Fiber Optics Illustrated Dictionary lasers and dopants (deliberate impurities such as se- lenium) may be added to promote the movement of electrons through the material. With the application of current, the component lases with a slightly wider range of frequencies and a slightly wider beam spread than traditional lasers. See gallium arsenide; Kazarinov, Rudolf; laser diode. semiconductor optical amplifier SOA. A solid state component for increasing gain in an optical transmis- sion. This is important for achieving power and dis- tance of sufficient magnitude for effective transmis- sions. See amplifier. SEQUEL See SQL. sequentialA nonoverlapping succession or series, in chronological or data order, with no significant in- tervening time or data. See concurrent, consecutive, parallel, serial. serial clock transmitexternal SCTE. A data stream common to serial cables used to connect computer modems. For example, on 25-pin serial communica- tions connections, one of the pins may be assigned to carry the external clock signal from the data ter- minal equipment (DTE). serial communication A means oftransferring data one element at a time, often through a single wire or trace in a circuit. While it may not seem very fast or efficient, serial communication is easily implemented and very commonly used in computing systems. The RS-232 standard is the most common specification for the physical/pin connections for serial communi- cations. See modem, parallel, RS-232. Serial Data Transport Interface SDTI. An emerg- ing packet data standard for the transport of audio, video, and data among various multimedia systems, including video servers, cameras, VCRs, editing sys- tems, etc., especially in professional broadcast video environments. SDTI provides a network environment for video data exchange, without a lot of network overhead. It supports single-direction, point-to-point, compressed-data connections. snTI has been well received due to the many advantages of digital sys- tems, including the reduction of generation loss com- mon to analog editing and transmission loss associ- ated with analog broadcasting. SDTI (SMPTE 305M) evolved from SDI (SMPTE 259M), which is used for transporting uncompressed audio/visual signals between digital broadcast and post-production devices. The two formats are me- chanically and electrically compatible to support co- existence in the same facility. SDTI, developed by the SMPTEPT20.04 Workgroup on Packetized Television Interconnections, extended this concept by enabling compressed video to be ex- changed without the need for frequent compression! decompression processes that could slow or degrade the transmission. SDTI has been recommended by the EBU/SMPTE Task Force for Harmonized Standards for the Exchange of Programme Material as Bitstreams. Serial Data Transport Protocol SDTP. A network data protocol which provides a means of transporting serial data streams over PPP links. SnTP arose out 852 of the work of the TR30.1 ad hoc committee in the mid-1990s to provide a standard means for synchro- nous data compression. SDTP specifies a transport protocol and an associated control protocol (PPP- SDTP and PPP-SDCP) to be used in conjunction with PPP protocols. See Point-to-Point Protocol, RFC 1963. serial interface cardA printed circuit card which fits into a slot in a computer or other computerized de- vice or piggybacks on a motherboard to provide stan- dardized electrical connections for the synchronous serial transmission of digital data. The connection on the card is typically a 25-pin D connector. On con- sumer desktop computer systems, most serial inter- face cards support data rates up to about 28,800 or 38,400 bps. A serial interface card is a common way to connect remote computer terminals and data mo- dems to a computer. See RS-232. SerialLine Interface Protocol, Serial Line IP SLIP. Originating with an early 1980s 3COM UNET TCP/ IP implementation, SLIP became a de facto standard encapsulation protocol for serial lines, used for point- to-point communications with TCP lIP. SLIP has now been superseded by Point-to-Point Protocol (PPP). See Point-to-Point Protocol, RFC 1055. SERN See Software Engineering Research Network. server A system which provides services to other computers connected to it through a network. A server may store and administer software applications, se- curity measures, access to peripherals or external sys- tems, etc. The server does not necessarily have to be an enhanced system, as servers can be specialized as print servers, mail servers, etc. (and several servers may be on a system), but servers performing the bulk ofcentralized or generalized tasks often have more memory, processing speed, and storage than other systems on the network. The software is probably the most important aspect ofa good server. Good network software is robust, configurable, and usually fully multitasking. There are many well-tuned network workstation options that are reliable and do not crash, except in the most unusual of circumstances. Shop around when select- ing network server software; paying a few hundred or thousand extra dollars in terms of the initial cost can often be recouped in six months or less through savings on downtime, software reinstallation, and ad- ministrative costs that accrue on unreliable systems. server agent In server/client systems, software that handles the major processing or protocols and serves a request from a client as a Web server, mail server, or FTP server. Server Message Block protocol 5MB. A client and! or server request-response network protocol for shar- ing resources such as files, ports, printers, and other useful services. 5MB was defined by ffiM in the mid- 1980s and is prevalent on Windows-based systems. It works in conjunction with NetBIOS over TCP/IP. In conjunction with Samba, it can also be used with Linux systems. 