Telecom Dictionary S: Phone Service Definitions

Safeguarding Statement - [A] statement affixed to a computer output or printout that states the highest classification being processed at the time the product was produced and requires control of the product, at that level, until determination of the true classification by an authorized person. Synonym [in INFOSEC] banner.

Safety Service - Any radiocommunication service used permanently or temporarily for the safeguarding of human life and property.

Sampled Data - Data in which the information content exists only for instantaneous values measured or determined at or for discrete intervals. Note: Sampled data can be analog or digital.

Sampling - See signal sampling. The process of obtaining a sequence of instantaneous values of a particular signal characteristic, usually at regular time intervals.

Sampling Frequency - See sampling rate. The number of samples taken per unit time, i.e., the rate at which signals are sampled for subsequent use, such as for modulation, coding, and quantization.

Sampling Interval - The reciprocal of the sampling rate, i.e. , the interval between corresponding points on two successive sampling pulses of the sampling signal.

Sampling Rate - The number of samples taken per unit time, i.e., the rate at which signals are sampled for subsequent use, such as for modulation, coding, and quantization. Deprecated synonym sampling frequency.

Sampling Theorem - Synonym Nyquist's theorem. A theorem, developed by H. Nyquist, which states that an analog signal waveform may be uniquely reconstructed, without error, from samples taken at equal time intervals. The sampling rate must be equal to, or greater than, twice the highest frequency component in the analog signal.

Sanitize - [In INFOSEC, the] process to remove information from media such that data recovery is not possible. It includes removing all classified labels, markings, and activity logs.

Sanitizing - Of a recording medium, erasing or overwriting all data in such a manner that it cannot be recovered, even by extraordinary, e.g., laboratory, means. Note: Sanitizing is distinguished from simply "deleting" or "erasing" data. For example, a software-based deletion command may simply mark a file or block of data to the effect that it may be overwritten during a subsequent "save" of different data. Until such a save is performed, the deleted data may be recovered, sometimes by the simple expedient of executing a software command. Likewise, digital data, for example, may be "erased," from a magnetic recording medium, but still be recoverable if subjected to laboratory techniques that can identify residual magnetism left after the erasure.

SAP - Abbreviation for service access point. - 1. A physical point at which a circuit may be accessed. 2. In an Open Systems Interconnection (OSI) layer, a point at which a designated service may be obtained.

Satellite - A body which revolves around another body of preponderant mass and which has a motion primarily and permanently determined by the force of attraction of that other body. Note: A parent body and its satellite revolve about their common center of gravity.

Satellite Access - In satellite communications systems, the establishment of contact with a communications satellite space station. Note: An example of satellite access is access at the moment at which an Earth station commences to use a satellite space station as a signal repeater, i.e. , to use its transponder. Each radio frequency (rf) carrier that is relayed by a satellite space station at any time occupies an access channel. Accesses, i.e., channels, are distinguishable by various system parameters, such as frequency, time, or code.

Satellite Communications - A telecommunications service provided via one or more satellite relays and their associated uplinks and downlinks.

Satellite Digital Audio Radio Service (SDARS) - 1. A satellite-based direct-broadcast radio service in which digitally encoded audio entertainment material is broadcast to Earth-based receivers, either directly from an orbiting satellite, or--in cases in which the receiver is in a shielded location--from the satellite to the receiver via a repeater station. 2. A radiocommunication service in which audio programming is digitally transmitted by one or more space stations directly to fixed, mobile, and/or portable stations, and which may involve complementary repeating terrestrial transmitters, telemetry, tracking and control facilities. 3. A radiocommunication service in which compact-disc quality programming is digitally transmitted by one or more space stations.

Satellite Earth Terminal - Synonym Earth terminal. In a satellite link, one of the non-orbiting communications stations that receives, processes, and transmits signals between itself and a satellite. Note: Earth terminals may be at mobile, fixed, airborne, and waterborne Earth terminal complexes.

Satellite Emergency Position-Indicating Radiobeacon - An Earth station in the mobile-satellite service the emissions of which are intended to facilitate search and rescue operations.

Satellite Link - A radio link between a transmitting Earth station and a receiving Earth station through one satellite. A satellite link comprises one uplink and one downlink.

Satellite Network - A satellite system or a part of a satellite system, consisting of only one satellite and the cooperating Earth stations.

Satellite Operation - See satellite PBX. A PBX system that is not equipped with attendant positions, and is associated with an attended main PBX system. Note: The main attendant provides attendant functions for the satellite system.

Satellite PBX - See satellite operation. A PBX system that is not equipped with attendant positions, and is associated with an attended main PBX system. Note: The main attendant provides attendant functions for the satellite system.

Satellite Period - See period (of a satellite). The time elapsing between two consecutive passages of a satellite through a characteristic point on its orbit.

Satellite Relay - An active or passive satellite repeater that relays signals between two Earth terminals.

Satellite System - A space system using one or more artificial Earth satellites.

Saturation - 1. In a communications system, the condition in which a component of the system has reached its maximum traffic handling capacity. Note: Saturation is equivalent to one erlang per circuit. 2. The point at which the output of a linear device, such as a linear amplifier, deviates significantly from being a linear function of the input when the input signal is increased.Note: Modulation often requires that amplifiers operate below saturation.

Scalability - The degree to which video and image formats can be sized in systematic proportions for distribution over communications channels of varying capacities.

Scan - 1. To examine sequentially, part by part. 2. To examine every reference in every entry in a file routinely as part of a retrieval scheme. 3. In radar, one complete rotation of the interrogating antenna. 4. In SONAR, to search 360° or a specific search sector by the use of phased array of transducers. 5. To sweep, i.e., rotate, a beam about a point or about an axis.

Scan Line - The line produced on a recording medium frame by a single sweep of a scanner. In an imaging system, the path traversed by a scanning spot during a single line sweep.

Scanner - A device that examines a spatial pattern, one part after another, and generates analog or digital signals corresponding to the pattern. Note: Scanners are often used in mark sensing, pattern recognition, and character recognition.

Scanning - 1. In telecommunications systems, examination of traffic activity to determine whether further processing is required. Note: Scanning is usually performed periodically. 2. In television, facsimile, and picture transmission, the process of successively analyzing the colors and densities of the object according to a predetermined pattern. 3. The process of tuning a device through a predetermined range of frequencies in prescribed increments and at prescribed times. Note: Scanning may be performed at regular or random increments and intervals. 4. In radar and radio direction-finding, the slewing of an antenna or radiation pattern for the purpose of probing in a different direction. Note 1: In radar, scanning may be mechanical, using a rotary microwave joint to feed the antenna, or electronic, using a phased array of radiators, the radiated pattern (beam) of which depends on the relative phases of the signals fed to the individual radiators. Note 2: In civilian air traffic control radar, scanning usually implies continuous rotation of the antenna or beam about a vertical axis. In military radars, scanning may occur about other than a vertical axis, and may not encompass a full 360°.

Scanning Direction - In facsimile transmitting equipment, the scanning of an object, such as a message surface or the developed plane in the case of a drum, along parallel lines in a specified pattern. Note 1: The scanning direction is equivalent to scanning over a right-hand helix on a drum. Note 2: The orientation of the message on the scanning plane will depend upon its dimensions. Note 3: In facsimile receiving equipment, scanning from right to left and top to bottom, is called "positive" reception and from left to right and top to bottom, is called "negative" reception. Note 4: Scanning direction conventions are included in CCITT Recommendations for phototelegraphic equipment.

Scanning Field - In facsimile systems, the total of the areas that are actually explored by the scanning spot during the scanning of the object by the transmitter or during scanning of the record medium by the receiver.

Scanning Line - In an imaging system, the path traversed by a scanning spot during a single line sweep.

Scanning Line Frequency - In facsimile, the frequency at which a fixed line perpendicular to the direction of scanning is crossed by a scanning spot. Note: The scanning line frequency is equivalent to drum speed in some mechanical systems. Synonym scanning line rate.

