and formats (semantic and syntactic) that determines the communication behavior of application entities in the performance of application functions. [T1.208-1989]

application service element (ASE): A coherent set of integrated functions to help accomplish application communication, e.g., within an application entity. [T1.667-1999]

application software: Synonym application.

applique: Circuit components added to an existing system to provide additional or alternate functions. Note: Applique may be used to modify carrier telephone equipment designed for ringdown manual operation to allow for use between points having dial equipment.

appointment book: A small (pocket-size or smaller), specialized, portable computing device, with a self-contained power source, designed to (a) accept as inputs; (b) store; and (c) read out, user-designated information, usually appointments, telephone numbers, etc.

approved circuit: Deprecated synonym for protected distribution system.

aramid yarn: Generic name for a tough synthetic yarn that is often used in optical cable construction for the strength member, protective braid, and/or rip cord for jacket removal. [FAA]

architecture: See computer architecture, network architecture.

archive file: A software file that has been set aside, often in a redundant storage medium, as a security measure or for later retrieval, e.g. , for research or verification.

archiving: The storing of files, records, and other data for reference and alternative backup.

area broadcast shift: The changing from listening to transmissions intended for one broadcast area to listening to transmissions intended for another broadcast area. Note 1: An area broadcast shift may occur when a ship or aircraft crosses the boundary between listening areas. Note 2: Shift times, on the date a ship or aircraft is expected to pass into another area, must be strictly observed or the ship or aircraft will miss messages intended for it. Synonym radio watch shift.

area code: See access code, code, country code, NXX code.

area loss: When optical fibers are joined by a splice or a pair of mated connectors, a power loss that is caused by any mismatch in size or shape of the cross section of the cores of the mating fibers. Note 1: Any of the above conditions may allow light from the core of the "transmitting" fiber to enter the cladding of the "receiving" fiber, where it is quickly lost. Note 2: Area loss may be dependent on the direction of propagation. For example, in coupling a signal from an optical fiber having a smaller core to an otherwise identical one having a larger core, there will be no area loss, but in the opposite direction, there will be area loss. [After FAA]

argument: 1. An independent variable. 2. Any value of an independent variable. Note: Examples of arguments include search keys, numbers that identify the location of a data item in a table, and the in sin .

arithmetic and logic unit (ALU): A part of a computer that performs arithmetic, logic, and related operations.

arithmetic overflow: 1. In a digital computer, the condition that occurs when a calculation produces a result that is greater than a given register or storage location can store or represent. 2. In a digital computer, the amount that a calculated value is greater than a given register or storage location can store or represent. Note: The overflow may be placed at another location. Synonym [loosely] overflow.

arithmetic register: A register that holds the operands or the results of operations such as arithmetic operations, logic operations, and shifts.

arithmetic shift: A shift, applied to the representation of a number in a fixed radix numeration system and in a fixed-point representation system, and in which only the characters representing the fixed-point part of the number are moved. An arithmetic shift is usually equivalent to multiplying the number by a positive or a negative integral power of the radix, except for the effect of any rounding; compare the logical shift with the arithmetic shift, especially in the case of floating-point representation.

arithmetic underflow: In a digital computer, the condition that occurs when a calculation produces a non-zero result that is less than the smallest non-zero quantity that a given register or storage location can store or represent.

arithmetic unit: In a processor, the part that performs arithmetic operations; sometimes the unit performs both arithmetic and logic operations.

Armed Forces Radio Service (AFRS): A radio broadcasting service that is operated by and for the personnel of the armed services in the area covered by the broadcast. Note: An example of an AFRS is the radio service operated by the U.S. Army for U.S. and allied military personnel on duty in overseas areas.

armor: Of a communications cable, a component intended to protect the critical internal components, e.g., buffer tubes or fibers, or electrical conductors, from damage from external mechanical attack, e.g., rodent attack or abrasion. [After FAA] Note: Armor usually takes the form of a steel or aluminum tape wrapped about an inner jacket that covers the critical internal components. An outer jacket usually covers the armor.

ARP: Abbreviation for address resolution protocol.

ARPANET: Abbreviation for Advanced Research Projects Agency Network. A packet-switching network used by the Department of Defense, later evolved into the Internet.

ARQ: Abbreviation for automatic repeat-request. Error control for data transmission in which the receiver detects transmission errors in a message and automatically requests a retransmission from the transmitter. Note: Usually, when the transmitter receives the ARQ, the transmitter retransmits the message until it is either correctly received or the error persists beyond a predetermined number of retransmissions. Synonyms error-detecting-and-feedback system, repeat-request system.

array: 1. An arrangement of elements in one or more dimensions. 2. In a programming language, an aggregate that consists of data objects with identical attributes, each of which may be uniquely referenced by subscription.

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

arrester: A device that protects hardware, such as systems, subsystems, circuits, and equipment, from voltage or current surges produced by lightning or electromagnetic pulses. Note: If the hardware is adequately protected, associated software may also be adequately protected. Synonyms surge protective device, surge suppressor.

ARS: Abbreviation for automatic route selection.

article: Synonym posting.

articulation index: A measure of the intelligibility of voice signals, expressed as a percentage of speech units that are understood by the listener when heard out of context. Note: The articulation index is affected by noise, interference, and distortion.

articulation score (AS): A subjective measure of the intelligibility of a voice system in terms of the percentage of words correctly understood over a channel perturbed by interference. Note: Articulation scores have been experimentally obtained as functions of varying word content, bandwidth, audio signal-to-noise ratio and the experience of the talkers and listeners involved.

artifact: In facsimile or television, a defect or distortion of the image, introduced along the sequence from origination and image capture to final display. Note 1: Artifacts may arise from the overload of channel capacity by excess signal bandwidth. Note 2: In general, artifacts may result from (a) sampling effects in temporal, spatial, or frequency domains, (b) processing by the transfer functions, (c) compromises and inadequacies in the system employed, (d) cascading of minor defects, or (e) any other departure of the total system from "complete transparency." [After SMPTE]

artificial intelligence (AI): The capability of a device to perform functions that are normally associated with human intelligence, such as reasoning and optimization through experience. Note: AI is the branch of computer science that attempts to approximate the results of human reasoning by organizing and manipulating factual and heuristic knowledge. Areas of AI activity include expert systems, natural language understanding, speech recognition, vision, and robotics.

artificial transmission line: A four-terminal electrical network, i.e. an electrical circuit, that has the characteristic impedance, transmission time delay, phase shift, and/or other parameter(s) of a real transmission line and therefore can be used to simulate a real transmission line in one or more of these respects. Synonym [loosely] art line.

art line: Synonym [loosely] artificial transmission line.

ARU: Abbreviation for audio response unit.

ASCII: Acronym for American Standard Code for Information Interchange. The standard code used for information interchange among data processing systems, data communications systems, and associated equipment in the United States. Note 1: The ASCII character set contains 128 coded characters. Note 2: Each ASCII character is a 7-bit coded unique character; 8 bits when a parity check bit is included. Note 3: The ASCII character set consists of control characters and graphic characters. Note 4: When considered simply as a set of 128 unique bit patterns, or 256 with a parity bit, disassociated from the character equivalences in national implementations, the ASCII may be considered as an alphabet used in machine languages. Note 5: The ASCII is the U.S. version of International Reference Alphabet (IRA) No. 5 (formerly International Alphabet No. 5, or "IA5") as specified in ITU-T Recommendation T.50.

ASP: Abbreviation for adjunct service point.

aspect ratio: 1. In facsimile or television, the ratio of the width to the height of a scanning field or image. Note: For example, the classical NTSC television standard specifies an aspect ratio of 4:3, and the new high-definition television standard specifies 16:9. 2. The ratio of the width to the height of any video or scanned image display.

assemble: To translate a computer program expressed in an assembly language into a machine language.

assembler: A computer program that is used to assemble. Synonym assembly program.

assembly: In logistics, an item forming a portion of an equipment that can be provisioned and replaced as an entity and which normally incorporates replaceable parts or groups of parts. [JP1]

assembly language: A computer-oriented language (a) in which instructions are symbolic and usually in one-to-one correspondence with sets of machine language instructions and (b) that may provide other facilities, such as the use of macro instructions. Synonym computer-dependent language.

assembly program: Synonym assembler.

assembly time: The elapsed time taken for the execution of an assembler.

asset: 1. In security, a resource or information that is to be protected. [After CC-99] 2. Any system or component (e.g., subsystem, hardware, firmware, software, database, or interconnection communications network or facility) that is part of a communications system or an information system. [After FAA]

assigned frequency: 1. The center of the assigned frequency band assigned to a station. [RR] 2. The frequency of the center of the radiated bandwidth. Note: The frequency of the rf carrier, whether suppressed or radiated, is usually given in parentheses following the assigned frequency, and is the frequency appearing in the dial settings of rf equipment intended for single-sideband or independent-sideband transmission. 3. The frequency coinciding with the center of an authorized bandwidth of emission. [47CFR] 4. The center of the frequency band assigned to a station. [47CFR]

assigned frequency band: The frequency band within which the emission of a station is authorized; the width of the band equals the necessary bandwidth plus twice the absolute value of the frequency tolerance. Where space stations are concerned, the assigned frequency band includes twice the maximum Doppler shift that may occur in relation to any point of the Earth's surface. [NTIA] [RR]

assignment: For NS/EP, the designation of priority level(s).