5MB is also known as Common In- ternet File System (eIRS). See Samba, 5MB Project, RFC 1001, RFC 1002. © 2003 by CRC Press LLC Service Advertising Protocol See Service Location Protocol. Service Agent A network utility which, when que- ried, provides information about a network service (printer, modem, etc.) such as its URL. Service Location Protocol SLP. An intelligent re- source discovery and registration protocol developed in the mid-1990s. Described as a "quieter" alterna- tive to Service Advertising Protocol (SAP), SLP in- cludes extended attributes information to reduce net- work traffic queries. Thus, a printer may be described in terms of its capabilities (such as duplex printing, PostScript-capable, tabloid paper) and found trans- parently, without the user querying for its IP address. See Service Advertising Protocol, Service Agent, SLIP, RFC 2165. service quality Standards of service established by businesses that include such things as service with- out outages, available lines without lag or busy sig- nals, technical support availability, good data integ- rity, etc.· This is not the same as quality of service (QoS), which has a more specific meaning. Service-Specific Connection-Oriented Protocol SSCOP. A B-ISDN signaling ATM adaptation layer (SAAL) mechanism for managing the establishment, monitoring, and release of data exchanged between signaling peers. In the context ofQ.2931, SSCOP pro- vides error and flow control signaling services some- what analogous to those provided by TCP for Inter- net Protocol (IP). Service-Specific Convergence Sublayer SSCS. A component of the ATM adaptation layer (AAL) that coordinates protocols of the next higher layer with the requirements of the next lower layer, the Com- mon Part Convergence Sublayer (CPCS). See asyn- chronous transfer mode. services-oo-demand SoD. Services provided to an audience on a request basis, rather than on a sched- uled broadcast basis. The concept is not new; in fact, it has been available for media services for over 100 years, but new digital technologies are providing au- tomated services, thus making available cost-effec- tive SoD delivery options which were not previously possible. See audio-on-demand, video-on-demand. Session Announcement Protocol SAP. A network protocol for sending announcements to users that is common to broadcast communications. It is consid- ered distinct from broadcast content and data triggers. SAP version 2 was submitted as an Experimental RFC by Handley et a1. in October 2000. A SAP an- nouncer periodically multi casts an announcement packet to a known multicast address and port. The SAP recipient "listens in" on a SAP address and port for multicast scopes and thus learns of all the sessions being announced so that the sessions may be joined. See enhanced TV, RFC 2974. Session Control Protocol SCPo A simple client/server network protocol to facilitate multiple conversations over a single rep connection. With SCP, parties can establish (or reject) a virtual session over a single transport connection. Session Description Protocol SDP. A network protocol used for announcements and other notifications for multimedia broadcasts. The protocol was described by the Multiparty Multimedia Session Control (MMUSIC) working group of the IETF and was sub- mitted as a Standards Track RFC by Handley and Jacobson in April 1998. SDP provides session sup- port for Internet multicast backbone (Mbone) services and more general realtime multimedia services. The protocol is not intended for the negotiation of media encodings. SDP is designed to convey session directory infor- mation to recipients that can be used in conjunction with avariety of transport protocols, such as Session Announcement Protocol, Hypertext Transport Proto- col, and others. See Session Initiation Protocol, RFC 2327 Session Initiation Protocol SIP. An application-layer signaling protocol for creating, changing, and termi- nating Internet-based telephony, conferencing, mes- saging, and events notification involving one or more participants. I SIP was originally developed with the IETF Multi- party Multimedia Session Control (MMUSIC) work- ing group and continued, as of September 1999, by the IETF SIP working group. Internet Drafts were submitted by the SIP working group in May and Oc- tober 2001. SIP invitations provide a means to convey session descriptions so that compatible media types can be agreed upon by participants. Proxy servers are used to help route requests to users and to assist in fire- wall traversal. SIP runs on top ofa number of differ- ent transport protocols. See Session Description Pro- tocol. See RFC 2543, RFC 3050, RFC 3087. Session Initiation Protocol Forum SIP Forum. A E!f:~;i~~~~~~r~~E;~r~::::J~::~~~1I SET secure electronic transaction. A phrase used in electronic commerce to signify a transaction which is protected by various network security measures such as authentication and verification procedures, digital certificates and signatures, secure servers, etc. set-top box colloq. A media device which sits on top ofa TV set or within a home entertainment compo- nent cabinet to hook into the system in some way. Set- top boxes provide a variety ofcapabilities, including conversion of cable TV signals, provision of Web TV services, etc. Some set-top boxes are proprietary units offered through lease or purchase by a service pro- vider or vendor. SETI Search for ExtraTerrestrial Intelligence. An interesting, federally funded scientific project in which arrays of radiotelescopes are used to search for signs of intelligence in other parts of the universe. The movie Contact (Warner Home Video) provides an idea ofa SETI-like project. The rationale of SETI is that signals can be sent far- ther and faster using radiowaves than by sending spacecraft (SETI is also in favor of spacecraft mis- sions and radio signals sent from spacecraft) and that it's worthwhile to send out signals in the hope that 853 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary other life fOnTIS may intercept them or that humans may intercept the transmissions of other life forms. Unfortunately, to date, no signs of sentient commu- nications have been detected, but SETIconcepts have resulted in the discovery of interesting radiowave sig- nals from distant celestial objects. The Columbus Optical (COSET!) Observatory, a pio- neering observatory located near Columbus, Ohio, conducts searches for extraterrestrial intelligence in the optical spectrum. http://www.seti.org/ http://www.coseti.org/ SF I. single frequency. 