Scanning Line Length - In facsimile systems, the total length of a scanning line, equal to the spot speed divided by the scanning line frequency. Note: The scanning line length is usually greater than the length of the available line.

Scanning Line Period - In facsimile systems, the time interval between (a) the instant at which the scanning spot probes or writes to a given spot on one scanning line, and (b) the instant at which the scanning spot probes or writes to the corresponding spot on the next scanning line.

Scanning Line Rate - Synonym scanning line frequency. In facsimile, the frequency at which a fixed line perpendicular to the direction of scanning is crossed by a scanning spot. Note: The scanning line frequency is equivalent to drum speed in some mechanical systems.

Scanning Pitch - The distance between the centers of consecutive scanning lines.

Scanning Rate - In facsimile and television systems, the rate of displacement of the scanning spot along the scanning line.

Scanning Spot - In facsimile systems, the area on the object, i.e., the original, covered instantaneously by the pickup system of the scanner.

Scan-Stop Lockup - In automatic link establishment (ALE) radios, the undesired condition in which the normal process of (a) scanning radio channels, (b) stopping on the desired channel, or (c) returning to scan is terminated by the equipment.

Scattering - Of a wave propagating in a material medium, a phenomenon in which the direction, frequency, or polarization of the wave is changed when the wave encounters discontinuities in the medium, or interacts with the material at the atomic or molecular level. Note: Scattering results in a disordered or random change in the incident energy distribution.

Scattering Center - In the microstructure of a transmission medium, a site at which electromagnetic waves are scattered. Note 1: Examples of scattering centers are vacancy defects; interstitial defects; inclusions, such as a gas molecules, hydroxide ions, iron ions, and trapped water molecules; and microcracks or fractures in dielectric waveguides. Note 2: Scattering centers are frozen in the medium when it solidifies and may not necessarily cause Rayleigh scattering, which varies inversely as the fourth power of the wavelength. For example, in glass optical fibers, there is a high attenuation band at 0.95 m, primarily caused by scattering and absorption by OH (hydroxyl) ions.

Scattering Coefficient - The factor that expresses the attenuation caused by scattering, e.g. , of radiant or acoustic energy, during its passage through a medium. Note: The scattering coefficient is usually expressed in units of reciprocal distance.

Scattering Cross Section - The area of an incident wavefront, at a reflecting surface or medium, such as an object in space, through which will pass radiant energy, that, if isotropically scattered from that point, would produce the same power at a given receiver as is actually provided by the entire reflecting surface.

Scattering Loss - The part of the transmission (power) loss that results from scattering within a transmission medium or from roughness of a reflecting surface.

SCC - Abbreviation for specialized common carrier. A common carrier offering a limited type of service or serving a limited market.

Scene Cut - Video imagery in which consecutive frames are highly uncorrelated.

Scene Cut Response - In video systems, the perceived impairments associated with a scene cut.

Schematic - 1. A diagram, drawing, or sketch that details the elements of a system, such as the elements of an electrical circuit or the elements of a logic diagram for a computer or communications system. 2. Pertaining to a diagram, drawing, or sketch that details the elements of a system, such as the elements of an electrical circuit or the elements of a logic diagram for a computer or communications system.

Scintillation - In electromagnetic wave propagation, a small random fluctuation of the received field strength about its mean value. Note: Scintillation effects become more significant as the frequency of the propagating wave increases.

Scrambler - A device that transposes or inverts signals or otherwise encodes a message at the transmitter to make the message unintelligible at a receiver not equipped with an appropriately set descrambling device. Note: Scramblers usually use a fixed algorithm or mechanism. However, a scrambler provides communications privacy that is inadequate for classified traffic.

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

Screen - 1. In a telecommunications, computing, or data processing system, to examine entities that are being processed to determine their suitability for further processing. 2. A nonferrous metallic mesh used to provide electromagnetic shielding. 3. To reduce undesired electromagnetic signals and noise by enclosing devices in electrostatic or electromagnetic shields. 4. A viewing surface, such as that of a cathode ray tube or liquid crystal display (LCD).

Screen Capture - In computers, the process or act by which the data currently displayed on a monitor, usually representing a single frame of information, are stored or processed in a graphical format. Note: A screen capture thus represents an instantaneous "snapshot" of the state of the display.

Screened-Host Gateway - Synonym bastion host. A host computer that, in a screened subnetwork, performs the functions of a firewall.

Screened Subnetwork - A subnetwork that permits access from both untrusted external networks and from trusted internal networks, but does not permit traffic flow between the two.

Screening Router - A network router capable of discriminating network-based traffic on the protocol type and the value of the protocol fields within the packet.

Script - A relatively short computer program that performs one specific task. Note: A script is normally used to automate complex or advanced features or procedures within a system. Scripts are commonly used to process user information from Web pages (for example, a search engine is a script).

Scripting Language - A programming language supported by and specific to a particular program. Note: A scripting program is normally used to automate complex or advanced features or procedures within the program.

Scroll - In a display device, to move the display window of the screen vertically to view the contents of a stored document. Note: Scrolling may be performed continuously or incrementally.

Scroll-Mode Terminal - A terminal in which the data are displayed (or pass by) a line at a time.

SCSI - Acronym for small computer system interface. An intelligent interface device that expands a microprocessor (CPU) bus to facilitate connections to multiple peripherals (e.g., CD-ROM drives, hard drives, or scanners) and exchange data with those peripherals via a separate communications bus. Note 1: The original SCSI was capable of supporting up to 7 devices at a data rate of 5 Mb/s over an 8-bit parallel bus. Subsequent SCSI versions feature a parallel communications bus having greater width and speed. Note 2: Since SCSI exchanges data with the peripherals over a separate communications bus rather than the processor bus, the CPU can devote the saved processing time to other tasks. Note 3: Pronounced "scuzzy."

SDARS ( Satellite Digital Audio Radio Service) - 1. A satellite-based direct-broadcast radio service in which digitally encoded audio entertainment material is broadcast to Earth-based receivers, either directly from an orbiting satellite, or--in cases in which the receiver is in a shielded location--from the satellite to the receiver via a repeater station. 2. A radiocommunication service in which audio programming is digitally transmitted by one or more space stations directly to fixed, mobile, and/or portable stations, and which may involve complementary repeating terrestrial transmitters, telemetry, tracking and control facilities. 3. A radiocommunication service in which compact-disc quality programming is digitally transmitted by one or more space stations.

SDLC - Abbreviation for synchronous data link control. In a data network, a bit-oriented protocol for the control of synchronous transmission over data links.

Search Engine - A specialized program that facilitates information retrieval from large segments of the Internet. Note 1: Search engines attempt to help a user locate desired information or resources by seeking matches to user-specified key words. The usual method for finding and isolating this information is to compile and maintain an index of Web resources that can be queried for the key words or concepts entered by the user. The indices are often built from specific resource lists, and may also be created from the output of Web crawlers, wanderers, robots, spiders, or worms. The indices are usually compiled during times of minimum network traffic. Note 2: Different engines are appropriate for different kinds of searches, and most can be optimized for specified results.

Search Time - In data processing systems, the time interval required to locate a particular data element, record, or file in a storage device.

Search Vehicles - Sources that offer a variety of methods to obtain information on the WWW (e.g. catalogs, directories, search engines, spiders).

SECAM - Acronym for système electronique couleur avec memoire. A television signal standard (625 lines, 50 Hz, 220 V primary power) used in France, eastern European countries, the former USSR, and some African countries.

Second (s) - In the International System of Units (SI), the time interval equal to 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.

Secondary Channel - In a system in which two channels share a common interface, a channel that has a lower data signaling rate (DSR) capacity than the primary channel.

Secondary Emission - Particles or radiation, such as photons, Compton recoil electrons, delta rays, secondary cosmic rays, and secondary electrons, that are produced by the action of primary radiation on matter.