assignment (of a radio frequency or radio frequency channel): Authorization given by an administration for a radio station to use a radio frequency or radio frequency channel under specified conditions. [NTIA] [RR]

associated common-channel signaling: Common-channel signaling in which the signal channel is associated with a specific trunk group and terminates at the same pair of switches as the trunk group. Note: In associated common-channel signaling, the signaling is usually accomplished by using the same facilities as the associated trunk group.

associated mode of signaling: The mode in which messages for a signaling relation involving two adjacent signaling points are conveyed over a directly interconnecting signaling link. [T1.226-1992]

associative storage: 1. A storage device whose storage locations are identified by their contents, or by a part of their contents, rather than by their names or positions. Note: Associative storage can also refer to this process as well as to the device. Synonym content-addressable storage. 2. Storage that supplements another storage.

assurance: 1. Grounds for confidence that an information-technology (IT) product or system meets its security objectives. 2. In INFOSEC, see information assurance.

asymmetrical modulator: Synonym unbalanced modulator.

asymmetric compression: A data compression technique that requires more processing capability to compress than to decompress. Note: Asymmetric compression is typically used for the mass distribution of programs on media such as CD ROM, where significant expense can be incurred for the production and compression of data but the retrieval (or the playback) system must be low in cost. [After Silicon]

asymmetric cryptographic algorithm: A cryptographic formula that uses two related keysa public key and a private keyeach of which has the characteristic algorithm that, given the public key, it is computationally infeasible to derive the private key. [After X9.62]

asymmetric digital subscriber line (ADSL): 1. An access technology that allows voice and high-speed data to be sent simultaneously over local exchange service copper facilities; the technology supports data rates of up to 1.544 Mb/s when receiving data (downstream rate) and up to 256 kb/s when sending data (upstream rate). 2. A modem technology that provides enhanced and affordable access to the Internet, live video, and a wide variety of other multimedia broadband services over existing copper twisted-pair wirelines; usually the ADSL operates with different data rates in the two directions. [After NCS TIP 99-1]

asymmetric encryption: An encryption system that utilizes two keys, one called a public key (which is known to both the sender and the recipient of encrypted data), and the other, called a private key (known only to the individual sending the data). Note: Data are encrypted with the private key and decrypted with the public key. Asymmetric encryption allows for the secure transfer of data. [After Bahorsky]

asynchronous communications system: A data communications system that uses asynchronous operation. Note: The time spacing between successive data characters or blocks may be of arbitrary duration. Synonym start-stop system. [T1.X1]

asynchronous multiplexed transmission system: A multiplexed transmission system in which the rates of the carrier and its tributaries are not traceable to the same master clocking source. Note: In this type of system, extra signal elements are usually appended to the individual tributary signals for the purpose of interleaving them at a rate consistent with the carrier rate. [T1.X1]

asynchronous network: A network in which the clocks do not need to be synchronous or mesochronous. Synonym nonsynchronous network.

asynchronous operation: 1. A sequence of operations in which operations are executed out of time coincidence with any event. 2. An operation that occurs without a regular or predictable time relationship to a specified event; e.g., the calling of an error diagnostic routine that may receive control at any time during the execution of a computer program. Synonym asynchronous working.

asynchronous time-division multiplexing (ATDM): Time-division multiplexing in which asynchronous transmission is used.

asynchronous transfer mode (ATM): A high-speed multiplexing and switching method utilizing fixed-length cells of 53 octets to support multiple types of traffic. Note: ATM, specified in international standards, is asynchronous in the sense that cells carrying user data need not be periodic.

asynchronous transmission: Data transmission in which the instant that each character, or block of characters, starts is arbitrary; once started, the time of occurrence of each signal representing a bit within the character, or block, has the same relationship to significant instants of a fixed time frame.

asynchronous working: Synonym asynchronous operation.

ATB: Abbreviation for all trunks busy.

AT Commands: A de facto standard for modem commands from an attached CPU, used in most 1,200 and 2,400 b/s modems.

ATDM: Abbreviation for asynchronous time-division multiplexing.

ATM: Abbreviation for asynchronous transfer mode.

ATM (asynchronous transfer mode) connection: A virtual channel connection (VCC) or a virtual path connection (VPC). [T1.627-1993]

ATM (asynchronous transfer mode) layer connection: An association established by the ATM Layer to support communication between two or more ATM service users (i.e., two or more next higher layer entities or two or more ATM Management entities). The communication over an ATM Layer connection may be either bidirectional or unidirectional. When bidirectional, two virtual channel connections (VCCS) are used. When unidirectional, only one VCC is used. Note: In the case where there is ATM Layer connection associated ATM Layer Management (e.g., F5 Flow), the ATM Layer connection is bidirectional even if the user communication is unidirectional. [T1.629-1993]

ATM (asynchronous transfer mode) transport system: A transport system composed of a carrier system terminated by an ATM multiplexer at each end. The relationship between input and output, i.e., virtual circuits, cannot change on a per call basis, i.e., permanent virtual circuits only are supported. This device does not allow blocking or signal compression. [T1.Rpt46-1996]

ATM link (asynchronous transfer mode link): A virtual path link (VPL) or a virtual channel link (VCL). [T1.627-1993]

atmospheric duct: A horizontal layer in the lower atmosphere in which the vertical refractive index gradients are such that radio signals (a) are guided or focused within the duct, (b) tend to follow the curvature of the Earth, and (c) experience less attenuation in the ducts than they would if the ducts were not present. Note: The reduced refractive index at the higher altitudes bends the signals back toward the Earth. Signals in a higher refractive index layer, i.e., duct, tend to remain in that layer because of the reflection and refraction encountered at the boundary with a lower refractive index material.

atmospheric noise: Radio noise caused by natural atmospheric processes, primarily lightning discharges in thunderstorms.

ATM switch (asynchronous transfer mode switch): 1. With reference to both analog and digital interfaces, the ATM functional unit and technology that operates with analog 2-wire interfaces and standard digital line rates on both the inputs and the outputs, e.g., DS1 on the input and DS3 on the output, and which passes the cells received on the input to the output. The relationship between input and output, can change on a per call basis, i.e., switched virtual circuits must be supported and permanent virtual circuits may be supported. (For the purposes of this document it is assumed that the cell formation function associated with an analog input is performed before the switch function occurs. However, this definition is not intended to constrain technical implementations.) This device allows blocking and concentration. The switch function is also required to perform 2-wire to 4-wire conversion as well as A/D conversion. [T1.Rpt46-1996] 2. With reference to digital interfaces only, the ATM functional unit and technology that operates with cell-based signals and over standard line rates on both the inputs and the outputs, e.g., DS1 on the input and DS3 on the output, and which passes the cells received on the input to the output. The relationship between input and output, can change on a per-call basis, i.e., switched virtual circuits must be supported and permanent virtual circuits may be supported. This device allows blocking and performs concentration. [T1.Rpt46-1996]

atomic time: See International Atomic Time.

ATOW: Acronym for acquisition and tracking orderwire. A downlink circuit that provides a terminal with information regarding uplink acquisition and synchronization status.

attachment: In e-mail, a computer file that is transmitted with an e-mail message. Note: Attachments are converted by an e-mail manager program, or by an add-on, to a MIME (multipurpose Internet mail extension) or binary format. The files are recovered by the recipient's e-mail manager program or by an add-on into their original, usually application-specific, format. [Bahorsky]

attachment unit interface (AUI): In a local area network, the interface between the medium access unit (MAU) and the data terminal equipment within a data station.

attack: 1. An attempt to violate computer security. Note: An example of an attack is malicious logic. [2382-pt.8] 2. [An] intentional act of attempting to bypass one or more of the following security controls of an information system (IS): nonrepudiation, authentication, integrity, availability, or confidentiality. [INFOSEC-99]

attack time: The time between (a) the instant that a signal at the input of a device or circuit exceeds the activation threshold of the device or circuit and (b) the instant that the device or circuit reacts in a specified manner, or to a specified degree, to the input. Note: Attack time occurs in devices such as clippers, peak limiters, compressors, and voxes.

attendant access loop: A switched circuit that provides an attendant with a manual means for call completion and control. Note: An attendant access loop might be given a specific telephone number. Synonym access loop.

attendant conference: A network-provided service feature that allows an attendant to establish a conference connection of three or more users.

attendant position: The part of a switching system used by an attendant, i.e., an operator, to assist users in call completion and use of special services.

