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Glossary S

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  • SAA: See System Application Architecture
  • Scattering: A property of glass which causes light to deflect from the fiber & contributes to losses.
  • SCSI: See Small Computer System Interface
  • SD: See Send Data
  • SDLC: See Synchronous Data Link Control
  • Security Management: Protects a network from invalid accesses. It is one of the management categories defined by the ISO.
  • SEF: See Source Explicit Forwarding
  • Segment: One portion of the bus of an Ethernet LAN, consisting of standard Ethernet coaxial cable up to 500 meters or thinnet (RG58) coaxial cable up to 185 meters.
  • Segmentation: The division of breaking larger segments (ring) into multiple small segments (rings)
  • Semaphore: A method with which multiple applications can coordinate related operations using messages.
  • Send Data: (SD) A signal in the RS232-C DTE-DCE interface. This signal is used by the DTE to send Data..
  • Sequenced Packet Protocol: (SPP) XNS (Xerox Network Systems) protocol governing sequenced data.
  • Sequential Packet Exchange: (SPX) Novell's implementation of SPP.
  • Serial Transmission: The most common transmission mode; information bits are sent sequentially on a single data channel.
  • Server: A computer that provides shared resources, such as files & printers, to the network.
  • Session: An active communications connection between nodes or applications.
  • Session Establishment: The process of starting a Session. It is a part of the Initial Handshaking. (see Initial Handshaking)
  • Session Layer: Layer 5 of the OSI (Open Systems Interconnection) model; it defines protocols governing communications between applications.
  • Shared memory: Portion of memory accessible to multiple processes.
  • Shield: A tape, screen or braid of metal, usually copper, aluminum, or other conductive material placed around or between electric circuits or cables or their components, to prevent signal leakage or interference.
  • Shielded: Wiring protected from electromagnetic & radio frequency interference by metal-backed mylar foil & plastic or PVC.
  • Shielded Twisted Pairs: (STP) General term for cabling systems that are designed specifically for data transmission, & where the cables are shielded.
  • Shielding: Protective cable covering that eliminates electromagnetic & radio frequency interference.
  • SHM: (Short-Haul Modem) See line Driver
  • Short-Haul Modem: (SHM) See line Driver
  • Shrink Tubing: Tubing which has been extruded, cross-linked & mechanically expanded, which when reheated will return to its original diameter.
  • Signal: Any visible or audible indication which can convey information. Also, the information conveyed through a communication system.
  • Signal Distortion: The discrepancy of the signal at the output point from the signal transmitted at the point of origin.
  • Signal Ground: One of the signals in the RS232-C DTE-DCE interface. The signal performs as the electrical reference point for other signals in the interface.
  • Signal Quality Error: (SQE) A signal quality error message sent by transceivers to the Ethernet controller card when there are network faults or collisions on the network.
  • Signal to Noise Ratio: (SNR) Commonly used interchangeably with ACR
  • Silicone: A material made from silicone & oxygen. Can be in thermosetting elastomer or liquid form. The thermosetting elastomer form is noted for high heat resistance.
  • Simple Mail Transfer Protocol: (SMTP) TCP/IP protocol governing electronic mail transmissions.
  • Simple Terminal: A terminal device with little or not computing power.
  • Simultaneous Peripheral Operation On-Line: (SPOOL) See Print Spooler.
  • Sine Wave: The basic signal waveform. In analog data communications, the carrier signal on the telephone line is a type of sine wave.
  • Single Mode Fiber: An optical fiber that will allow only one mode to propagate; this fiber is a step index fiber.
  • Singlemode: Describing an optical waveguide that is designed to propagate light of only a single wavelength & perhaps a single phase; essentially, an optical fiber that allows the transmission of only one light beam, or data carrying lightwave channel, & is optimized for a particular lightwave frequency, compared with multimode.
  • Skew Rays: A ray that does not intersect the fiber axis. Generally, a light ray that enters the fiber core at a very high angle.
  • Skin Effect: The tendency of alternating current to travel only on the surface of a conductor as its frequency increases.
  • Small Computer System Interface: (SCSI) An interface type for computer adapters & controller boards.
  • Small Office/Home Office: (SOHO) Small scale equipment (4 port hubs and so forth).
  • SMTP: See Simple Mail Transfer Protocol
  • SNA: Systems Network Architecture
  • SNMP: Simple Network Management Protocol.
  • SNR: See Signal to Noise Ratio
  • Sockets: An API (Application Program Interface) for communications between Unix & TCP/IP.
  • Software: Binary information the computer understands to perform certain tasks.
  • SOH: See Start of Header
  • SOHO: See Small Office/Home Office
  • Source: the device( usually LED or laser) used to convert an electrical information-carrying signal into a corresponding optical signal for transmission by an optical wave guide.
  • Source Explicit Forwarding: (SEF) Security feature that allows transmissions only from specified stations to be forwarded by bridges.
  • Space: Absence of signal. In telegraph communications, a space represents the open condition or no current flowing. A space impulse is equivalent to a binary 0.
  • Spanning Tree Algorithm: (STA) An IEEE 802.1 standard algorithm used by bridges to automatically compute a set of routes between all bridges in the network to avoid loops. It is required in a bridge network if redundant links are needed for backup purposes.
  • Spectral Bandwidth: The difference between wavelengths at which the radiant intensity of illumination is half its peak intensity.
  • Spectrum: Frequencies that exists in a continuous range & have a common characteristic. A spectrum may be inclusive of many spectrums.
  • Spike: See Transient
  • SPOOL: (Simultaneous Peripheral Operation On-Line) See Print Spooler.