Secondary Frequency Standard - A frequency standard that does not have inherent accuracy, and therefore must be calibrated against a primary frequency standard. Note: Secondary standards include crystal oscillators and rubidium standards. A crystal oscillator depends for its frequency on its physical dimensions, which vary with fabrication and environmental conditions. A rubidium standard is a secondary standard even though it uses atomic transitions, because it takes the form of a gas cell through which an optical signal is passed. The gas cell has inherent inaccuracies because of gas pressure variations, including those induced by temperature variations. There are also variations in the concentrations of the required buffer gases, which variations cause frequency deviations.

Secondary Radar - A radiodetermination system based on the comparison of reference signals with radio signals retransmitted from the position to be determined. Note: An example of secondary radar is the transponder-based surveillance of aircraft. Synonym secondary surveillance radar.

Secondary Radiation - See secondary emission. Particles or radiation, such as photons, Compton recoil electrons, delta rays, secondary cosmic rays, and secondary electrons, that are produced by the action of primary radiation on matter.

Secondary Service Area - The service area of a broadcast station served by the skywave and not subject to objectionable interference and in which the signal is subject to intermittent variations in strength.

Secondary Station - In a communications network, a station that (a) is responsible for performing unbalanced link-level operations as instructed by the primary station and (b) interprets received commands and generates responses.

Secondary Surveillance Radar - Synonym secondary radar. A radiodetermination system based on the comparison of reference signals with radio signals retransmitted from the position to be determined. Note: An example of secondary radar is the transponder-based surveillance of aircraft.

Secondary Time Standard - A time standard that requires periodic calibration against a primary time standard.

Second Dialtone - 1. Dialtone presented to the call originator after an access code has been dialed for access to a second, outside, telecommunications system or service. 2. Dialtone returned to the call originator after she/he has dialed an access number and has reached a switch providing access to modem, to a fax machine, to another telephone, etc.

Second Window - Of silica-based optical fibers, the transmission window at approximately 1.3

m. Note: The second window is the minimum-dispersion window in silica-based glasses.

SECOR - : Acronym for secure voice cord board . A desk-mounted patch panel that provides the capability for controlling (a) sixteen 50-kb/s wideband or sixteen 2400-b/s narrowband user lines and (b) 5 narrowband trunks to DSN or other narrowband facilities.

SECTEL - Acronym for secure telephone. See STU. A U.S. Government-approved telecommunications terminal that protects the transmission of sensitive or classified information in voice, data, and facsimile systems.

Sectoring - In magnetic or optical disk storage media, the division of tracks into a specified number of segments, for the purpose of organizing the data stored thereon.

Secure Communications - Telecommunications deriving security through use of type 1 products and/or protected distribution systems.

Secure Internet Protocols - A set of security rules or standards that determines how computers talk to each other on a network.

Secure Telephone Unit - See STU. A U.S. Government-approved telecommunications terminal that protects the transmission of sensitive or classified information in voice, data, and facsimile systems.

Secure Transmission - 1. In transmission security, see secure communications . 2. In spread-spectrum systems, the transmission of binary coded sequences that represent information that can be recovered only by persons or systems that have the proper key for the spread-spectrum code-sequence generator, i.e., have a synchronized generator that is identical to that used for transmission.

Secure Voice Cord Board - See SECORD. A desk-mounted patch panel that provides the capability for controlling (a) sixteen 50-kb/s wideband or sixteen 2400-b/s narrowband user lines and (b) 5 narrowband trunks to DSN or other narrowband facilities.

Security - 1. A condition that results from the establishment and maintenance of protective measures that ensure a state of inviolability from hostile acts or influences. 2. With respect to classified matter, the condition that prevents unauthorized persons from having access to official information that is safeguarded in the interests of national security. 3. Measures taken by a military unit, an activity or installation to protect itself against all acts designed to, or which may, impair its effectiveness.

Security Filter - 1. In communications security, the hardware, firmware, or software used to prevent access to specified data by unauthorized persons or systems, such as by preventing transmission, preventing forwarding messages over unprotected lines or circuits, or requiring special codes for access to read-only files. 2. [An] AIS trusted subsystem that enforces security policy on the data that passes through it.

Security Kernel - 1. In computer and communications security, the central part of a computer or communications system hardware, firmware, and software that implements the basic security procedures for controlling access to system resources. 2. A self-contained usually small collection of key security-related statements that (a) works as a part of an operating system to prevent unauthorized access to, or use of, the system and (b) contains criteria that must be met before specified programs can be accessed. 3. Hardware, firmware, and software elements of a trusted computing base that implement the reference monitor concept.

Security Management - In network management, the set of functions (a) that protects telecommunications networks and systems from unauthorized access by persons, acts, or influences and (b) that includes many subfunctions, such as creating, deleting, and controlling security services and mechanisms; distributing security-relevant information; reporting security-relevant events; controlling the distribution of cryptographic keying material; and authorizing subscriber access, rights, and privileges.

Seek - To position selectively the access mechanism of a direct access [storage] device.

Seek Time - The time required for the access arm of a direct-access storage device to be positioned on the appropriate track. Synonym positioning time.

Segment - In a distributed queue dual bus (DQDB) network, a protocol data unit (PDU) that (a) consists of 52 octets transferred between DQDB-layer peer entities as the information payload of a slot, (b) contains a header of 4 octets and a payload of 48 octets, and (c) is either a pre-arbitrated segment or a queued arbitrated segment.

Segmented Encoding Law - An encoding law in which an approximation to a curve defined by a smooth encoding law is obtained by a number of linear segments. Synonym piecewise linear encoding law.

Segregation - In a communications system or network, the protection given to information to conceal it from persons having access to the system or network.

Seizing - The temporary dedication of various parts of a communications system to a specific use, usually in response to a user request for service. Note: The parts seized may be automatically connected, such as by direct distance dialing (DDD), or may require operator intervention.

Seizure Signal - In telephone systems, a signal used by the calling end of a trunk or line to indicate a request for service. Note: A seizure signal also locks out the trunk or line to other demands for service.

Selcall - Acronym for selective calling. Calling from one station in which call identification is sent to signal automatically one or more remote stations and to establish links among them. Note 1: Selective calling may be used to un-mute the speakers at designated stations or to initiate a handshake for link establishment. Note 2: Selective calling is specified in CCIR Recommendations for HF and VHF/UHF radio, generally for ship-to-shore, ship-to-ship, aircraft-to-aircraft, and aircraft-to-ground communications.

Selection Position - Synonym decision instant. In the reception of a digital signal, the instant at which a decision is made by a receiving device as to the probable value of a signal condition.

Selective Calling - See selcall. Calling from one station in which call identification is sent to signal automatically one or more remote stations and to establish links among them. Note 1: Selective calling may be used to un-mute the speakers at designated stations or to initiate a handshake for link establishment. Note 2: Selective calling is specified in CCIR Recommendations for HF and VHF/UHF radio, generally for ship-to-shore, ship-to-ship, aircraft-to-aircraft, and aircraft-to-ground communications.

Selective Fading - Fading in which the components of the received radio signal fluctuate independently.

Selective Jamming - See electronic warfare. Any military action involving the use of electromagnetic and directed energy to control the electromagnetic spectrum or to attack the enemy. The three major subdivisions within electronic warfare are: electronic attack, electronic protection, and electronic warfare support.

Selective Ringing - In a party line, ringing only the desired user instrument. Note: Without selective ringing, all the instruments on the party line will ring at the same time, selection being made by the number of rings.

Selectivity - A measure of the ability of a receiver to discriminate between a wanted signal on one frequency and unwanted signals on other frequencies.

Self-Authentication - 1. A procedure in which a transmitting station, i.e., a calling station, establishes its own validity without the participation of the receiving station, i.e., the called station. Note: The calling station establishes its own authenticity and the called station is not required to challenge the calling station. Self-authentication is usually used only when one-time authentication systems are used to derive the authentication. 2. Implicit authentication, to a predetermined level, of all transmissions on a secure communications system.