attention character: In [a] trusted-computing-base (TCB) design, a character entered from a terminal that tells the TCB the user wants a secure communications path from the terminal to some trusted node to provide a secure service for the user. [INFOSEC-99]

attention signal: The attention signal to be used by AM, FM, and TV broadcast stations to actuate muted receivers for inter-station receipt of emergency cuing announcements and broadcasts involving a range of emergency contingencies posing a threat to the safety of life or property. [47CFR]

attenuation: The decrease in intensity of a signal, beam, or wave as a result of absorption of energy and of scattering out of the path to the detector, but not including the reduction due to geometric spreading. [After JP1] Note 1: Attenuation is usually expressed in dB. Note 2: "Attenuation" is often used as a misnomer for "attenuation coefficient, " which is expressed in dB per kilometer. Note 3: A distinction must be made as to whether the attenuation is that of signal power or signal electric field strength.

attenuation coefficient: The rate of diminution of average power with respect to distance along a transmission path. Note: The attenuation coefficient is often calculated as the sum of the absorption coefficient and the scattering coefficient. Synonym attenuation rate.

attenuation constant: 1. The real part of the propagation constant in any electromagnetic propagation medium. Note 1: The attenuation constant is usually expressed as a numerical value per unit length. Note 2: The attenuation constant may be calculated or experimentally determined for each medium. 2. For a particular propagation mode in an optical fiber, the real part of the axial propagation constant.

attenuation distortion: The difference in loss at specified frequencies relative to the loss at 1004 Hz, unless otherwise specified. [NECA/FCC-5]

attenuation-limited operation: The condition that prevails when attenuation, rather than bandwidth, limits the performance of a communications link.

attenuation rate: Synonym attenuation coefficient.

attenuator: 1. In electrical systems, a network that reduces the amplitude of a signal without appreciably distorting its waveform. Note 1: Electrical attenuators are usually passive devices. Note 2: The degree of attenuation may be fixed, continuously adjustable, or incrementally adjustable. Fixed attenuators are often called pads, especially in telephony. Note 3: The input and output impedances of an attenuator are usually matched to the impedances of the signal source and load, respectively. 2. In optical systems, a device that reduces the amplitude of a signal without appreciably distorting its waveform. Note 1: Optical attenuators are usually passive devices. Note 2: The degree of attenuation may be fixed, continuously adjustable, or incrementally adjustable.

attribute: 1. In database management, a property inherent in an entity or associated with that entity for database purposes. 2. In network management, a property of a managed object that has a value. Note 1: Mandatory initial values for attributes can be specified as part of the managed object class definition. Note 2: Attributes may be either mandatory or conditional.

attribute authority: In computer security, an entity that is trusted by at least two entities to create and assign attribute certificates. [After X9.45]

attribute certificate: A set of attributes and a public key certificate identifier that are made unforgeable by use of the digital signature created with a private key. [After X9.30]

ATV: Abbreviation for advanced television.

audible: Synonym for audible ringing tone.

audible ringing: An audible signal (information tone) transmitted to the calling party when the called party is alerted. [T1.401-1988]

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. 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 ringing, ringback tone.

audio channel: A means for delivering audio signals from one point to another. An audio waveform submitted to the channel input results in a similar (not necessarily identical) waveform at the channel output. The audio channel may be comprised of the following components: encoders (compressors) and decoders (decompressors), buffers, multiplexors and demultiplexors, modulators and demodulators, transmission facilities, switches, multipoint conference units, and other components necessary to achieve the desired channel characteristics.

audio dubbing: 1. In videotape editing, a process or technique employed to (a) enhance (e.g. , remove noise from, or introduce some kind of special effect into), (b) add to, or (c) replace totally, the originally recorded audio (if any), without modifying the originally recorded video signal. 2. In audiotape editing or mixing, a process analogous to any of the above, performed (a) on a single audio channel, or (b) on one or more audio channels of a multi-track recording without modifying the other channel(s). 3. The copying of one or more audio signals from one storage medium, location, or format to another with or without modification or enhancement.

audio-follow-video: A video recording, mixing or switching technique or process in which the audio signal associated with any given video signal is recorded, switched, or mixed with that video signal.

audio frame: A presentation unit of the audio channel; a group of consecutive audio samples. The preferred number of samples in an audio frame depends on the audio sample rate. These audio frames have no relationship to the frames designated by certain audio/speech codecs. [T1.801.04-1997]

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.

audio response unit (ARU): A device that provides synthesized voice responses to dual-tone multifrequency signaling input by processing calls based on (a) the call-originator input, (b) information received from a host data base, and (c) information in the incoming call, such as the time of day. Note: ARUs are used to increase the number of information calls handled and to provide consistent quality in information retrieval.

audit: 1. To conduct an independent review and examination of system records and activities in order to test the adequacy and effectiveness of data security and data integrity procedures, to ensure compliance with established policy and operational procedures, and to recommend any necessary changes. 2. Independent review and examination of records and activities to assess the adequacy of system controls, to ensure compliance with established policies and operational procedures, and to recommend necessary changes in controls, policies, or procedures. [INFOSEC-99]

audit record field: A field containing information regarding all entities in a transaction, and indicators of the types of processing performed by those entities. [After X9.17/95]

audit review file: A file created by executing statements included in a computer program for the explicit purpose of providing data for auditing.

audit trail: 1. A record of both completed and attempted accesses and service. 2. Data in the form of a logical path linking a sequence of events, used to trace the transactions that have affected the contents of a record. 3. [In INFOSEC, a] chronological record of system activities to enable the reconstruction and examination of the sequence of events and/or changes in an event. Note: Audit trail may apply to information in an information system (IS), to message routing in a communications system, or to the transfer of COMSEC material. [INFOSEC-99]

AUI: Abbreviation for attachment unit interface.

aurora: Sporadic radiant emission from the upper atmosphere that usually occurs about the North and South magnetic poles of the Earth. Note 1: Auroras are most intense at times of intense magnetic storms caused by sunspot activity. The distribution of auroral intensity with altitude shows a pronounced maximum near 100 km above the Earth. Auroras may occasionally be observed within 40° or less of the equator. Note 2: Auroras interfere with radio communications. Note 3: In the Northern hemisphere, the aurora is called the Aurora Borealis (Northern Lights). In the Southern hemisphere, the aurora is called the Aurora Australis (Southern Lights).

authenticate: 1. To establish, usually by challenge and response, that a transmission attempt is authorized and valid. 2. [To] verify the identity of a user, user device, or other entity, or the integrity of data stored, transmitted, or otherwise exposed to unauthorized modification in an information system (IS), or establish the validity of a transmission. [INFOSEC-99] 3. A challenge given by voice or electrical means to attest to the authenticity of a message or transmission. [JP1]

authentication: 1. [Any] Security measure designed to establish the validity of a transmission, message, or originator, or a means of verifying an individual's authorization to receive specific categories of information. [INFOSEC-99] [After JP 1-02] 2. A security measure designed to protect a communications system against acceptance of a fraudulent transmission or simulation by establishing the validity of a transmission, message, or originator. [JP 1-02] 3. Evidence by proper signature or seal that a document is genuine and official. [JP 1-02]

authentication algorithm: An encryption process or tool in which the results of text encryption depend on all relevant authentication elements. [After X9.24]

authentication element: A contiguous group of characters or bits that are corruption-protected by being processed by the authentication algorithm. [After X9.24]

authentication exchange: A mechanism intended to ensure the identity of an entity by means of an information exchange. [2382-pt.8]

authentication information: Information used to establish the validity of a claimed identity of an entity. [2382-pt.8]

authentication key: A (data-encryption algorithm) key used to authenticate data in accordance with specific encryption standards. [After X9.26]

authentication system: [A] cryptosystem or process used for authentication. [INFOSEC-99]

authenticator: 1. A symbol or group of symbols, or a series of bits, selected or derived in a prearranged manner and usually inserted at a predetermined point within a message or transmission for the purpose of attesting to the validity of the message or transmission. [JP 1-02] 2. A letter, numeral, group of letters or numerals, or any combinations of these, attesting to the authenticity of a message or transmission. [After JP1] 3. [In INFOSEC,] a means used to confirm the identity of a station, originator, or individual. [INFOSEC-99]

authorization: 1. The rights granted to a user to access, read, modify, insert, or delete certain data, or to execute certain programs. 2. Access privileges granted to a user, program, or process. [INFOSEC-99]

authorization certificate: Any of several types of attribute certificates containing information used in the authorization process. Note: Authorization information may also be contained in a public key certificate, in which case that public key certificate also serves as an authorization certification [After X9.45]

authorized bandwidth: 1. Authorized bandwidth is, for purposes of this Manual, the necessary bandwidth (bandwidth required for transmission and reception of intelligence) and does not include allowance for transmitter drift or Doppler shift. [NTIA] 2. The maximum bandwidth authorized to be used by a station as specified in the station license. This shall be occupied bandwidth or necessary bandwidth, whichever is greater. [47CFR] 3. The maximum width of the band of frequencies permitted to be used by a station. This is normally considered to be the necessary or occupied bandwidth, whichever is greater. [47CFR]

authorized frequency: 1. A frequency that is allocated and assigned by a competent authority to a specific user for a specific purpose. 2. The frequency, or frequency range, assigned to a station by the Commission [FCC] and specified in the instrument of authorization. [47CFR] See assigned frequency.