  • SPP: See Sequenced Packet Protocol
  • SPX: See Sequential Packet Exchange
  • SQE: See Signal Quality Error
  • SQL: See Structured Query Language
  • STA: See Spanning Tree Algorithm
  • Standing Wave: The stationary pattern of waves produced by two waves of the same frequency traveling in opposite directions on the same transmission line. The existence of voltage & current maxima & minima along a transmission line is a result of reflected energy from an impedance mismatch.
  • Standing Wave Ratio: A ratio of the maximum amplitude to the minimum amplitude of a standing wave slated in current or voltage amplitudes.
  • Star: A LAN topology in which all the nodes are connected to one central point like a switch or hub.
  • Star Coupler: Optical components which allows emulation of a bus topology in fiber optic systems.
  • Star Topology: A network topology in which nodes are connected to a common device such as a switch, hub or concentrator.
  • StarLAN: A local area network design & specification, within the IEEE standards, characterized by 1 Mbps baseband data transmission over two twisted-pair wiring.
  • Start Bit: The first bit in a character in asynchronous transmission to indicate the beginning of a character. By convention, the Start bit is a Zero ("Space" condition on the line).
  • Start of Header: (SOH) SOH is a Data Link Control character. SOH precedes the main message & contains auxiliary information related to a message. STX ends the SOH function.
  • Start of Text: (STX) A control character used to indicate the beginning of a message; it immediately follows the header in transmission blocks.
  • Start-Stop: A technique used in asynchronous transmission so that a group of bits representing a character is preceded by a Start Bit & ended with a Stop Bit.
  • Start-Stop Protocol: A commonly-used term for Data Link Control Procedures used by terminals (DTE's) using Start-Stop techniques.
  • Static Charge: An electrical charge that is bound to an object. An unmoving electrical charge.
  • Station: Any DTE that receives or transmits messages on a data link, including network nodes & user devices.
  • Station Activation: A sequence in the data transfer between two DTE's. It is the reverse of station activation. Upon completion of this sequence, the DTE's become non-operational.
  • Statistical Multiplexing: A multiplexing technique: Bandwidth is dynamically allocated on the basis of need
  • Stay Cord: A component of a cable, usually of high tensile strength, used to anchor the cable ends at their points of termination & keep any pull on the cable from being transferred to the electrical conductors.
  • Step-Index Fiber: A fiber in which the core is of a uniform refractive index, & there's a sharp decrease in the index of refraction at the cladding.
  • Stop Bit: The last bit in a character in asynchronous transmission to indicate the ending of the character. By conversion, the Stop Bit is a One ("Mark" condition on the line). Some equipment us4es 1.5 or 2 bits for stop.
  • Storage Device: A device for holding (strong) programs or data. Storage devices may be temporary (memory with the computer) or permanent (disk storage).
  • Storage Media: Usually refers to devices, such as, magnetic disks, magnetic tapes, LSI memory chips, for storing programs or data.
  • Store-and-Forward: A message switching technique in which messages are temporarily stored at intermediate points before being retransmitted to the next destination.
  • Stored Procedures: Compiled code residing on an intelligent database server that reduces the processing burden on the client.
  • STP: See Shielded Twisted Pairs
  • Strain Gauge: A device for determining the amount of strain (change in dimensions) when a stress is applied
  • StreetTalk: (Virtual Network Software) protocol for discovering & maintaining network resource information distributed among different servers.
  • String: A process consisting of a series of threads or a series of characters.
  • Structural Return Loss: Backward reflected energies from uneven parts of the cable structure are termed structural return loss.
  • Structured Networking: A new design of the network fabric in which physical connections are decoupled from logical associations in the network, & a network center serves as a centralized location for hubs & servers. Structured networking allows networks to be created to organizations structure rather than the building structure.
  • Structured Query Language: (SQL) A widely used programming language for manipulating database information.
  • STX: See Start of Text
  • Sublayer: A portion of an OSI model layer. For exam
  • Subnet: A network that has been connected to larger & more powerful network system. Subnets are connected to a larger net to by a bridge or router.
  • Suite: A related group of protocols, such as the TCP/IP protocol suite. The TCP/IP protocol suite includes IP,ARP,ICMP, TCP, UDP, Telnet, FTP, SMTP, & others.
  • Surge: An oversupply of voltage from the power company, lasting as long as several seconds. A strong surge can damage electronic equipment.
  • Sweep Test: A method to determine the frequency response of cable by generating an RF voltage whose frequency is varied at a rapid constant rate over a given range.
  • Swell: A persistent voltage surplus. The overvoltage is not enough to cause a surge but can damage sensitive equipment.
  • Switch: Multiport LAN device designed to increase network performance by allowing only essential traffic on the attached individual LAN segment. Packets are filtered or forwarded based upon their source & destination addresses.
  • Switched LAN: A term which encompasses all switched approaches to providing LAN information transport, including frame switching, hybrid switching & cell switching.
  • Switched Line: A communications link for which the physical path may vary with each usage, such as the public telephone network.
  • Synchronous: Data transmission where characters & bits are transmitted at a fixed rate with the transmitter & receiver synchronized.
  • Synchronous Data Link Control: (SDLC) an IBM standard protocol, superseding bisynchronous (BSC)
  • Synchronous Transmission: Transmission of data that uses a clock signal to synchronize the transmitter and receiver. Synchronized transmission eliminates the need for start & stop bits. See also asynchronous transmission.
  • System Application Architecture: (SAA) An IBM developed set of standards that provides identical user interfaces for applications running on PCs, minicomputers, & mainframes.
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