Self-Delineating Block - A block in which a bit pattern or a flag identifies the beginning or end of a block.

Self-Synchronizing Code - A code in which the symbol stream formed by a portion of one code word, or by the overlapped portion of any two adjacent code words, is not a valid code word. Note 1: A self-synchronizing code permits the proper framing of transmitted code words provided that no uncorrected errors occur in the symbol stream. Note 2: External synchronization is not required. Note 3: High-level data link control (HDLC) and Advanced Data Communication Control Procedures (ADCCP) frames represent self-synchronizing code words.

Semiautomated Tactical Command and Control System - A machine-aided command and control system in which human intervention is required in varying degrees to operate the system.

Semiautomatic Switching System - 1. In telephone systems, a switching system in which telephone operators receive call instructions orally from users and complete them by automatic equipment. 2. At tape-relay intermediate stations, the manual routing or rerouting of taped messages without rekeying them.

Semiconductor Laser - Synonym injection laser diode. A laser that uses a forward-biased semiconductor junction as the active medium. Note: Stimulated emission of coherent light occurs at a p-n junction where electrons and holes are driven into the junction. Synonyms diode laser, laser diode.

Semiduplex Operation - 1. A method which is simplex operation at one end of the circuit and duplex operation at the other. Footnote: In general, duplex operation and semiduplex operation require two frequencies in radiocommunication; simplex operation may use either one or two. 2. Operation of a communications network in which a base station operates in a duplex mode with a group of remote stations operating in a half-duplex mode. Note: The terms "half-duplex" and "simplex" are used differently in wire and radio communications.

Sender - A device that accepts address information from a register or routing information from a translator, and then transmits the proper routing information to a trunk or to local equipment. Note: Sender and register functions are often combined in a single unit.

Sending-End Crossfire - In teletypewriter (TTY) systems, interference, in a given channel, caused by transmissions from one or more adjacent TTY channels transmitting from the end at which the crossfire, i.e., interference, is measured.

Sensitive Information - Information, the loss, or misuse, or unauthorized access to or modification of which could adversely affect the national interest or the conduct of federal programs, or the privacy to which individuals are entitled to under 5 U.S.C. Section 552a (the Privacy Act), but that has not been specifically authorized under criteria established by an Executive Order or an Act of Congress to be kept secret in the interest of national defense or foreign policy.

Sensitivity - In an electronic device, e.g., a communications system receiver, or detection device, e.g., PIN diode, the minimum input signal required to produce a specified output signal having a specified signal-to-noise ratio, or other specified criteria. Note 1: The signal input may be expressed as power in dBm or as field strength in microvolts per meter, with input network impedance stipulated. Note 2: "Sensitivity" is sometimes improperly used as a synonym for "responsivity."

Sensor - A device that responds to a physical stimulus, such as thermal energy, electromagnetic energy, acoustic energy, pressure, magnetism, or motion, by producing a signal, usually electrical.

Sentinel - See flag. In data transmission or processing, an indicator, such as a signal, symbol, character, or digit, used for identification. Note: A sentinel may be a byte, word, mark, group mark, or letter that signals the occurrence of some condition or event, such as the end of a word, block, or message.

Separate Channel Signaling - Signaling in which the whole or a part of one or more channels in a multichannel system is used to provide for supervisory and control signals for the message traffic channels. Note: The same channels, such as frequency bands or time slots, that are used for signaling are not used for message traffic. Contrast with common-channel signaling.

Sequence - An arrangement of items according to a specified set of rules, for example, items arranged alphabetically, numerically, or chronologically.

Sequential Access - Synonym serial access. 1. Pertaining to the sequential or consecutive transmission of data into or out of a device, such as a computer, transmission line, or storage device. 2. A process by which data are obtained from a storage device or entered into a storage device in such a way that the process depends on the location of those data and on a reference to data previously accessed.

Sequential Logic Element - A device that has at least one output channel and one or more input channels, all characterized by discrete states, such that the state of each output channel is determined by the previous states of the input channels.

Sequential Transmission - Synonym serial transmission. The sequential transmission of the signal elements of a group representing a character or other entity of data. Note: The characters are transmitted in a sequence over a single line, rather than simultaneously over two or more lines, as in parallel transmission. The sequential elements may be transmitted with or without interruption.

Serial - 1. Pertaining to a process in which all events occur one after the other; for example, the serial transmission of the bits of a character according to the CCITT V.25 protocol. 2. Pertaining to the sequential or consecutive occurrence of two or more related activities in a single device or channel. 3. Pertaining to the sequential processing of the individual parts of a whole, such as the bits of a character or the characters of a word, using the same facilities for successive parts.

Serial Access -: 1. Pertaining to the sequential or consecutive transmission of data into or out of a device, such as a computer, transmission line, or storage device. 2. A process by which data are obtained from a storage device or entered into a storage device in such a way that the process depends on the location of those data and on a reference to data previously accessed. Synonym sequential access.

Serial Computer - 1. A computer that has a single arithmetic and logic unit. 2. A computer, some specified characteristic of which is serial; for example, a computer than manipulates all bits of a word serially.

Serializer - See parallel-to-serial conversion. Conversion of a stream of multiple data elements, received simultaneously, into a stream of data elements transmitted in time sequence, i.e. , one at a time.

Serial Line Internet Protocol (SLIP) - See Dialup Service

Serial Port - A port through which data are passed serially, i.e. , one bit at a time, and that requires only one input channel to handle a set of bits, e.g., all the bits of a byte. Contrast with parallel port.

Serial-to-Parallel Conversion - Conversion of a stream of data elements received in time sequence, i.e., one at a time, into a data stream consisting of multiple data elements transmitted simultaneously. Contrast with parallel-to-serial conversion.

Serial Transmission - The sequential transmission of the signal elements of a group representing a character or other entity of data. Note: The characters are transmitted in a sequence over a single line, rather than simultaneously over two or more lines, as in parallel transmission. The sequential elements may be transmitted with or without interruption. Synonym sequential transmission.

Series T Junction - A three-port waveguide junction that has an equivalent circuit in which the impedance of the branch waveguide is predominantly in series with the impedance of the main waveguide at the junction.

Server - A network device that provides service to the network users by managing shared resources. Note 1: The term is often used in the context of a client-server architecture for a local area network (LAN). Note 2: Examples are a printer server and a file server.

Service - In the Open Systems Interconnection--Reference Model (OSI--RM), a capability of a given layer, and the layers below it, that (a) is provided to the entities of the next higher layer and (b) for a given layer, is provided at the interface between the given layer and the next higher layer.

Service Access - In personal communications service (PCS), the ability for the network to provide user access to features and to accept user service requests specifying the type of bearer services or supplementary service that the users want to receive from the PCS network.

Service Access Point (SAP) - 1. A physical point at which a circuit may be accessed. 2. In an Open Systems Interconnection (OSI) layer, a point at which a designated service may be obtained.

Service Bit - A system overhead bit used for providing a network service, such as a request for a repetition or for a numbering sequence. Note: A service bit is not a check bit.

Service Channel - Synonym orderwire circuit. A voice or data circuit used by technical control and maintenance personnel for coordination and control actions relative to activation, deactivation, change, rerouting, reporting, and maintenance of communication systems and services. Synonyms engineering channel, engineering orderwire.

Service Class - See class of service. 1. A designation assigned to describe the service treatment and privileges given to a particular terminal. 2. A subgrouping of telephone users for the purpose of rate distinction. Note: Examples of class of service subgrouping include distinguishing between (a) individual and party lines, (b) Government and non-Government lines, (c) those permitted to make unrestricted international dialed calls and those not so permitted, (d) business, residence, and coin-operated, (e) flat rate and message rate, and (f) restricted and extended area service. 3. A category of data transmission provided by a public data network in which the data signaling rate, the terminal operating mode, and the code structure, are standardized. Note: Class of service is defined in CCITT Recommendation X.1.