2. See SuperFrame. SF signaling See single-frequency signaling. SFOCS See Submarine Fiber Optics Communica- tions Systems. SFfP See Simple File Transfer Protocol. SGML Standard Generalized Markup Language. A markup standard adapted by the International Orga- nization on Standardization (ISO) in 1986 which is not a language, but is designed for specifying the con- tent and structure of a document or document language, with the assumption that the actual output or display of the document may vary according to the output device. SGML allows the development of cross-platform applications and documents, and a document can be processed by an SGML compiler by referencing a document tag definition (DTD). HyperText Markup Language, widely used on the World Wide Web, is a descendant of SGML that in- corporates some of its capabilities. Yuri Rubinsky (1952-1996) was one of the pioneers who enthusiastically did much to promote the use of SGML through educational programs. SGMP See Simple Gateway Monitoring Protocol. SGRAM synchronous graphics random access memory. A type of memory optimized for use in memory-hungry graphics applications, particularly 3D rendering and ray tracing. shadow mask A type of cathode-ray tube (CRT) color display technology which incorporates a thin, perfo- rated metal plate mounted close to the front of the inside of the tube to create a mask through which red, green, or blue (RGB) phosphors can be selectively excited. See cathode-ray tube. Service Environments and Systems The following definitions are related to Service ar- chitectures that can be used in conjunction with Intelligent Networks (INs). See also Intelligent Network. ServiceAccess Code SAC. A specific telephone prefix access code (e.g., 800) for aspecific category of service such as toll free numbers. See Service Management System. Service Control Point SCPo A point that provides access to an Intelligent Network (IN) database, willch is connected to a Service Management System (SMS), and which accesses Internet Protocol (IP) as needed. SCPs are a mechanism for providing advanced services by processing the format or content of transmitted information. Information contained in the SCP may be downloaded by phone service carriers. Architecturally, the Service Control Point is somewhat self-contained, providing services to Intelligent Networks (e.g., those based upon Signaling System No.7) that architecturally separate switch and service functions. Service Switching Points (SSPs) can relay value- added service calls to the SCP which can appropriately complete the call. The ITU-T describes the SSP in Recommendation Q.1205. Service Creation Environment SCE. In the context of Service Control Points interfacing with Intelligent Networks (INs), the SCE enables new telecommunications services to be quickly designed and implemented without necessitating changes in the IN switchingsystem. 854 Service Creation Environment Point SCEP. A protocol used for defining and developing a service, for example, for implementing security features and validating services in networks (often in conjunction with LDAP). The SCEP creates service in conjunction with Service Independent Building-blocks (SIBs) and interfaces with the Service Management Point (SMP). The ITU- T describes the SSP in Recommendation Q.1205. Service Data Point SDP. A standard database designed for use with Intelligent Networks (INs). Contemporary phone services are complex compared to historic analog residential/business telephone lines. Users now have different services from different carriers and different variations of those services offered through bundles, specials, prepaid cards, and carrier offerings in specific regions. These services are stored, configured, and managed through software that must be associated with specific subscribers or paid calling card access. The SDP works in conjunction with Service Control Points (SCPs) to manage this enormous task. In SDP, the customer information is separated from the logic that manages the services. Tills compartmentalization is important for the privacy of subscribers and also enables updates, maintenance, and new services to be easily integrated into the system without side effects in other areas. Fast processing and large data storage facilities are needed to make the system viable. The ITU-T describes the SDP in Recommendation Q.1205. © 2003 by CRC Press LLC Shannon, Daude Elwood (1916-2001)Acelebrated American theorist who contributed significantly to the study and understanding of information theory. The history of communications emphasizes the in- ventors, programmers, and hobbyists who have de- veloped the mechanisms and operations of informa- tion systems, but few people at the time had taken a broad look at what information is, how it relates to the technology (e.g., channel capacity), and what the process ofconveying information entails from amore abstract, theoretical, statistical, and broadly practical viewpoint. Shannon, while working at Bell Laboratories, is credited with bringing together and clearly stating fundamental theories of information in 1948. Shannon is often credited as wholly developing in- formation theory, and his work may have been done independently ofErlang and Kotel'nikov, who devel- oped similartheories many thousands of miles away, but it is derived at least in part from the work of Harry Nyquist, who is cited in Shannon's writings. Nyquist developed principles of communications rates and digital sampling in the 1920s. Shannon has received many awards for his work, in- cluding the National Medal of Science (1966) and the John Fritz medal (1983). See Erlang, Agner; Hagelbarger, David; Kotel'nikov, Vladimir; Nyquist, Harry; sampling