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.