Service Data Unit (SDU) - In layered systems, a set of data that is sent by a user of the services of a given layer, and is transmitted to a peer service user semantically unchanged.

Service Feature - In telephony, any of a number of special functions that may be specified initially, or added to, the user's basic service. Note: Modern telephone switches are capable of providing a wide variety of service features, such as call forwarding and call waiting.

Service Identification - The information that uniquely identifies an NS/EP telecommunications service to the service vendor and the service user.

Service Integrity - The degree to which a service is provided without excessive impairment, once obtained.

Service Outage - The state of a service when (a) a network failure impairs or prevents the initiation of new requests for the service, (b) continued use of the service is impaired or not possible, or (c) certain service parameters fall outside prescribed limits.

Service Probability - The probability of obtaining a specified (or higher) grade of service during a given period of time.

Service Profile - Synonym UPT service profile. In universal personal telecommunications service, a record that contains all information related to a UPT user, which information is required to provide that user with UPT service such as subscriptions to basic and supplementing services and call-routing preferences. Note: Each UPT service profile is associated with a single UPT number.

Service Profile Management - Synonym UPT service profile management. In universal personal telecommunications service, authorized access to and manipulation of the UPT service profile. Note: UPT service profile management can be performed by the UPT user, by the UPT customer (subscriber), or by the UPT service provider.

Service Program - Synonym utility program. A computer program that is in general support of the operations and processes of a computer. Note: Examples of utility programs include diagnostic programs, trace programs, input routines, and programs used to perform routine tasks, i.e., perform everyday tasks, such as copying data from one storage location to another. Synonyms service routine, utility routine, utility program.

Service Provider - 1. An organization that provides services for content providers and for users of a computer network. Note: The services may include access to the computer network, content hosting, server of a private message handling system, news server, etc. 2. A company, organization, administration, business, etc., that sells, administers, maintains, charges for, etc., the service. The service provider may or may not be the provider of the network.

Service Routine - Synonym utility program. A computer program that is in general support of the operations and processes of a computer. Note: Examples of utility programs include diagnostic programs, trace programs, input routines, and programs used to perform routine tasks, i.e., perform everyday tasks, such as copying data from one storage location to another. Synonyms service program, utility routine, utility program.

Service Signals - Signals that enable data systems equipment to function correctly, and possibly to provide ancillary facilities. Synonym housekeeping signals.

Service Termination Point - The last point of service rendered by a commercial carrier under applicable tariffs. Note 1: The service termination point is usually on the customer premises. Note 2: The customer is responsible for equipment and operation from the service termination point to user end instruments. Note 3: The service termination point usually corresponds to the demarcation point.

Service User - An individual or organization, including a service vendor, that is provided a telecommunications service for which a priority level has been requested or assigned.

Session Initiation Protocol (SIP) - Communication protocol that operates similarly to H.323 but is less complex and more Internet- and Web-friendly. Fully modular and designed from the ground up for functioning over IP networks, it can be tailored more easily than H.323 for Internet applications. SIP and H.323 can and do coexist. See also MGCP.

Set - 1. A finite or infinite number of objects, entities, or concepts, that have a given property or properties in common. 2 . To configure all or part of a device into a specified state.

Seven-Bit Byte - Synonym septet. A byte composed of seven binary elements.

Seven-Hundred (700) Service - A personal telephone service that allows individuals to receive, via a single number, telephone calls in various locations (e.g. , home, office, or car) from call originators using the same common carrier.

Sexadecimal - Synonym hexadecimal. 1. Characterized by a selection, choice or condition that has sixteen possible different values or states. 2. Pertaining to a fixed-radix numeration system in which the radix is sixteen.

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

SF - Abbreviation for single-frequency. See single-frequency signaling. In telephony, signaling in which dial pulses or supervisory signals are conveyed by a single voice-frequency tone in each direction. Note 1: An SF signaling unit converts E & M signaling to a format (characterized by the presence or absence of a single voice-frequency tone), which is suitable for transmission over an ac path, e.g. , a carrier system. The SF tone is present in the idle state and absent during the seized state. In the seized state, dial pulses are conveyed by bursts of SF tone, corresponding to the interruptions in dc continuity created by a rotary dial or other dc dialing mechanism. Note 2: The SF tone may occupy a small portion of the user data channel spectrum, e.g., 1600 Hz or 2600 Hz ("in-band" SF signaling), usually with a notch filter at the precise SF frequency, to prevent the user from inadvertently disconnecting a call if user data has a sufficiently strong spectral content at the SF frequency. The SF tone may also be just outside the user voice band, e.g., 3600 Hz. Note 3: The Defense Data Network (DDN) transmits dc signaling pulses or supervisory signals, or both, over carrier channels or cable pairs on a 4-wire basis using a 2600-Hz signal tone. The conversion into tones, or vice versa, is done by SF signal units.

SFTS - Abbreviation for standard frequency and time signal. See standard time and frequency signal service. In the United States, standard time and frequency signals, broadcast on very precise carrier frequencies by the U.S. Naval Observatory and the National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards (NBS). Note: The Radio Regulations (RR) define an identical international service as standard frequency and time signal service

.SGML - Abbreviation for Standard Generalized Mark-up Language. A file format for storage of text and graphics files.

Shadow Loss - 1. The attenuation caused to a radio signal by obstructions in the propagation path. 2. In a reflector antenna, the relative reduction in the effective aperture of the antenna caused by the masking effect of other antenna parts, such as a feed horn or a secondary reflector, which parts obstruct the radiation path.

Shannon (Sh) - The unit of information derived from the occurrence of one of two equiprobable, mutually exclusive, and exhaustive events. Note: A bit may, with perfect formatting and source coding, contain 1 Sh of information. However, the information content of a bit is usually be less than 1 Sh.

Shannon's Law - A statement defining the theoretical maximum rate at which error-free digits can be transmitted over a bandwidth-limited channel in the presence of noise, usually expressed in the form C = W log₂(1 + S /N ), where C is the channel capacity in bits per second, W is the bandwidth in hertz, and S /N is the signal-to-noise ratio. Note: Error-correction codes can improve the communications performance relative to uncoded transmission, but no practical error correction coding system exists that can closely approach the theoretical performance limit given by Shannon's law.

Shaping Network - A network inserted in a circuit for the purpose of improving or modifying the waveform of signals.

Sharpness - The informal, subjective evaluation of detail in the clarity or resolution seen in an image. Note: Often sharpness and resolution are assumed to be directly related, because images possessed of greater sharpness are assumed to have greater resolution. An increase in subjective sharpness is usually reported when objects are more clearly delineated from each other and from background having hard, sharply defined edges. A major contribution to subjective sharpness is this high contrast at edge transitions, as is emphasized by both edge enhancement and aperture correction, for example. In many practical systems, increasing the contrast at edge transitions is often accompanied by a reduction in fine detail, and under these conditions sharpness and resolution may describe opposite characteristics.

Sheath - Of a communications or power cable, the outer covering or coverings of tough material, often plastic, that is resistant to environmental hazards such as abrasion, liquid intrusion, solar radiation, etc., and is used to protect cable component(s) such as optical fibers or metallic conductors that transport the signal or power. Note: There may be more than one sheath surrounding a given cable. For example, some cable designs use an inner sheath surrounded by metallic armor, over which is an outer sheath. Synonym jacket.

Shell - In a computer environment, an operating system command interpreter, i.e., a software utility that reads an input specifying an operation, and that may perform, direct, or control the specified operation. Note 1: For example, a shell may permit a user to switch among application programs without terminating any of them. Note 2: A shell may take its input from either a user terminal or from a file.

Shell Access - Basic Dialup Access to the Internet offering e-mail and mail forwarding, telnet, FTP and IRC.

SHF - Abbreviation for super high frequency. See electromagnetic spectrum.

Shield - 1. A housing, screen, sheath, or cover that substantially reduces the coupling of electric, magnetic, or electromagnetic fields into or out of circuits or transmission lines. 2. A protective cover that prevents the accidental contact of objects or persons with parts or components operating at hazardous voltage levels.