authorized signatory: The highest level issuer of authorization certificates in an organization. Note: Authorized signatories are designated in a signatory certificate, which is issued to an organization by an agreed signatory authority. [After X9.45]

authorized user: In security, a user who may, according to an organization's security policy, perform an operation. [After CC-99]

AUTODIN: Acronym for automatic digital network. See Defense Data Network.

automated data medium: Synonym machine-readable medium.

automated information system (AIS): 1. An assembly of computer hardware, software, firmware, or any combination of these, configured to accomplish specific information-handling operations, such as communication, computation, dissemination, processing, and storage of information. 2. [In INFOSEC,] any equipment or interconnected system or subsystems of equipment that is used in the automatic acquisition, storage, manipulation, management, movement, control, display, switching, interchange, transmission or reception of data and includes computer software, firmware, and hardware. Note: Included are computers, word processing systems, networks, or other electronic information handling systems, and associated equipment. [NIS] 3. See information systems security.

automated information systems security: 1. Measures and controls that ensure confidentiality, integrity, and availability of the information processed and stored by automated information systems. Note 1: The unauthorized disclosure, modification, or destruction may be accidental or intentional. Note 2: Automated information systems security includes consideration of all hardware and software functions, characteristics and features; operational procedures; accountability procedures; and access controls at the central computer facility, remote computer, and terminal facilities; management constraints; physical structures and devices, such as computers, transmission lines, and power sources; and personnel and communications controls needed to provide an acceptable level of risk for the automated information system and for the data and information contained in the system. Automated information systems security also includes the totality of security safeguards needed to provide an acceptable protection level for an automated information system and for the data handled by an automated information system. 2. In INFOSEC, synonym computer security.

automated maritime telecommunications system (AMTS): An automatic, integrated and interconnected maritime communications system serving ship stations on specified inland and coastal waters of the United States. [47CFR]

automated radio: A radio that can be automatically controlled by electronic devices and that requires little or no human intervention.

automated security monitoring: Use of automated procedures to ensure security controls are not circumvented or the use of these tools to track actions taken by subjects suspected of misusing the information system (IS). [INFOSEC-99]

automated tactical command and control system: A command and control system, or part thereof, that manipulates the movement of information from source to user without human intervention. Note: In an automated tactical command and control system, automated execution of a decision without human intervention is not mandatory.

automatic answering: A service feature in which the called terminal automatically responds to the calling signal and the call may be established whether or not the called terminal is attended by an operator.

automatic callback: A service feature that permits a user, when encountering a busy condition, to instruct the system to retain the called and calling numbers and to establish the call when there is an available line. Note 1: Automatic callback may be implemented in the terminal, in the switching system, or shared between them. Note 2: Automatic callback is not the same as camp-on.

automatic call distributor (ACD): A device that distributes incoming calls to a specific group of terminals. Note: If the number of active calls is less than the number of terminals, the next call will be routed to the terminal that has been in the idle state the longest. If all terminals are busy, the incoming calls are held in a first-in-first-out queue until a terminal becomes available.

automatic calling: Calling in which the elements of the selection signal are entered into the data network contiguously at the full data signaling rate. The selection signal is generated by the data terminal equipment. Note: A limit may be imposed by the design criteria of the network to prevent more than a permitted number of unsuccessful call attempts to the same address within a specified period.

automatic calling unit (ACU): A device that enables equipment, such as computers and card dialers, to originate calls automatically over a telecommunications network.

automatic data handling (ADH): 1. A generalization of automatic data processing to include the aspect of data transfer. [JP1] 2. Combining data processing and data transfer.

automatic data processing (ADP): 1. An interacting assembly of procedures, processes, methods, personnel, and equipment to perform automatically a series of data processing operations on data. Note: The data processing operations may result in a change in the semantic content of the data. 2. Data processing by means of one or more devices that use common storage for all or part of a computer program, and also for all or part of the data necessary for execution of the program; that execute user-written or user-designated programs; that perform user-designated symbol manipulation, such as arithmetic operations, logic operation, or character-string manipulations; and that can execute programs that modify themselves during their execution. Note: Automatic data processing may be performed by a stand-alone unit or by several connected units. 3. Data processing largely performed by automatic means. [JP 1-02] 4. That branch of science and technology concerned with methods and techniques relating to data processing largely performed by automatic means. [JP 1-02]

automatic data processing equipment (ADPE): Any equipment or interconnected system or subsystems of equipment that is used in the automatic acquisition, storage, manipulation, management, movement, control, display, switching, interchange, transmission, or reception, of data or information (i) by a Federal agency, or (ii) under a contract with a Federal agency which (i) requires the use of such equipment, or (ii) requires the performance of a service or the furnishing of a product which is performed or produced making significant use of such equipment. Such term includes (i) computer, (ii) ancillary equipment, (iii) software, firmware, and similar procedures, (iv) services, including support services, and (v) related resources as defined by regulations issued by the Administrator for General Services. . . . [Public Law 99-500, Title VII, Sec. 822 (a) Section 111(a) of the Federal Property and Administrative Services Act of 1949 (40 U.S.C. 759(a)) revised. ]

automatic dialing: See automatic calling unit.

Automatic Digital Network (AUTODIN): Formerly, a worldwide data communications network of the Defense Communications System, now replaced by the Defense Switched Network (DSN).

automatic error correction: See error-correcting code.

automatic exchange: In a telephone system, an exchange in which communications among users are effected by means of switches set in operation by the originating user equipment without human intervention at the central office or branch exchange.

automatic frequency control (AFC): A device or circuit that maintains the frequency of an oscillator within the specified limits with respect to a reference frequency.

automatic function: A machine function or series of machine functions controlled by a program and carried out without assistance of an operator.

automatic gain control (AGC): A process or means by which gain is automatically adjusted in a specified manner as a function of a specified parameter, such as received signal level.

automatic identified outward dialing (AIOD): A service feature of some switching or terminal devices that provides the user with an itemized statement of usage on directly dialed calls. Note: AIOD is facilitated by automatic number identification (ANI) equipment to provide automatic message accounting (AMA).

automatic link establishment (ALE): 1. In high-frequency (HF) radio, the capability of a station to make contact, or initiate a circuit, between itself and another specified radio station, without human intervention and usually under processor control. Note: ALE techniques include automatic signaling, selective calling, and automatic handshaking. Other automatic techniques that are related to ALE are channel scanning and selection, link quality analysis (LQA), polling, sounding, message store-and-forward, address protection, and anti-spoofing. 2. In HF radio, a link control system that includes automatic scanning, selective calling, sounding, and transmit channel selection using link quality analysis data. Note: Optional ALE functions include polling and the exchange of orderwire commands and messages.

automatic link transfer: Automatic rerouting of the radio portion of a call for signal quality, traffic management, or other reasons. [T1.702-1995]

automatic message accounting (AMA): A service feature that automatically records data regarding user-dialed calls.

automatic message exchange (AME): In an adaptive high-frequency (HF) radio network, an automated process allowing the transfer of a message from message injection to addressee reception, without human intervention. Note: Through the use of machine-addressable transport guidance information, i.e., the message header, the message is automatically routed through an on-line direct connection through single or multiple transmission media.

automatic message processing system (AMPS): Any organized assembly of resources and methods used to collect, process, and distribute messages largely by automatic means. [JP1]

automatic number identification (ANI): 1. A service feature in which the directory number or equipment number of a calling station is automatically obtained. Note: ANI is used in message accounting. 2. The code that provides the billing number of the line or trunk that originated a call. [T1.104-1988] 3. A system that identifies the billing account for a call. For 911 systems, the ANI identifies the calling party and may be used as a call back number. [47CFR]

automatic operation: The functioning of systems, equipment, or processes in a desired manner at the proper time under control of mechanical or electronic devices that operate without human intervention.

automatic personal deregistration: The process by which a user's location registration is automatically canceled without any explicit action from the user. [T1.Rpt34-1994]

automatic personal registration: The process by which a user's location registration is automatically updated without any explicit action from the user. [T1.Rpt34-1994]

automatic redial: A service feature that allows the user to dial, by depressing a single key or a few keys, the most recent telephone number dialed at that instrument. Note: Automatic redial is often associated with the telephone instrument, but may be provided by a PBX, or by the central office. Synonym last number redial.

automatic reload: See bootstrap.

automatic remote rekeying: [In INFOSEC, a] procedure to rekey a distant crypto-equipment electronically without specific actions by the receiving terminal operator. [INFOSEC-99] Note: Automatic remote rekeying may also apply to non-crypto devices.

automatic remote reprogramming and rekeying: The procedure by which distant equipment is reprogrammed or rekeyed electronically without specific actions by the receiving terminal.

automatic repeat-request (ARQ): See ARQ.

automatic ringdown circuit: A circuit providing priority telephone service, typically for key personnel; the circuit is activated when the telephone handset is removed from the cradle causing a ringing signal to be sent to the distant unit(s). See verified off-hook.

automatic route selection (ARS): Electronic or mechanical selection and routing of outgoing calls without human intervention.