theorum. Service Environments and Systems, cont. Service Discovery Protocol SDP. A wireless network resource discovery protocol from Bluetooth that enables applications/devices to seek out and find compatible devices with services that may be useful. Thus, using SDP, Bluetooth devices can connect wireless to nearby services such as printing, digital cameras, etc. SDP was specified by the Bluetooth SIG in 1999. SDP defines how a Bluetooth client application shell discovers available services and their defined characteristics, withoutprior knowledge of the services, as the device enters the accessible "regionof access" of the services. SDP also makes it possible to detect when a service is no longer available. In March 2001, RidgeRun, Inc., announced support for the Bluetooth technology by releasing an Open Source SDP enabling Bluetooth users to get broader access to on-demand wireless services. Service ManagementAccess Point SMAP. In a network architecture, the SMAP interfaces with the Service Management Point and assists in managing user access and services. The ITV- Tdescribes the SSP in Recommendation Q.1205. See Intelligent Network. Service Management Point SMP. In an Intelligent Network (IN) architecture (e.g., SS7), a centralized manager interfacing with a number of other elements such as the Service Creation Environment Point (SeEP), a Service Management Access Point (SMAP), and various Service Control Points (SCPs) and other elements to satisfy value-added service calls from the Service Control Point (SCP). The ITD-T describes SSP in Recommendation Q.l205. See Intelligent Network. Service Management System SMS. A centralized interactive computer system dedicated to coordinating network service-related information. The national 800 numbers are managed in a central database computer from ffiM. This is one type of Service Control Point providing value-added services to Intelligent Networks (INs) known as SMS/800. See SMS/800. Service Profile Identifier SPID. When hooking up ISDN BRI services, the carrier provides the user with a SPill for each number installed, typically two. The SPill points to a memory location in the carrier's central office where ISDN parameters, including which services are enabled for a particular subscriber, are stored. As not all phone carriers have automatic SPill detection, some newer modems can determine what type of ISDN service is connected and configure the SPills accordingly. When connected to carriers with automatic SPill detection, they can configure themselves whether or not a computer is attached to the modem. Modems with these capabilities help compensate for some of the problems traditionally associated with the installation of ISDN services. It is recommended that the subscriber keep a record ofSPID numbers filed away somewhere, as it's easier to look up a lost SPID than to get it again from the phone carrier. Service Switching and Control Point SSCP. An entity in the physical plane that controls network resources and sessions, including the Service Control function, Service Data function, and Service Switching/Call Control functions. Service Switching Point SSP. An entity in the physical plane, the SSP is apoint providing local access and an ISDN interface for a Signaling Transfer Point (STP), which, in tum, provides packet switching for message- based signaling protocols in an Intelligent Network (IN). The SSP can be implemented as a central office switching system capable of communicating with a Service Control Point (SCP) in order to enable switched calls to be routed through a somewhat separate service environment that will then complete the call. The ITV- T describes the SSP in Recommendation Q .1205. See Intelligent Network, Service Control Point. 855 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary Shannon-Hartley Capacity Theorem A theorem that facilitates calculations for assessing an environ- ment with additive white Gaussian noise and its im- plications for relative capacities in communications channels. In other words, capacity has been math- ematically defined in relation to bandwidth, energy per bit, and noise power density, to yield useful in- formation about total signal and noise power, and bandwidth efficiency (in bits per second per Hertz) in communication technologies. Shared Facility Centers A telework organization similar to a branch office, but co-owned or partially community- or freelance professionals-funded, situ- ated in a residential or rural village area, and made commercially viable by the implementation of new communications technologies. See ADVANCE Project, telework. shared tenant services STS. A category of commu- nications services applying to residents of multiple family dwelling units. In these types of residences, cable services, antennas and other communications facilities are often shared among a group of residents within a complex, section, or building. The distinc- tion is important because, historically, many of these residents have been served by independent private carriers considered distinct from local exchange car- riers (LECs) and thus are not bound by the same regu- latory framework. With the advance of technology, private service provider services are becoming more like LEC services and thus are debated and evaluated in the context of changing technologies. ShareView 3000 A Macintosh-based videoconfer- encing system from Creative Labs which supports audio, video, whiteboarding, application and docu- ment sharing, and file transfers over analog phone lines. An IBM-licensed PC-version called ShareVision PC3000 is also available. See Cameo Personal Video System, Connect 918, MacMICA, IRIS, VISIT Video. sheath n. A close-fitting protective covering, usually tubular, often made of plastic. Sheaths can be used to bundle wires, to insulate, to protect from moisture or wear, or to provide identifying colors or symbols. They are c-ommonly used on conducting wires and fiber cables. See conduit. sheave The round, usually rotating track in a string- ing block used to direct a line conductor that is being installed, removed, or temporarily redirected for maintenance or repair. See stringing roller. shell, command shell A computer user interface in- put and display environment which translates user commands into operating system instructions. Shenzhen Bordering Hong Kong, this is a major sup- plier of optical cross-connect, multiplexing, and single- and multimode optical fiber products. In ad- dition to supplying components, the Chinese city puts major emphasis on linking its populace through fi- ber optic backbone network systems. ShermanAntitrustActAn important 1890 U.S. act passed to prevent the establishment of monopolies that could hinder U.S. trade and competition based upon free enterprise. 