Shielded Pair - A 2-wire transmission line surrounded by a sheath of conductive material that protects it from the effects of external fields and confines fields produced within the line.

Shielded Twisted Pair - A transmission line composed of a twisted 2-wire metallic transmission line surrounded by a sheath of conductive material that protects it from the effects of external fields and confines fields produced within the line.

Shielding - See shield. 1. A housing, screen, sheath, or cover that substantially reduces the coupling of electric, magnetic, or electromagnetic fields into or out of circuits or transmission lines. 2. A protective cover that prevents the accidental contact of objects or persons with parts or components operating at hazardous voltage levels.

Shielding Effectiveness - The factor that expresses the attenuation caused by scattering, e.g., of radiant or acoustic energy, during its passage through a medium. Note: The scattering coefficient is usually expressed in units of reciprocal distance.

Shift - 1. The movement of some or all of the characters or bits of a word by the same number of character or bit positions in the direction of a specified end of a word. 2. In radar, the ability to move the origin of a radial display away from the center of the cathode ray tube.

Shift Register - A storage device, usually in a central processing unit (CPU), in which device a serially ordered set of data may be moved, as a unit, into a discrete number of storage locations. Note 1: Shift registers may be configured so that the stored data may be moved in more than one direction. Note 2: Shift registers may be configured so that data may be entered and stored from multiple inputs. Note 3: Shift registers may be grouped into arrays of two or more dimensions in order to perform more complex data operations.

Ship Earth Station - A mobile Earth station in the maritime mobile-satellite service located on board ship.

Ship's Emergency Transmitter - A ship's transmitter to be used exclusively on a distress frequency for distress, urgency or safety purposes.

Ship Station - A mobile station in the maritime mobile service located on board a vessel which is not permanently moored, other than a survival craft station.

Shock Excitation - Synonym impulse excitation. The production of oscillation in a circuit or device by impressing a stimulus (signal) for a period that is extremely short compared to the duration of the oscillation that it produces.

Short Haul Toll Traffic - A general term applied to message toll traffic between nearby points. In common usage, this term is ordinarily applied to message toll traffic between points less than 20 to 50 miles apart.

Shortwave - In radio communications, pertaining to the band of frequencies approximately between 3 MHz and 30 MHz. Note: "Shortwave" is not a term officially recognized by the international community.

Short Wavelength - In optical communication, optical radiation having a wavelength less than approximately 1

Shot Noise - The noise caused by random fluctuations in the motion of charge carriers in a conductor. Note: There is often a minor inconsistency in referring to shot noise in an optical system: many authors refer to shot noise loosely when speaking of the mean square shot noise current (amperes²) rather than noise power (watts).

International System of Units (SI)

SI Prefixes. The common metric prefixes are;
Multiplacation Factor	Prefix Name	Prefix	Symbol
1 000 000 000 000 =	10¹²	tera	T
1 000 000 000 =	10⁹	giga	G
1 000 000 =	10⁶	mega	M
1 000 =	10³	kilo	k
100 =	10²	hecto	h
10 =	10¹	deka	da
0.1 =	10^-1	deci	d
0.01 =	10^-2	centi	c
0.001 =	10^-3	milli	m
0.000 001 =	10^-6	micro
0.000 000 001 =	10^-9	nano	n
0.000 000 000 001 =	10^-12	pico	p

SI Base Units
Quantity	Unit Name	Unit Symbol
length	meter	m
mass	kilogram	kg
time	second	s
electric current	ampere	A
thermodynamic temperature	kelvin	K
amount of substance	mole	mol
luminous intensity	candela	cd

**SI derived units.** Derived units are formed by combining base units and other derived units according to the algebraic relations linking the corresponding quantities. The symbols for derived units are obtained by means of the mathematical signs for multiplication, division, and use of exponents. For example, the SI unit for velocity is the *meter per second* (m/s or m•s^-1), and that for angular velocity is the *radian per second* (rad/s or rad•s^-1). Some derived SI units have been given special names and symbols, as listed in this table.
Quantity	Unit Name	Unit Symbol	Expression in Terms of Other SI Units
Absorbed dose, specific energy imparted, kerma, absorbed dose index	gray	Gy	J/kg
Activity (of a radionuclide)	becquerel	Bq	1/s
Celsius temperature	degree Celsius	C	K
Dose equivalent	sievert	Sv	J/kg
Electric capacitance	farad	F	C/V
Electric charge, quantity of electricity	coulomb	C	A•s
Electric conductance	siemens	S	A/V
Electric inductance	henry	H	Wb/A
Electric potential, potential difference, electromotive force	volt	V	W/A
Electric resistance	ohm		V/A
Energy, work, quantity of heat	joule	J	N•m
Force	newton	N	kg•m/s²
Frequency (of a periodic phenomenon)	hertz	Hz	1/s
Illuminance	lux	lx	lm/m²
Luminous flux	lumen	lm	cd•sr
Magnetic flux	weber	Wb	V•s
Magnetic flux density	tesla	T	Wb/m²
Plane angle	radian	rad	m/m
Power, radiant flux	watt	W	J/s
Pressure, stress	pascal	Pa	N/m²
Solid angle	steradian	sr	m²/m²

Sideband - In amplitude modulation (AM), a band of frequencies higher than or lower than the carrier frequency, containing energy as a result of the modulation process. Note: Amplitude modulation results in two sidebands. The frequencies above the carrier frequency constitute what is referred to as the "upper sideband"; those below the carrier frequency, constitute the "lower sideband." In conventional AM transmission, both sidebands are present. Transmission in which one sideband is removed is called " single-sideband transmission."

Signal - 1. Detectable transmitted energy that can be used to carry information. 2. A time-dependent variation of a characteristic of a physical phenomenon, used to convey information. 3. As applied to electronics, any transmitted electrical impulse. 4. Operationally, a type of message, the text of which consists of one or more letters, words, characters, signal flags, visual displays, or special sounds, with prearranged meaning and which is conveyed or transmitted by visual, acoustical, or electrical means.

Signal Compression - 1. In analog (usually audio) systems, reduction of the dynamic range of a signal by controlling it as a function of the inverse relationship of its instantaneous value relative to a specified reference level. Note 1: Signal compression is usually expressed in dB. Note 2: Instantaneous values of the input signal that are low, relative to the reference level, are increased, and those that are high are decreased. Note 3: Signal compression is usually accomplished by separate devices called "compressors." It is used for many purposes, such as (a) improving signal-to-noise ratios prior to digitizing an analog signal for transmission over a digital carrier system, (b) preventing overload of succeeding elements of a system, or (c) matching the dynamic ranges of two devices. Note 4: Signal compression (in dB) may be a linear or nonlinear function of the signal level across the frequency band of interest and may be essentially instantaneous or have fixed or variable delay times. Note 5: Signal compression always introduces distortion, which is usually not objectionable, if the compression is limited to a few dB. Note 6: The original dynamic range of a compressed signal may be restored by a circuit called an " expander." 2. In facsimile systems, a process in which the number of pels scanned on the original is larger than the number of encoded bits of picture information transmitted.

Signaling - 1. The use of signals for controlling communications. 2. In a telecommunications network, the information exchange concerning the establishment and control of a connection and the management of the network, in contrast to user information transfer. 3. The sending of a signal from the transmitting end of a circuit to inform a user at the receiving end that a message is to be sent.

Signaling Message - 1. An assembly of signaling information pertaining to a call that includes, but is not limited to, management transactions, that is transferred (between signaling points) as an entity. 2. An assembly of signaling information that pertains to a call management transaction, and the like and that is transferred as an entity.

SIGINT - Acronym for signals intelligence. 1. A category of intelligence comprising, either individually or in combination, all communications intelligence, electronics intelligence, and foreign instrumentation signals intelligence, however transmitted. 2. Intelligence derived from communications, electronics, and foreign instrumentation signals.