Automatic Secure Voice Communications Network (AUTOSEVOCOM): A worldwide, switched, secure voice network developed to fulfill DOD long-haul, secure voice requirements. [JP1]

automatic sequential connection: A service feature in which the terminals at each of a set of specified addresses are automatically connected, in a predetermined sequence, to a single terminal at a specified address.

automatic signaling service: Synonym hotline.

automatic sounding: The testing of selected channels or paths by providing a very brief beacon-like identifying broadcast that may be used by other stations to evaluate connectivity, propagation, and availability, and to identify known working channels for possible later use for communications or calling. Note 1: Automatic soundings are primarily intended to increase the efficiency of the automatic link establishment (ALE) function, thereby increasing system throughput. Note 2: Sounding information is used for identifying the specific channel to be used for a particular ALE connectivity attempt.

automatic switching system: 1. In data communications, a switching system in which all the operations required to execute the three phases of information-transfer transactions are automatically executed in response to signals from a user end-instrument. Note: In an automatic switching system, the information-transfer transaction is performed without human intervention, except for initiation of the access phase and the disengagement phase by a user. 2. In telephony, a system in which all the operations required to set up, supervise, and release connections required for calls are automatically performed in response to signals from a calling device.

Automatic Voice Network (AUTOVON): Formerly, the principal long-haul, unsecure voice communications network within the Defense Communications System, now replaced by the Defense Switched Network (DSN).

automation: 1. The implementation of processes by automatic means. [JP 1-02] 2. The investigation, design, development, and application of methods of rendering processes automatic, self-moving, or self-controlling. 3. The conversion of a procedure, a process, or equipment to automatic operation. [JP 1-02]

autonomous system: Internet (TCP/IP) terminology for a collection of gateways (routers) that fall under one administrative entity and that cooperate using a common interior gateway protocol (IGP). [Bahorsky] Note: Routers pertaining to different autonomous systems must agree on a common exterior gateway protocol in order to communicate with each other effectively.

auto-reply: In e-mail systems, a message sent automatically upon receipt of incoming e-mail. Note: Auto-replies are used to acknowledge delivery of e-mail and to provide receipts for e-mail messages.

AUTOSEVOCOM: Acronym for Automatic Secure Voice Communications Network.

AUTOVON: Acronym for Automatic Voice Network. Superseded by Defense Switched Network.

auxiliary operation: An offline operation performed by equipment not under control of the processing unit.

auxiliary power: Electric power that is provided by an alternate source and that serves as backup for the primary power source at the station main bus or prescribed sub-bus. Note 1: An offline unit provides electrical isolation between the primary power source and the critical technical load whereas an online unit does not. Note 2: A Class A power source is a primary power source, i.e., a source that assures an essentially continuous supply of power. Note 3: Types of auxiliary power services include Class B, a standby power plant to cover extended outages of the order of days; Class C, a 10-to-60-second quick-start unit to cover short-term outages of the order of hours; and Class D, an uninterruptible non-break unit using stored energy to provide continuous power within specified voltage and frequency tolerances.

auxiliary storage: 1. Storage that is available to a processor only through its input/output channels. 2. In a computer, any storage that is not internal memory, i.e., is not random access memory (RAM). Note: Examples of auxiliary storage media are magnetic diskettes, optical disks including CD ROM, and magnetic tape cassettes.

availability: 1. The degree to which a system, subsystem, or equipment is operable and in a committable state at the start of a mission, when the mission is called for at an unknown, i.e., a random, time. Note 1: The conditions determining operability and committability must be specified. Note 2: Expressed mathematically, availability is 1 minus the unavailability. 2. The ratio of (a) the total time a functional unit is capable of being used during a given interval to (b) the length of the interval. Note 1: An example of availability is 100/168 if the unit is capable of being used for 100 hours in a week. Note 2: Typical availability objectives are specified in decimal fractions, such as 0.9998. 3. Timely, reliable access to data and information services for authorized users. [INFOSEC-99]

available line: 1. In voice, video, or data communications, a circuit between two points that is ready for service, but is in the idle state. 2. In facsimile transmission, the portion of the scanning line that can be specifically used for image signals. Synonym useful line.

available state: A state where a (bidirectional or unidirectional) service is usable. Note: Each direction of a service is assumed to be in the available state unless a transition to the unavailable state is observed without a subsequent transition to the available state. In this standard the transitions between the available and unavailable states are: (a) transition to the unavailable state occurs at the beginning of 10 consecutive severely errored seconds (SES); (b) transition to the available state occurs at the beginning of 10 consecutive seconds none of which is an SES. [T1.514-1995]

available time: From the point of view of a user, the time during which a functional unit can be used. Note: From the point of view of operating and maintenance personnel, the available time is the same as the uptime, i.e., the time during which a functional unit is fully operational.

avalanche multiplication: A current-multiplying phenomenon that occurs in a semiconductor photodiode that is reverse-biased just below its breakdown voltage. Note: Under such a condition, photocurrent carriers, i.e. , electrons, are swept across the junction with sufficient energy to ionize additional bonds, creating additional electron-hole pairs in a regenerative action. [After FAA]

avalanche photodiode (APD): A photodiode that operates with a reverse-bias voltage that causes the primary photocurrent to undergo amplification by cumulative multiplication of charge carriers. Note: As the reverse-bias voltage increases toward the breakdown, hole-electron pairs are created by absorbed photons. An avalanche effect occurs when the hole-electron pairs acquire sufficient energy to create additional pairs when the incident photons collide with the ions, i.e., the holes and electrons. Thus, a signal gain is achieved. [After 2196]

avatar: An interactive representation of a human in a virtual reality environment. [Bahorsky]

average picture level (APL): In video systems, the average level of the picture signal during active scanning time integrated over a frame period; defined as a percentage of the range between blanking and reference white level.

average rate of transmission: Synonym effective transmission rate.

AVI (.avi): Abbreviation for audio video interleaved. A file-name extension used to indicate a compressed video file in the AVI standard for a common operating system. Note: This file format for digital video and audio compression indicates that (a) the audio and video data are stored in alternate blocks, and (b) the file format is cross-platform compatible, allowing .avi video files to be played under various operating systems.

AVK: Abbreviation for audio video kernel. Digital video interface (.dvi) software designed for playing motion video and audio across several different varieties of hardware and operating systems. [After Silicon]

avoidance routing: The assignment of a circuit path to avoid certain critical or trouble-prone circuit nodes.

AVSS: Abbreviation for audio-video support system. A digital video interface system software (for DOS) that plays motion video and audio. [After Silicon]

AWG: Abbreviation for American wire gauge. A standard system for measuring and classifying the thickness of wire conductors; also referred to as the "Brown and Sharpe (B & S)" wire gauge.

AWGN: Abbreviation for additive white gaussian noise. See white noise.

axial propagation constant: In an optical fiber, the propagation constant evaluated along the optical axis of the fiber in the direction of transmission. Note: The real part of the axial propagation constant is the attenuation constant. The imaginary part is the phase constant. [After 2196]

axial ratio: Of an electromagnetic wave having elliptical polarization, the ratio of the magnitudes of the major axis and the minor axis of the ellipse described by the electric field vector.

axial ray: A light ray that travels along the optical axis.

b: Abbreviation for bit.

B: Abbreviation for bel, byte.

babble: In transmission systems, the aggregate of crosstalk induced in a given line by all other lines.

backbone: 1. The high-traffic-density connectivity portion of any communications network. 2. In packet-switched networks, a primary forward-direction path traced sequentially through two or more major relay or switching stations. Note: In packet-switched networks, a backbone consists primarily of switches and interswitch trunks.

back door: Synonym trapdoor.

background: 1. In Web pages, an image or color, usually defined in HTML, that serves as the underlying color or imagery for a Web page. Note: Browsers may also be configured to dictate backgrounds. 2. In computer display systems, the underlying color or imagery for the main screen or for the fundamental window of a computer's operating system.

background noise: The total system noise in the absence of information transmission.

background processing: The execution of lower priority computer programs when higher priority programs are not using the system resources. Note: Priorities may be assigned by system software, application software, or the operator.

backhaul: 1. The practice of routing telecommunications traffic beyond its intended destination, and then back to the intended destination, usually for the purpose of taking advantage of tariffs or prices that are lower than those afforded by direct routing. 2. In security, to bring a call that has been routed along a particular path (usually via satellite) to some earlier position in the switching sequence (also using satellites). Note: The technique supports CALEA, the Communications Assistance to Law Enforcement Act and LAES, Lawfully Authorized Electronic Surveillance. 3. In commercial telephone networks, referring to a trunk or line between two central offices that may not share the same facilities or geographic location, and which trunk is used to support special services, e.g., orderwires, off-premises extensions, common exchange number routing, or least-cost routing of customer centrex services. 4. In mission programs requiring contingency networking, referring to a link that provides feedback to the mission control center to verify that the transmission via a primary communications has been successful. Note: In cases where the primary path fails to deliver the transmission, the communications may occur over the backhaul connection or an alternative network connection. 5. In contingency networking, an alternative connection that is routed via a diversified path, e.g., an alternative frequency, satellite facility, cable, trunk, or time slot.