856 SHF super high frequency. About 3 to 30 Ghz, used for satellite transmissions. ship to shore telephone See marine telephone. shock, electric A sudden, often hazardous, electri- cal stimulation to a living body which may greatly affect nerves and cause convulsive contractions through muscles, possibly endangering the heart muscle. It may also cause severe burning, confusion, and unconsciousness. Light electric shocks are uncomfortable, but not al- ways dangerous and are sometimes used as perim- eter boundaries for livestock or secure areas. Light electric shocks are also used in animal experiments for studying the nervous system and are occasionally used in riot control and law enforcement. It is unwise to open up or attempt to repair cathode- ray tubes (CRTs), which may store a considerable charge, without careful preparation and knowledge of safety procedures. Electric shocks must be taken seriously and, if severe, may require contacting emergency services or the application of cardiopulmonary resuscitation (CPR). Never touch someone who is experiencing shock from an electrical source until the electricity is turned off or the source of the contact knocked away with a nonconducting material. Consult emergency first aid sources for information. Shockley, William Bradford (1910- )An English- born American physicist who worked in the Bell Tele- phone Laboratories from 1936. He discovered the rectifying properties of impure germanium crystals at a time when vacuum tube rectifiers had replaced the old galena and carborundum crystal detectors. This led Shockley to explore the various impurities in germanium and he found electron drift toward the positive or negative pole under controlled conditions. When these solid-state rectifiers were combined, the transistor was born and vacuum tubes superseded. One of the most significant consequences of transis- tors at the time was miniaturization of communica- tions devices and room-sized computing machines. Shoemaker detector A type of electrolytic detector that incorporates a battery and, consequently, requires no outside power source. It consists ofa glass tube with a platinum-sealed point, with a zinc strip rather than the platinum point coming in contact with a mild sulphuric acid solution. Shoemaker detectors were used commercially in wireless telephone receivers in the early 1900s. See electrolytic detector. short circuit, short An unintended or harmful cross connection, of low resistance, of electrical circuits. Short circuits can occur from an excess of solder, in- correctly connected wires, conductive debris (such as a screw falling into a circuit box), worn-out insu- lation in bundled wire, water, or physical bumping of electric conductors, etc. The result is often a sud- den flow of current in the wrong direction or of the wrong magnitude, which can potentially damage components. Some systems are configured to shut down or blow a fuse or breaker in the event of an ex- cess of current or other abnormal electrical activity. See burst, spike. © 2003 by CRC Press LLC shorthaulA short travel or installation distance. The actual length depends upon the situation or medium employed. A short haul for a SCSI cable is about 3 feet or less; above 6 feet, serious signal degradation occurs and special hardware is needed for distances over 12 feet (see Fibre Channel). A short haul for other media may be several yards or thousands of miles. short haul modem A softwarelhardware combina- tion used for short distance communications up to a couple of dozen miles, usually over a copper single- channel line. See baseband modem. ShortMessage Service SMS. A global, wireless, low bandwidth, two-way service first distributed in Eu- rope in the early 1990s and later in North America. SMS provides the capability of transmitting alpha- numeric messages between mobile systems and ex- ternal systems that support paging, email, and voice mail. The handsets used in these services can send or receive at any time, regardless of whether a data or voice call is in progress. SMS is appropriate for applications like stock quotes, paging, short fax and email messages, online quick banking, etc. Short Message Service Center SMSC. A relay and administrative center for Short Message Service (SMS) which provides store and forward services. This is somewhat like an enhanced alphanumeric paging system with two-way service and guaranteed delivery. See Short Message Service. Short Wavelength Fast Ethernet Standard TW EIA/ANSI-785. An industry standard for the trans- mission of data at 100 Mbps over 850-nm optical links approved in June 2001. This technology is also known as 100BASE-SX and provides a cost-effec- tive upgrade path from 10 to 100 Mbps through the use of short wavelength opto-electronic devices. Overall this standard can lower the cost of implement- ing Fiber to the Desktop (FTTD) and Fiber to the Home (FTTH) services. shortwave, shortwave Long-range radio transmis- sion frequencies in approximately the 1.6 to 30 MHz range, above the commercial broadcast bands. Short- wave signals are easier to apprehend at night, due to lowered atmospheric noise and the fact that many shortwave broadcasters prefer to send in the evening hours. Coordinated Universal Time (UTC) is often used as the reference time for