Signal - 1. Detectable transmitted energy that can be used to carry information. 2. A time-dependent variation of a characteristic of a physical phenomenon, used to convey information. 3. As applied to electronics, any transmitted electrical impulse. 4. Operationally, a type of message, the text of which consists of one or more letters, words, characters, signal flags, visual displays, or special sounds, with prearranged meaning and which is conveyed or transmitted by visual, acoustical, or electrical means.

Signal Center - A combination of signal communication facilities operated by the Army in the field and consisting of a communications center, telephone switching central and appropriate means of signal communications.

Signal Compression - 1. In analog (usually audio) systems, reduction of the dynamic range of a signal by controlling it as a function of the inverse relationship of its instantaneous value relative to a specified reference level. Note 1: Signal compression is usually expressed in dB. Note 2: Instantaneous values of the input signal that are low, relative to the reference level, are increased, and those that are high are decreased. Note 3: Signal compression is usually accomplished by separate devices called "compressors." It is used for many purposes, such as (a) improving signal-to-noise ratios prior to digitizing an analog signal for transmission over a digital carrier system, (b) preventing overload of succeeding elements of a system, or (c) matching the dynamic ranges of two devices. Note 4: Signal compression (in dB) may be a linear or nonlinear function of the signal level across the frequency band of interest and may be essentially instantaneous or have fixed or variable delay times. Note 5: Signal compression always introduces distortion, which is usually not objectionable, if the compression is limited to a few dB. Note 6: The original dynamic range of a compressed signal may be restored by a circuit called an "expander." 2. In facsimile systems, a process in which the number of pels scanned on the original is larger than the number of encoded bits of picture information transmitted.

Signal Contrast - In facsimile, the ratio of the level of the white signal to the level of the black signal. Note: Signal contrast is usually expressed in dB.

Signal Conversion Equipment - Synonym modem. 1. In general, a device that both modulates and demodulates signals. 2. In computer communications, a device used for converting digital, signals into, and recovering them from, quasi-analog signals suitable for transmission over analog communications channels. Note: Many additional functions may be added to a modem to provide for customer service and control features. Synonym signal conversion equipment. 3. In FDM carrier systems, a device that converts the voice band to, and recovers it from, the first level of frequency translation.

Signal Distance - 1. A measure of the difference between a given signal and a reference signal. Note: For analog signals, the signal distance is the root mean square difference between the given signal and a reference signal over a symbol period. 2. Synonym Hamming distance. The number of digit positions in which the corresponding digits of two binary words of the same length are different. Note 1: The Hamming distance between 1011101 and 1001001 is two. Note 2: The concept can be extended to other notation systems. For example, the Hamming distance between 2143896 and 2233796 is three, and between "toned" and "roses" it is also three.

Signal Distortion - See distortion. 1. In a system or device, any departure of the output signal waveform from that which should result from the input signal waveform's being operated on by the system's specified, i.e. , ideal, transfer function. Note: Distortion may result from many mechanisms. Examples include nonlinearities in the transfer function of an active device, such as a vacuum tube, transistor, or operational amplifier. Distortion may also be caused by a passive component such as a coaxial cable or optical fiber, or by inhomogeneities, reflections, etc., in the propagation path. 2. In start-stop teletypewriter signaling, the shifting of the significant instants of the signal pulses from their proper positions relative to the beginning of the start pulse. Note: The magnitude of the distortion is expressed in percent of an ideal unit pulse length.

Signal Droop - In an otherwise essentially flat-topped rectangular pulse, distortion characterized by a decline of the pulse top. See illustration under waveform.

Signal Element - A part of a signal, distinguished by its nature, magnitude, duration, transition, or relative position. Note: Examples of signal elements include signal transitions, significant conditions, significant instants, and binary digits (bits).

Signal Expansion - Restoration of the dynamic range of a compressed signal. Contrast with signal compression.

Signal Frequency Shift - 1. Any change in frequency. 2. Any change in the frequency of a radio transmitter or oscillator. Note: In the radio regime, frequency shift is also called rf shift. 3. See frequency-shift telegraphy. 4. In facsimile, a frequency modulation system where one frequency represents picture black and another frequency represents picture white. Frequencies between these two limits may represent shades of gray. 5. An intentional frequency change used for modulation purposes.

Signaling Connection Control Part (SCCP) - 1. A component part of SS7 protocol that provides additional functions to the message transfer part (MTP) to provide for Connectionless and Connection Oriented network services to transfer signaling information between exchanges and specialized centers in telecommunication networks. The combination of the MTP and the SCCP is called Network Service part. 2. The part of SS7 that provides additional functions to the MTP to cater to both connectionless as well as connection-oriented network services and to achieve an OSI-compatible network service.

Signaling Data Link - A combination of two data channels operating together in a single signaling system.

Signaling Link - A transmission facility that consists of a signaling data link and its transfer control functions, used for reliable transfer of a signaling message.

Signaling Message - 1. An assembly of signaling information pertaining to a call that includes, but is not limited to, management transactions, that is transferred (between signaling points) as an entity. [T1.226-1992] 2. An assembly of signaling information that pertains to a call management transaction, and the like and that is transferred as an entity.

Signaling Path - In a transmission system, a path used for system control, synchronization, checking, signaling, and service signals used in system management and operations rather than for the data, messages, or calls of the users.

Signaling Rate - The aggregate rate at which data pass a point in the transmission path of a data transmission system. Note 1: The DSR is usually expressed in bits per second. Note 2: The data signaling rate is given by data signaling rate (DSR)

where m is the number of parallel channels, n_i is the number of significant conditions of the modulation in the I-th channel, and T_i is the unit interval, expressed in seconds, for the I-th channel. Note 3: For serial transmission in a single channel, the DSR reduces to (1/T )log₂n ; with a two-condition modulation, i.e. , n =2, the DSR is 1/T . Note 4: For parallel transmission with equal unit intervals and equal numbers of significant conditions on each channel, the DSR is (m /T )log₂n ; in the case of a two-condition modulation, this reduces to m /T . Note 5: The DSR may be expressed in bauds, in which case, the factor log₂n_i in the above summation formula should be deleted when calculating bauds. Note 6: In synchronous binary signaling, the DSR in bits per second may be numerically the same as the modulation rate expressed in bauds. Signal processors, such as four-phase modems, cannot change the DSR, but the modulation rate depends on the line modulation scheme, in accordance with Note 4. For example, in a 2400 b/s 4-phase sending modem, the signaling rate is 2400 b/s on the serial input side, but the modulation rate is only 1200 bauds on the 4-phase output side.

Signaling System No. 7 (SS7) - A common-channel signaling system defined by the CCITT in the 1988 Blue Book, in Recommendations Q.771 through Q.774. Note: SS7 is a prerequisite for implementation of an Integrated Services Digital Network (ISDN).

Signaling Time Slot - In TDM carrier systems, a time slot starting at a particular phase or instant in each frame and allocated to the transmission of signaling (supervisory and control) data.

Signal Intelligence - 1. A category of intelligence comprising, either individually or in combination, all communications intelligence, electronics intelligence, and foreign instrumentation signals intelligence, however transmitted. 2. Intelligence derived from communications, electronics, and foreign instrumentation signals.

Signal Level - In a communications system, the signal power or intensity at a specified point and with respect to a specified reference level, e.g., 1 mW.

Signal Message - In communications systems, a message, i.e., an assembly of signaling information, that (a) includes associated message alignment and service indications, (b) pertains to a call, and (c) is transferred via the message transfer part.

Signal-Plus-Noise-Plus-Distortion to Noise-Plus-Distortion Ratio - See SINAD. - 1. The ratio of (a) total received power, i.e., the received signal-plus-noise-plus-distortion power to (b) the received noise-plus-distortion power. 2. The ratio of (a) the recovered audio power, i.e., the original modulating audio signal plus noise plus distortion powers from a modulated radio frequency carrier to (b) the residual audio power, i.e., noise-plus-distortion powers remaining after the original modulating audio signal is removed. Note: The SINAD is usually expressed in dB.