backscattering: 1. Radio wave propagation in which the direction of the incident and scattered waves, resolved along a reference direction (usually horizontal) are oppositely directed. A signal received by backscattering is often referred to as "backscatter." [JP 1-02] 2. In optics, the scattering of light into a direction generally opposite to the original one.

back-to-back connection: 1. A direct connection between the output of a transmitting device and the input of an associated receiving device. Note: When used for equipment measurements or testing purposes, such a back-to-back connection eliminates the effects of the transmission channel or medium. 2. A direct connection between the output of a receiving device and the input to a transmitting device. Note: The term "direct," as used in both definitions, may be construed as permitting a passive device such as a pad (attenuator) to accommodate power level constraints.

backup: 1. [A] copy of files and programs made to facilitate recovery, if necessary. [INFOSEC-99] 2. See backup file.

backup file: A copy of a file made for purposes of later reconstruction of the file, if necessary. Note: A backup file may be used for preserving the integrity of the original file and may be recorded on any suitable medium. Synonym job-recovery control file.

backward channel: 1. In data transmission, a secondary channel in which the direction of transmission is constrained to be opposite to that of the primary, i.e. , the forward (user-information) channel. Note: The direction of transmission in the backward channel is restricted by the control interchange circuit that controls the direction of transmission in the primary channel. 2. In a data circuit, the channel that passes data in a direction opposite to that of its associated forward channel. Note 1: The backward channel is usually used for transmission of supervisory, acknowledgement, or error-control signals. The direction of flow of these signals is opposite to that in which user information is being transferred. Note 2: The backward-channel bandwidth is usually less than that of the primary channel, i.e., the forward (user information) channel.

backward indicator bit (BIB): A signal unit (or sequence of bits) that (by changing its status) is used to request retransmission by the remote end whenever it receives a signal unit that is out of sequence. [T1.226-1992]

backward recovery: The reconstruction of an earlier version of a file by using a newer version of data recorded in a journal.

backward sequence number (BSN): A field in a signal unit that contains the forward sequence number of a correctly received signal unit being acknowledged in the signal unit that is being returned to the sender. [After T1.226-1992]

backward signal: A signal sent from the called to the calling station, i.e., from the original data sink to the original data source. Note: Backward signals are usually sent via a backward channel and may consist of supervisory, acknowledgment, or control signals.

backward supervision: The use of supervisory signal sequences from a secondary to a primary station.

bacterium: A program that attempts to propagate itself by e-mailing a copy of itself to e-mail addresses found on a recipient's hard drive. This is done without the consent or knowledge of the recipient. Note: In many cases, this type of program does not attempt to cause any direct damage to user's system, but rather attempts to overload the e-mail distribution system, much as a successful chain letter would do to the postal system. Synonym [in cryptosystems] chain letter.

bad sectoring: A technique for copy protection in which bad sectors are intentionally written on a diskette. [2382-pt.8]

balance: In electrical circuits and networks, to adjust the impedance to achieve specific objectives, such as to reach specified return loss objectives at a hybrid junction of two-wire and four-wire circuits.

balanced: Pertaining to electrical symmetry.

balanced code: 1. In PCM systems, a code constructed so that the frequency spectrum resulting from the transmission of any code word has no dc component. 2. In PCM, a code that has a finite digital sum variation.

balanced line: A transmission line consisting of two conductors in the presence of ground, capable of being operated in such a way that when the voltages of the two conductors at all transverse planes are equal in magnitude and opposite in polarity with respect to ground, the currents in the two conductors are equal in magnitude and opposite in direction. Note: A balanced line may be operated in an unbalanced condition. Synonym balanced signal pair.

balanced modulator: A modulator constructed so that the carrier is suppressed and any associated carrier noise is balanced out. Note 1: The balanced modulator output contains only the sidebands. Note 2: Balanced modulators are used in AM transmission systems.

balanced signal pair: Synonym balanced line.

balance return loss: 1. A measure of the degree of balance between two impedances connected to two conjugate sides of a hybrid set, coil, network, or junction. 2. A measure of the effectiveness with which a balancing network simulates the impedance of a two-wire circuit at a hybrid coil.

balancing network: 1. In a hybrid set, hybrid coil, or resistance hybrid, a circuit used to match, i.e. , to balance, the impedance of a uniform transmission line, i.e., twisted metallic pair, over a selected range of frequencies. Note: A balancing network is required to ensure isolation between the two ports of the four-wire side of the hybrid. 2. A device used between a balanced device or line and an unbalanced device or line for the purpose of transforming from balanced to unbalanced or from unbalanced to balanced.

balun: Abbreviation for balanced to unbalanced. In radio frequency usage, a device used to couple a balanced device or line to an unbalanced device or line.

band: 1. In communications, the frequency spectrum between two defined limits. 2. A group of tracks on a magnetic drum or on one side of a magnetic disk. 3. A set of frequencies authorized for use in a geographical area defined for common carriers for purposes of communications system management.

band-elimination filter: Synonym band-stop filter.

bandpass filter: A filter that ideally passes all frequencies between two non-zero finite limits and bars all frequencies not within the limits. Note: The cutoff frequencies are usually taken to be the 3-dB points.

bandpass limiter: A device that imposes hard limiting on a signal and contains a filter that suppresses the unwanted products (harmonics) of the limiting process.

band-rejection filter: Synonym band-stop filter.

band-stop filter: A filter that attenuates, usually to very low levels, all frequencies between two non-zero, finite limits and passes all frequencies not within the limits. Note: A band-stop filter may be designed to stop the specified band of frequencies but usually only attenuates them below some specified level. Synonyms band-elimination filter, band-rejection filter, band-suppression filter, notched filter.

band-suppression filter: Synonym band-stop filter.

bandwidth (BW): 1. The difference between the limiting frequencies within which performance of a device, in respect to some characteristic, falls within specified limits. 2. The difference between the limiting frequencies of a continuous frequency band.

bandwidth balancing mechanism: In a distributed-queue dual-bus network, a procedure in which a node occasionally skips the use of empty queued arbitrated slots, and which procedure effects sharing of the bandwidth mechanisms.

bandwidth compression: 1. The reduction of the bandwidth needed to transmit a given amount of data in a given time. 2. The reduction of the time needed to transmit a given amount of data in a given bandwidth. Note: Bandwidth compression implies a reduction in normal bandwidth of an information-carrying signal without reducing the information content of the signal.

bandwidth•distance product: Of an optical fiber, under specified launching and cabling conditions, at a specified wavelength, a figure of merit equal to the product of the fiber's length and the 3-dB bandwidth of the optical signal. Note 1: The bandwidth•distance product is usually stated in megahertz•kilometer (MHz•km) or gigahertz•kilometer (GHz•km). Note 2: The bandwidth•distance product, which is normalized to 1 km, is a useful figure of merit for predicting the effective fiber bandwidth for other lengths, and for concatenated fibers. Synonym bandwidth•length product.

bandwidth•length product: Synonym for bandwidth•distance product.

bandwidth-limited operation: The condition prevailing when the system bandwidth limits performance. Note: Bandwidth-limited operation occurs when the system distorts the signal waveform beyond specified limits. For linear systems, bandwidth-limited operation is equivalent to distortion-limited operation.

bandwidth (of an optical fiber): 1. The lowest modulation frequency at which the RMS peak-to-valley amplitude (optical power) difference of an intensity-modulated monochromatic signal decreases, at the output of the fiber, to a specified fraction (usually one-half) of the RMS peak-to-valley amplitude (optical power) difference of a nearly-zero (arbitrarily low) modulation frequency, both modulation frequencies having the same RMS peak-to-valley amplitude (optical power) difference at the fiber input. Note 1: In multimode fibers, multimode distortion is usually the most significant parameter limiting fiber bandwidth, although material dispersion may also play a significant role, especially in the first (850-nm) window. Note 2: In multimode fibers, the bandwidthdistance product (loosely, "fiber bandwidth" ) is customarily specified by vendors for the bandwidth as limited by multimode distortion only. The spectral width of the optical source is assumed to be extremely narrow. In practice, the effective fiber bandwidth will also be limited by dispersion, especially in the first (850-nm) window, where material dispersion is relatively high, because optical sources have a finite spectral width. Laser diodes typically have a spectral width of several nanometers, FWHM. LEDs typically have a spectral width of 35 to 100 nm, FWHM. Note 3: The effective risetime of multimode fibers may be estimated fairly accurately as the square root of the sum of the squares of the material-dispersion-limited risetime and the multimode-distortion-limited risetime. Note 4: In single-mode fibers, the most important parameters affecting fiber bandwidth are material dispersion and waveguide dispersion. Practical fibers are designed so that material dispersion and waveguide dispersion cancel one another at the wavelength of interest. Note 5: Regarding effective fiber bandwidth as it affects overall system performance, it should be recognized that optical detectors such as PIN diodes are square-law devices. Their photocurrent is proportional to the optical power of the detected signal. Because electrical power is a function of the square of the current, when the optical power decreases by one-half (a 3-dB decrease), the electrical power decreases by three-fourths (a 6-dB decrease). 2. Loosely, synonym bandwidthdistance product.