broadcasts. The Inter- net has sites that list broadcast times and frequencies for various shortwave stations around the world. See microwave, ionospheric wave, radio. SHT Short Hold Time. Shugart A historic disk development company. Shugart floppy disk drives were used on some of the earliest microcomputers. shunt n. A switch, pipe, detour sign, or other divert- ing mechanism. shunt, electrical In electrical circuits, a means to di- vert some or all of the current. A shunt is sometimes used to divert part ofa current in order to prevent damage to sensitive measuring instruments. Tempo- rary shunts are sometimes established with jumper wires or alligator clip connections. shunt circuit, bypass circuit, detour circuit A cir- cuit configuration through which a specific portion of the current is redirected or subdivided. Often used for diagnostic purposes, temporary arrangements, or circuits in which variable conditions are accommo- dated or where the original current can be more ef- fectively used by dividing it. Shunts are sometimes incorporated into the internal workings of diagnos- tic instruments. shunting error A condition in which current divided through two circuits, as through a component and a measuring instrument, will vary depending upon the frequency. See reversing error. Amateur Radio Technology in Space Two members of the Spacelab Life Sciences c~ McArthur and Searfoss, training with amateur radio equipment for Space Shuttle mission STS-58 in Sep- tember 1993. {NASA/JSC image detail.} Endeavour Astronaut Linda M Godwin uses the Shuttle AmateurRadio Experiment, as did several of the otherSTS-59 crew members, to communicatefrom spacewith ham radio operators and students on Earth. {NASA/JSC image, April 1994.} Shuttle Amateur Radio Experiment SAREX, SAREX-2, SAREX-II. A series of amateur radio ex- periments consisting of equipment and procedures carried as payload aboard a number of U.S. space shuttles. SAREX was designed to provide an op- portunity for school and community groups to 857 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary communicate with astronauts in space and for con- ducting communications experiments with ground- based amateur radio operators. SAREX was later in- tegrated into the International Space Station project and has even been used for emergency messaging. Amateur radio has a long history of cooperation with U.S. aeronautical/space programs leading up to SAREX. In November, 1983, Astronaut Owen Garriott (W5LFL) took his amateur (ham) radio into orbit on the Space Shuttle Columbia STS-9 mission and communicated with Earth amateur radio stations. This tightened the relationship between ham opera- tors and shuttle crewmembers and the experiments continued on future shuttle missions of Columbia, Challenger, Atlantis, Discovery, and Endeavor. Many astronauts are licensed amateur radio operators. In several cases, the entire crew ofa space shuttle mis- sion comprised ham operators (e.g., STS-74). As examples of specific missions, a SAREX payload was carried aboard the Space Shuttle Columbia in 1993 and the Space Shuttle Endeavour in 1993 and 1994. The SAREX-2 amateur radio system was used to contact elementary and middle school children from space on October 22, 1993. SAREX - II was used in 1994 to contact school and Boy Scout groups and also to communicate with Russian cosmonauts aboard the MIR space station. Amateur radio communications were monitored in realtime by ama- teur radio stations via rebroadcasts and the telebridge system. SAREX-II was further used to communicate with the Star City training center outside Moscow, Russia, on 16 April 1994. SAREX evolved into Ama- teur Radio on the International Space Station program (ARISS). Slow Scan Video Converter ~ ~ ;~~~~:~; ::' i:::~t",'i ; \,~'~I 'f"(~~ l~Aia' i ~ J The major unit in an early SAREXcontaining aslow scan video converter, control circuits. andpowersup- plies. The small tape recorder enables recording of video image data on tape for retransmission through the SAREX equipment. [NASA/JSC image. June 1985.J The American Relay Radio League (ARRL) chronicles the full history of the interesting SAREX and pioneer amateur radio experiments in their 858 publications and on their Web sites. NASA provides instructional support materials for SAREX partici- pants on their Division of Education Educational Ser- vices Web site. See Amateur Radio on the interna- tional Space Station, America Relay Radio League, AM SAT. Si symb. silicon. See silicon. SIA 1. Securities Industries Association. 2. See Semi- conductor Industry Association. Sibley, Hiram (1807-1888) Sibley founded the New York and Mississippi Valley Printing Telegraph Com- pany which took on the name of Western Union, sug- gested by Ezra Cornell in the mid-1850s, when it be- gan westward expansion. Sibley remained president during the expansion and Western Union installed the first transcontinental cable in 1861. After the failure of Western Union's first Atlantic cable, Sibley trav- eled to Russia to investigate the installation of a Si- berian-Alaskan communications line, and the Rus- sians offered to sell Alaska to Western Union. Sibley turned down the offer, but alerted the U.S. govern- ment to the opportunity. Along with his colleague, Ezra Cornell, Sibley helped to found Cornell Univer- sity. See Western Union. side circuit In telephone installations and other cir- cuits where additional endstations are desired, but where resources are limited, a side circuit is a means to build an additional circuit using the resources of two adjacent circuits. See phantom circuit. side lobe SL. In a directional antenna, any segment generating a lobe in the antenna pattern other than the primary/main lobe. Antenna patterns are often diagramed with a radiating region around the seg- ment, a region of influence that appears lobe-shaped. In a Cartesian coordinate system, the lobe shows up as a pronounced hump or "shoulder." Side lobe peaks are often described in terms oftheir ratio to the main lobe peak and will vary, depending