Signal-Plus-Noise-to-Noise Ratio ((S +N )/N ) - At a given point in a communications system, the ratio of (a) the power of the desired signal plus the noise to (b) the power of the noise. Note: The (S +N )/N ratio is usually expressed in dB.

Signal Processing - The processing--such as detection, shaping, converting, coding, and time positioning--of signals, that results in their transformation into other forms, such as other waveshapes, power levels, and coding arrangements.

Signal Processing Gain - 1. The ratio of (a) the signal-to-noise ratio of a processed signal to (b) the signal-to-noise ratio of the unprocessed signal. Note: Signal processing gain is usually expressed in dB. 2. In a spread-spectrum communications system, the signal gain, signal-to-noise ratio, signal shape, or other signal improvement obtained by coherent band spreading, remapping, and reconstitution of the desired signal.

Signal Reference Subsystem - The portion of a facility grounding system that (a) provides reference planes, such as ground-return circuits, for all of the signal paths in the facility and (b) is isolated from other circuits, especially isolated from circuits that carry fault, lightning discharge, and power distribution currents.

Signal Regeneration - Signal processing that restores a signal so that it conforms to its original characteristics.

Signal-Return Circuit - A current-carrying return path from a load back to the signal source, i.e., the low side of the closed loop energy transfer circuit between a source-load pair.

Signal Sampling - The process of obtaining a sequence of instantaneous values of a particular signal characteristic, usually at regular time intervals.

Signal Transition - 1. In the modulation of a carrier, a change from one significant condition to another. Note 1: Examples of signal transitions are a change from one electrical current, voltage, or power level to another; a change from one optical power level to another; a phase shift; or a change from one frequency or wavelength to another. Note 2: Signal transitions are used to create signals that represent information, such as "0" and "1" or "mark" and "space." 2. For channel-associated signaling, a change in state of the A bit for 2-state signaling, A and/or B bit in 4-state signaling, or the A, B, C, and/or D bit for 16-state signaling.

Significant Condition - In the modulation of a carrier, one of the values of the signal parameter chosen to represent information. Note 1: Examples of significant conditions are an electrical current, voltage, or power level; an optical power level; a phase value; or a frequency or wavelength chosen to represent a "0" or a "1"; or a "mark" or a "space." Note 2: The duration of a significant condition is the time interval between successive significant instants. Note 3: A change from one significant condition to another is called a " signal transition." Note 4: Signal transitions are used to create signals that represent information, such as "0" and "1" or "mark" and "space." Note 5: Significant conditions are recognized by an appropriate device. Each significant instant is determined when the appropriate device assumes a condition or state usable for performing a specific function, such as recording, processing, or gating.

Significant Instant - In a signal, any instant at which a significant condition of a signal begins or ends. Note: Examples of significant instants include the instant at which a signal crosses the baseline or reaches 10% or 90% of its maximum value.

Silent Transfer Destination-After activating either Silent Transfer feature, you will want to tell The Stick where to route the call. The "Destination" program feature gives you the option of routing all after-hours calls to either the fax or modem. Factory preset is to the port labeled "FAX".

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

Simulate - To represent certain features of the behavior of a physical or abstract system by the behavior of another system. Note 1: For example, delay lines may be used to simulate propagation delay and phase shift caused by an actual transmission line. Note 2: A simulator may imitate only a few of the operations and functions of the unit it simulates.

SINAD - Abbreviation for signal-plus-noise-plus-distortion to noise-plus-distortion ratio. 1. The ratio of (a) total received power, i.e., the received signal-plus-noise-plus-distortion power to (b) the received noise-plus-distortion power. 2. The ratio of (a) the recovered audio power, i.e., the original modulating audio signal plus noise plus distortion powers from a modulated radio frequency carrier to (b) the residual audio power, i.e., noise-plus-distortion powers remaining after the original modulating audio signal is removed. Note: The SINAD is usually expressed in dB.

Single Bill Fee - Many long distance companies are now adding a fee to the phone bills of customers who are billed for long distance service through their local phone companies. TDS Long Distance does not charge this fee.

Single-Mode Optical Fiber - An optical fiber in which only the lowest order bound mode can propagate at the wavelength of interest. Note 1: The lowest order bound mode is ascertained for the wavelength of interest by solving Maxwell's equations for the boundary conditions imposed by the fiber, e.g., core (spot) size and the refractive indices of the core and cladding. Note 2: The solution of Maxwell's equations for the lowest order bound mode will permit a pair of orthogonally polarized fields in the fiber, and this is the usual case in a communication fiber. Note 3: In step-index guides, single-mode operation occurs when the normalized frequency, V, is less than 2.405. For power-law profiles, single-mode operation occurs for a normalized frequency, V, less than approximately

Single-Phase Voltage and Frequency - Europe and most other countries in the world use a voltage which is twice that of the US. It is between 220 and 240 volts, whereas in Japan and in most of the Americas the voltage is between 100 and 127 volts.

The system of three-phase alternating current electrical generation and distribution was invented by a nineteenth century creative genius named Nicola Tesla. He made many careful calculations and measurements and found out that 60 Hz (Hertz, cycles per second) was the best frequency for alternating current (AC) power generating. He preferred 240 volts, which put him at odds with Thomas Edison, whose direct current (DC) systems were 110 volts. Perhaps Edison had a useful point in the safety factor of the lower voltage, but DC couldn't provide the power to a distance that AC could.

When the German company AEG built the first European generating facility, its engineers decided to fix the frequency at 50 Hz, because the number 60 didn't fit the metric standard unit sequence (1,2,5). At that time, AEG had a virtual monopoly and their standard spread to the rest of the continent. In Britain, differing frequencies proliferated, and only after World War II the 50-cycle standard was established. A big mistake, however.

Not only is 50 Hz 20% less effective in generation, it is 10-15% less efficient in transmission, it requires up to 30% larger windings and magnetic core materials in transformer construction. Electric motors are much less efficient at the lower frequency, and must also be made more robust to handle the electrical losses and the extra heat generated. Today, only a handful of countries (Antigua, Guyana, Peru, the Philippines, South Korea and the Leeward Islands) follow Tesla’s advice and use the 60 Hz frequency together with a voltage of 220-240 V.

Originally Europe was 110 V too, just like Japan and the US today. It has been deemed necessary to increase voltage to get more power with less losses and voltage drop from the same copper wire diameter. At the time the US also wanted to change but because of the cost involved to replace all electric appliances, they decided not to. At the time (50s-60s) the average US household already had a fridge, a washing-machine, etc., but not in Europe.

The end result is that now, the US seems not to have evolved from the 50s and 60s, and still copes with problems as light bulbs that burn out rather quickly when they are close to the transformer (too high a voltage), or just the other way round: not enough voltage at the end of the line (105 to 127 volt spread !).

Note that currently all new American buildings get in fact 220 volts split in two 110 between neutral and hot wire. Major appliances, such as virtually all drying machines and ovens, are now connected to 220 volts. Americans who have European equipment, can connect it to these outlets.

COUNTRY	SINGLE-PHASE VOLTAGE	FREQUENCY
Afghanistan	220 V	50 Hz
Albania	230 V	50 Hz
Algeria	230 V	50 Hz
American Samoa	120 V	60 Hz
Andorra	230 V	50 Hz
Angola	220 V	50 Hz
Anguilla	110 V	60 Hz
Antigua	230 V	60 Hz
Argentina	220 V	50 Hz
Armenia	230 V	50 Hz
Aruba	127 V	60 Hz
Australia	240 V	50 Hz
Austria	230 V	50 Hz
Azerbaijan	220 V	50 Hz
Azores	230 V	50 Hz
Bahamas	120 V	60 Hz
Bahrain

Telecom Dictionary - Definitions of terms

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S

International System of Units (SI)