bang: The exclamation point character (!) used in early addressing schemes to separate the names of individual machines. Note: Bang-style addressing is no longer common. Synonym scream.

banner: 1. An HTML tag that creates scrolling text on a Web page. [Bahorsky] 2. Scrolling or fixed text used as an advertisement on a Web page, e.g., banner advertising. [Bahorsky] 3. In computer networking, a page that is automatically prefixed to, and printed out as part of, a printing job , as a means of identifying uniquely the output belonging to a given user of a shared printer. Note: A banner may, in addition to identifying the owner of the printout, also identify the file name, queue , and server involved. Synonym banner page. 4. [A] display on an information system (IS) that sets parameters for system or data use. [INFOSEC-99]

banner page: Synonym banner.

bar code: A code representing characters by sets of parallel bars of varying thickness and separation that are read optically by transverse scanning. Note: Bar code uses include identifying merchandise, sorting mail, and inventorying supplies.

barrage jamming: Jamming accomplished by transmitting a band of frequencies that is large with respect to the bandwidth of a single emitter. Note: Barrage jamming may be accomplished by presetting multiple jammers on adjacent frequencies or by using a single wideband transmitter. Barrage jamming makes it possible to jam emitters on different frequencies simultaneously and reduces the need for operator assistance or complex control equipment. These advantages are gained at the expense of reduced jamming power at any given frequency.

base: 1. In the numeration system commonly used in scientific notation, the real number that is raised to a power denoted by the exponent and then multiplied by the coefficient to determine the value of the number represented without the use of exponents. Note: An example of a base is the number 6.25 in the expression 2.70 × 6.25^1.5 42.19. The 2.70 is the coefficient and the 1.5 is the exponent. In the decimal numeration system, the base is 10 and in the binary numeration system, the base is 2. The value e 2.718 is the natural base. 2. A reference value. 3. A number that is multiplied by itself as many times as indicated by an exponent.

base address: 1. An address that is used as the origin in the calculation of addresses in the execution of a computer program. [From Weik '96] 2. A given address from which an absolute address is derived by combination with a relative address. Note: Base addresses are primarily used by computer programmers rather than by computer users. [From Weik '96]

baseband: 1. The original band of frequencies produced by a transducer, such as a microphone, telegraph key, or other signal-initiating device, prior to initial modulation. Note 1: In transmission systems, the baseband signal is usually used to modulate a carrier. Note 2: Demodulation re-creates the baseband signal. Note 3: Baseband describes the signal state prior to modulation, prior to multiplexing, following demultiplexing, and following demodulation. Note 4: Baseband frequencies are usually characterized by being much lower in frequency than the frequencies that result when the baseband signal is used to modulate a carrier or subcarrier. 2. In facsimile, the frequency of a signal equal in bandwidth to that between zero frequency and maximum keying frequency.

baseband local area network: A local area network in which information is encoded, multiplexed, and transmitted without modulation of carriers.

baseband modulation: Intensity modulation of an optical source, e.g. , LED or ILD, directly, without first modulating the signal of interest onto an electrical carrier wave. [After FAA]

baseband signaling: Transmission of a digital or analog signal at its original frequencies; i.e. , a signal in its original form, not changed by modulation.

basecom: Abbreviation for base communications.

base communications (basecom): Communications services, such as the installation, operation, maintenance, augmentation, modification, and rehabilitation of communications networks, systems, facilities, and equipment, including off-post extensions, provided for the operation of a military post, camp, installation, station, or activity. Synonym communications base station.

base Earth station: An Earth station in the fixed-satellite service or, in some cases, in the land mobile-satellite service, located at a specified fixed point or within a specified area on land to provide a feeder link for the land mobile-satellite service. [NTIA] [RR] [47CFR]

base key: A key used to derive (to compute cryptographically) or to decrypt transaction keys. [After X9.24]

base station: 1. A land station in the land mobile service. [NTIA] [RR] [47CFR] 2. In personal communication service, the common name for all the radio equipment located at one fixed location, and that is used for serving one or several cells.

basic access: A characterization of a simple standardized combination of access channels that constitute the access arrangements for the majority of ISDN users; specifically, any of the following combinations of access channels: (1) one D-channel, (2) one B-channel plus one D-channel, (3) two B-channels plus one D-channel. [After T1.601-1988]

basic call: A call between two users that does not include additional features (e.g., a plain telephone call).

basic call process (BCP): The sequence of activities used in processing a basic call attempt. [T1.667-1999]

basic exchange telecommunications radio service (BETRS): A commercial service that can extend telephone service to rural areas by replacing the local loop with radio communications. Note: In the BETRS, non-government ultra high frequency (UHF) and very high frequency (VHF) common carrier and the private radio service frequencies are shared.

basic group: See group.

basic mode link control: Control of data links by use of the control characters of the 7-bit character set for information processing interchange as given in ISO Standard 646-1983 and CCITT (ITU-T) Recommendation V.3-1972.

basic rate interface (BRI): An ITU-T Integrated Services Digital Network (ISDN) multipurpose user interface standard that denotes the capability of simultaneous voice and data services provided over two clear 64-kb/s channels and one clear 16-kb/s channel (2B+D) access arrangement to each user location.

basic service: 1. A pure transmission capability over a communication path that is virtually transparent in terms of its interaction with customer-supplied information. 2. The offering of transmission capacity between two or more points suitable for a user's transmission needs and subject only to the technical parameters of fidelity and distortion criteria, or other conditioning.

basic service element (BSE): 1. An optional unbundled feature, generally associated with the basic serving arrangement (BSA), that an enhanced-service provider (ESP) may require or find useful in configuring an enhanced service. 2. A fundamental (basic) communication network service; an optional network capability associated with a BSA. Note: BSEs constitute optional capabilities to which the customer may subscribe or decline to subscribe.

basic serving arrangement (BSA): 1. The fundamental tariffed switching and transmission (and other) services that an operating company must provide to an enhanced service provider (ESP) to connect with its customers through the company network. 2. In an open-network-architecture context, the fundamental underlying connection of an enhanced service provider (ESP) to and through the operating company's network including an ESP access link, the features and functions associated with that access link at the central office serving the ESP and/or other offices, and the transport (dedicated or switched) within the network that completes the connection from the ESP to the central office serving its customers or to capabilities associated with the customer's complementary network services. Note: Each component may have a number of categories of network characteristics. Within these categories of network characteristics are alternatives from among which the customer must choose. Examples of BSA components are ESP access link, transport and/or usage.

basic status: In data transmission, the status of the capability of a secondary station to send or receive a frame containing an information field.

bastion host: A host computer that, in a screened subnetwork, performs the functions of a firewall. Synonym screened-host gateway. [2382-pt.35]

batched communications: Synonym batched transmission.

batched transmission: The transmission of two or more messages from one station to another without intervening responses from the receiving station. Synonym batched communications.

batch processing: 1. The processing of data or the accomplishment of jobs accumulated in advance in such a manner that the user cannot further influence the processing while it is in progress. 2. The processing of data accumulated over a period of time. 3. Loosely, the execution of computer programs serially. 4. Pertaining to the technique of executing a set of computer programs such that each is completed before the next program of the set is started. 5. Pertaining to the sequential input of computer programs or data.

baud (Bd): 1. A unit of modulation rate. Note: One baud corresponds to a rate of one unit interval per second, where the modulation rate is expressed as the reciprocal of the duration in seconds of the shortest unit interval. 2. A unit of signaling speed equal to the number of discrete signal conditions, variations, or events per second. Note 1: If the duration of the unit interval is 20 milliseconds, the signaling speed is 50 bauds. If the signal transmitted during each unit interval can take on any one of n discrete states, the bit rate is equal to the rate in bauds times log₂n . The technique used to encode the allowable signal states may be any combination of amplitude, frequency, or phase modulation, but it cannot use a further time-division multiplexing technique to subdivide the unit intervals into multiple subintervals. In some signaling systems, non-information-carrying signals may be inserted to facilitate synchronization; e.g., in certain forms of binary modulation coding, there is a forced inversion of the signal state at the center of the bit interval. In these cases, the synchronization signals are included in the calculation of the rate in bauds but not in the computation of bit rate. Note 2: Baud is sometimes used as a synonym for bit-per-second . This usage is deprecated.

Baudot code: A synchronous code in which five equal-length bits represent one character. Note 1: The Baudot code, which was developed circa 1880, has been replaced by the start-stop asynchronous International Alphabet No. 2 (IA No. 2). Note 2: IA No. 2 is not, and should not be identified as, the Baudot code. Note 3: The Baudot code has been widely used in teletypewriter systems.

BCC: Abbreviation for block check character.

BCD: Abbreviation for binary-coded decimal code. See binary-coded decimal notation.