up the shape of the antenna, its electrical characteristics, and the fre- quencies transmitted. Depending upon the configuration, an antenna may have several side lobes. In general, low side lobes are desired. Antennas that are likely to be physically ad- jacent to one another (toll booths, cell phones, even satellites, etc.) are often designed to deliberately suppress side lobes. In radar sensing, side lobe signals can muddy the ra- dar signal and cause confusion. Sometimes an omni- directional antenna separate from the main antenna is used to send out a reference pulse that is lower than the signal from the main antenna, but stronger than the signal from its side lobes. This enables the tran- sponder to make a few calculations and determine whether a signal came from the main antenna or the side lobe. It can then respond or not respond as ap- propriate. Antenna pattern modeling programs are used for gen- erating and assessing the configuration and influence of side lobes (including thermal emissions). Side lobe interference in antennas can be a problem, but it is not restricted to larger physical structures; side lobe issues are also important to small-scale © 2003 by CRC Press LLC transmissions phenomena such as laser light paths. In optical network links, side lobes can result from various structures integrated into the optical fiber to enhance its transmission qualities. For example, Bragg gratings suffer undesired effects from side lobes in their spectral response. This can reduce the usable bandwidth. side lobe dispersion An undesirable condition in which a directional antenna loses power due to dis- persion of the signals or wave patterns out of the "sides" of the antenna (e.g., portions that are not part of the main lobe). side-looking airborne radar SLAR. A self-illumi- nating (through microwaves) electronic image-cre- ation system derived from a radar beam transmitted perpendicular to the ground track during acquisition from an aircraft. Thus, the signal hits the terrain at a rather flattened angle and the view of the terrain is vertical, revealing fine surface features useful in in- terpretation of the data. The imaging is provided in strips or mosaics, as is true for many satellite imag- ing systems. SLAR imagery is used by geologists, Earth resource scientists, cartographers, engineers, and others. SLAR encompasses real-aperture and synthetic-aperture radar (SAR). SLAR is not used for very precise topographic mapping, as the resolution is only up to about 30m. sideband The frequencies on either side of the main frequency or carrier band in a communications sig- nal. These frequencies are within the modulation en- velope ofa transmission wave, but were originally not used because ofproblems with noise. Later, as technology improved and the demand for airspace continued to grow, sideband transmissions became more interesting, and it was found that one sideband could be transmitted, sometimes even without the carrier wave, and the original wave mathematically "rebuilt" at the receiving end. The advantages in- cluded lower power requirements for the transmis- sion and a narrower wave overall, leaving more room for other transmissions. sidetoneIna telephone receiver, transmitting currents are directed into the receiver to make it possible for the speaker to hear his or her own voice (somewhat like an echo) as a form offeedback mechanism. This has to be carefully controlled so that it doesn't be- come excessive, and various anti-sidetone circuits are applied to minimize feedback and reduce transmis- sion of acoustical noise. In early telephones, the sidetone was loud enough to be distracting. In 1920, G .A. Campbell designed a circui t to reduce the excess current from the line and the local receiver, thus reducing sidetone and improv- ing electrical efficiency. Sidetones are also used in telegraph keying systems as well, to provide feedback to the person keying in the (Morse) code. Sieve A protocol- and OS-independent, extensible mail filtering language proposed as a Standards Track RFC by T. Showalter, in Jan. 2001. Sieve is a lan- guage for filtering email messages at time of final delivery that can be implemented on either the mail server or a mail client. It is suitable for a variety of systems, including Internet Message Access Proto- col (IMAP) servers. Sieve uses IMAIL-compliant messages to enable a user to create filters for organizing incoming elec- tronic mail (email). The language is intended to be powerful enough to be useful, while not being so powerful that it can break out of its operating envi- ronment or wreak havoc on asystem through uncon- trolled variable, loops, or programming bugs. Sieve also facilitates the use of graphical user interfaces (GUIs) for creating and manipulating email filters. See FLAMES, RFC 3028. Siemens Telecom Networks Aprovider oftelecom- munications services and network equipment empha- sizing robust, secure technologies to regional Bell operating companies and independent telephone and holding companies. In March 2001, Siemens AG announced that it was cleared by the Federal Trade Commission to merge with Efficient Networks, Inc. (Texas-based suppliers of high-speed DSL networks). In December 200 1, to- gether with Cingular Wireless, the second largest mobile carrier in the U.S., Siemens announced that the two companies would be working to upgrade their services/systems to support 3G (third generation) mobile communications using Enhanced Data Rates for Global Evolution (EDGE) technology. Siemens, Werner(1816-1892) An American inven- tor who, along with his brother, William, developed the dynamo, a device to convert mechanical energy into electrical energy without the use ofpermanent magnets. In the 1870s, he demonstrated that the ve- locity ofelectrical conductivity through a wire could approximately equal that of light. ~~i~;:::~::~lrEf:!