B channel: 1. A communications channel used for the transmission of an aggregate signal generated by multichannel transmitting equipment. 2. The CCITT (now, ITU-T) designation for a clear channel, 64-kb/s service capability provided to a subscriber under the Integrated Services Digital Network offering. Note: The B channel, also called the bearer channel, is intended for transport of user information, as opposed to signaling information.

BCH code: Abbreviation for Bose-Chaudhuri-Hochquenghem code. A multilevel, cyclic, error-correcting, variable-length digital code used to correct errors up to approximately 25% of the total number of digits. Note: BCH codes are not limited to binary codes, but may be used with multilevel phase-shift keying whenever the number of levels is a prime number or a power of a prime number, such as 2, 3, 4, 5, 7, 8, 11, and 13. A BCH code in 11 levels has been used to represent the 10 decimal digits plus a sign digit.

BCI: Abbreviation for bit-count integrity.

Bd: Abbreviation for baud.

beacon: See radiobeacon station.

beam: 1. The main lobe of an antenna radiation pattern. 2. A column of light. Note: A beam may be parallel, divergent, or convergent. [After FAA]

beam diameter: Of an electromagnetic beam, along any specified line that (a) intersects the beam axis and (b) lies in any specified plane normal to the beam axis, the distance between the two diametrically opposite points at which the irradiance is a specified fraction, e.g., ½ or 1/, of the beam's peak irradiance. Note 1: Beam diameter is usually used to characterize electromagnetic beams in the optical regime, and occasionally in the microwave regime, i.e., cases in which the aperture from which the beam emerges is very large with respect to the wavelength. Note 2: Beam diameter usually refers to a beam of circular cross section, but not necessarily so. A beam may, for example, have an elliptical cross section, in which case the orientation of the beam diameter must be specified, e.g., with respect to the major or minor axis of the elliptical cross section.

beam divergence: Of an electromagnetic beam, in any plane that intersects the beam axis, the increase in beam diameter with distance from the aperture from which the beam emerges. Note 1: Beam divergence is usually used to characterize electromagnetic beams in the optical regime, i.e., cases in which the aperture from which the beam emerges is very large with respect to the wavelength. Note 2: Beam divergence usually refers to a beam of circular cross section, but not necessarily so. A beam may, for example, have an elliptical cross section, in which case the orientation of the beam divergence must be specified, e.g., with respect to the major or minor axis of the elliptical cross section.

beamsplitter: A device for dividing an optical beam into two or more separate beams. Note: An example of a beamsplitter is a partially reflecting mirror.

beam steering: Changing the direction of the main lobe of a radiation pattern. Note: In radio systems, beam steering may be accomplished by switching antenna elements or by changing the relative phases of the rf signals driving the elements. In optical systems, beam steering may be accomplished by changing the refractive index of the medium through which the beam is transmitted or by the use of mirrors or lenses.

beamwidth: 1. In the radio regime, of an antenna pattern, the angle between the half-power (3-dB) points of the main lobe, when referenced to the peak effective radiated power of the main lobe. Note: Beamwidth is usually expressed in degrees. It is usually expressed for the horizontal plane, but may also be expressed for the vertical plane. 2. For the optical regime, see beam divergence.

bearer channel: See B channel.

bearer service: A telecommunications service that allows transmission of user-information signals between user-network interfaces. See B channel, service access.

beating: See heterodyne.

beeping: Synonym paging, radio paging.

Bell-La Padula security model: [A] formal state-transition model of a computer security policy that describes a formal set of access controls based on information sensitivity and subject authorizations. [INFOSEC]

benign: [In INFOSEC a] condition of cryptographic data that cannot be compromised by human access. [INFOSEC-99]

benign environment: [In security, a] nonhostile environment that may be protected from external hostile elements by physical, personnel, and procedural security countermeasures. [INFOSEC-99]

beta test: A secondary software product test involving select or voluntary participants before the product is marketed to the general public. Note: Participants agree to provide detailed feedback to the product's manufacturer in return for their being allowed to preview, use, and contribute to the final form of the product. [After Bahorsky]

B8ZS: Abbreviation for bipolar with eight-zero substitution. A T-carrier line code in which bipolar violations are deliberately inserted if user data contains a string of 8 or more consecutive zeros. Note 1: B8ZS is used to ensure a sufficient number of transitions to maintain system synchronization when the user data stream contains an insufficient number of "ones" to do so. Note 2: B8ZS is used in the European hierarchy at the T1 rate.

bel (B): A unit of measure of ratios of power levels, i.e., relative power levels. Note 1: The number of bels for a given ratio of power levels is calculated by taking the logarithm, to the base 10, of the ratio. Mathematically, the number of bels is calculated as B = log₁₀(P ₁/P ₂) where P ₁ and P ₂ are power levels. Note 2 : The dB, equal to 0.1 B, is a more commonly used unit.

bell (BEL) character: A transmission control character that is used when there is a need to call for user or operator attention in a communications system, and that usually activates an audio or visual alarm or other attention-getting device.

Bell Operating Company (BOC): Historically, any of the 22 operating companies that were divested from AT&T by court order. Note: Cincinnati Bell Telephone Co. and Southern New England Bell Telephone Co. were not included.

bend loss: See macrobend loss, microbend loss.

BER: Abbreviation for bit error ratio.

BERT: Acronym for bit error ratio tester.

BETRS: Abbreviation for basic exchange telecommunications radio service.

between-the-lines entry: Unauthorized access to a momentarily inactive terminal, of a legitimate user, assigned to a communications channel.

BEX: Abbreviation for broadband exchange.

b frame: An MPEG video frame type that provides bidirectional interframe compression. Note: A b frame derives its content from the closest 1 or P frame, one in the past and one in the future. Generating b frames requires greater computing power than does generating 1 or P frames. The use of b frames enables compression ratios of 200:1. Robust MPEG encoders employ a combination of b, 1, and P frame encoding. [After Silicon]

bias: 1. A systematic deviation of a value from a reference value. 2. The amount by which the average of a set of values departs from a reference value. 3. Electrical, mechanical, magnetic, or other force (field) applied to a device to establish a reference level to operate the device. 4. In telegraph signaling systems, the development of a positive or negative dc voltage at a point on a line that should remain at a specified reference level, such as zero. Note: A bias may be applied or produced by (i) the electrical characteristics of the line, (ii) the terminal equipment, and (iii) the signaling scheme.

bias distortion: 1. Signal distortion resulting from a shift in the bias. 2. In binary signaling, distortion of the signal in which all the significant intervals have uniformly longer or shorter durations than their theoretical durations. Note: Bias distortion is expressed in percent of the system-specified unit interval.

biased: In the generation of pseudorandom or random numbers, pertaining to or describing a condition that renders the more likely occurrence of some numbers or patterns than others. [After X9.17]

biconical antenna: An antenna consisting of two conical conductors, having a common axis and vertex, and extending in opposite directions. Note 1: In a biconical antenna, excitation is applied at the common vertex. Note 2: If one of the cones is reduced to a plane, the antenna is called a discone.

bidirectional asymmetry: In data transmission, the condition that exists when information flow characteristics are different in each direction.

bidirectional connection: Association established between two higher layer entities for which information is transmitted between both entities. [After T1.629-1999]

bidirectional coupler: See directional coupler.

bidirectional symmetry: The condition that exists when information flow characteristics are the same in each direction.

big-endian: A format for transmission or storage of binary data in which the most significant byte (bit) appears first. [After X9.42]

BIH: French abbreviation for International Time Bureau. See International Atomic Time.

bilateral control: Synonym bilateral synchronization.

bilateral synchronization: A synchronization control system between exchanges A and B in which the clock at exchange A controls the data received at exchange B and the clock at exchange B controls the data received at exchange A. Note: Bilateral synchronization is usually implemented by deriving the timing from the incoming bit stream. Synonym bilateral control.

billboard antenna: An array of parallel dipole antennas with flat reflectors, usually positioned in a line or plane. Note 1: The spacing and dimensions of the dipoles depend on the wavelength. Note 2: The main lobe of a fixed billboard antenna may, within limits, be steered by appropriate phasing of the respective signals to individual elements of the array. Synonym broadside antenna.

binary: 1. Pertaining to a selection, choice, or condition that has two possible different values or states. 2. Pertaining to a fixed radix numeration system that has a radix of 2.

binary code: A code, the elements of which can assume either one of two possible states.

binary-coded decimal (BCD): Pertaining to the representation of a decimal digit by a unique arrangement of no fewer than four binary digits.

binary-coded decimal code: Synonym binary-coded decimal notation.

binary-coded decimal interchange code: See binary-coded decimal notation.

binary-coded decimal (BCD) notation: A binary notation in which each of the decimal digits is represented by a binary numeral. Synonyms binary-coded decimal code, binary-coded decimal representation.

binary-coded decimal representation: Synonym binary-coded decimal notation.

binary digit (bit): See bit.

binary element: A constituent element of data that takes either of two values or states. Note: Binary element should not be confused with binary digit.

binary exponential backoff: See truncated binary exponential backoff.

binary modulation: The process of varying a parameter of a carrier as a function of two finite, discrete states.