~: {is: :;~::~ ••• rather than the amplitude of the signal itself by first integrating and then delta modulating the signal. It is sometimes called delta-sigma modulation, depend- ing upon the sequence of the processes. It is a form of oversampling and noise-shaping signal conversion scheme. The system came into practical use about two decades after delta modulation was developed in the 1940s, when VLSI technology began to emerge, making it possible to develop practical embodiments of the sys- tem. SDM helped overcome some of the limitations associated with delta modulation, a simple scheme for sampling an analog signal and encoding the samples based upon the previous state. SDM is useful for quantizing and encoding audio sig- nals for transmission over telephone circuits and is also used in image processing. In sigma-delta modulation, an analog signal is quan- tized and the sum of the previous difference and the current signal compared rather than directly compar- ing the current and previous signal. This has greater spectrum independence than delta modulation and "conditions" the shape of the quantized signal in terms of its maximum possible range. SDM is also 859 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary known as pulse density modulation (PDM). While the resolution of SDM is good, the filtering introduces latency and it is not as fast as might be desired for newer, high-speed communications tech- nologies (e.g., multiplexed systems). Thus, research into improving the speed has yielded some deriva- tive schemes. For example, reduced-sample-rate sigma-delta modulation (RSRSD) schemes require only a fraction of the speed necessary for sigma-delta encoding without trading offbandwidth. RSRSD may be applicable to AID and D/ Aconversion for appli- cations such as Extended Digital Subscriber Line (xDSL). There are hobby kits available for demonstrating delta and sigma-delta modulation/demodulation processes. See AID conversion, delta modulation, pulse width modulator. Signal Corps Radio Laboratories SCRL. A facil- ity established at Fort Monmouth in March 1918 as Camp Alfred Vail. The lab was active in classified research including the development of radio direc- tion finding equipment and aviation radio communi- cations. Following World War I, aviation communi- cation research was moved to the Signal Corps Air- craft Radio Laboratory in Ohio. A few years later, a number of labs in Washington, D.C. and New York state were moved to the Fort Monmouth location and it was redesignated as the Signal Corps Laboratories. The lab was used for radio communications devel- opment and adjunct meteorological services. In the early 1930s, marine and aerial sensing were added and radar studies stepped up in the late 1930s. Signal Transfer Point STP. A non-terminal node point which provides access to a database and packet switching for message-based signaling protocols for the Service Control Point (SCP) in an Intelligent Net- work (IN). STPs are widely used in Common Chan- nel Signaling systems. To provide redundancy in a telephone network, Ser- vice Control Points (SCPs) and STPs are usually paired up. Depending on the implementation, the STPs may be arranged hierarchically, with the lower layer serving the smaller or local region and the higher layer serving the larger region. The Intelligent Network is based around Signaling System 7. See Intelligent Network, Service Switching Point. signal-to-noise SIN. A ratio frequently used in electronics and communications to quantify the proportion ofa signal (or communication) that is de- sired and useful to the proportion ofa signal that is undesired and distracting or destructive to the desired signal. It is preferable to have a high signal-to-noise ratio. In electronics the signal can be an electrical or opti- cal signal or a data stream and the noise can be vari- ous types of interference such as crosstalk, echo, spu- rious signals, etc. When Internet Relay Chat (IRC) was first developed, it had a pretty good signal-to-noise ratio in terms of the content of conversations. As the Net has grown, this has changed to the point that public online dis- cussions are often avoided in deference to private 860 chats and moderated email discussions. See noise. Signaling System 1 SS 1. Historically, an interna- tional CCITT-standardized supervision single-tone signaling protocol to signal call requests between tele- phone switchboards. A 500-Hz signal tone at 20 Hz was used. Bell had a similar standard for manual ring down signaling. These days Signaling System 1 is more often associated with newer digital signal- ing systems, i.e., DSS 1. See Digital Subscriber Sig- naling 1. Signaling System2 SS2. Historically, aCCIIT-stan- dardized supervision dual-tone signaling protocol to handle pulse-dial selection. Dual 600/750 Hz signal tones were used. It was somewhat similar to historic Bell radiotelephone dial signaling systems. Signaling System 3 SS3. Historically, aCCIIT-stan- dardized supervision single-tone signaling protocol used in one-way telephone circuits and not intended for use for multinational transit connections. This system was still in use until about the late 1970s. Signaling System 4 SS4. Historically, an interna- tional CCIIT-standardized supervision dual-tone sig- naling protocol for international and terminal tran- sit. SS4 was the first to fully support global direct dialing signaling. SS4 is not entirely obsolete; it is sometimes used for signaling for Voice over Packet (VoP) devices. Signaling System 5 SS5. Historically, an interna- tional CCIIT-standardized supervision dual-tone sig- naling protocol used in T1 and E1 telephone trunks. Dual 2400/2600 Hz tones were used. SS5 is similar to Bell DDD trunks with supervision used in North America. SS5 is not entirely obsolete. Although it has generally been superseded by SS6 and SS7 for sub- scriber systems, testing and maintenance devices for central office equipment sometimes use SS5 signal- ing on digital trunk interfaces for testing gateways and SS5 is still used on analog international gateways. Signaling System 6 SS6. Historically, aCCIIT out- of-band signaling system developed in the 1960s, which is being superseded by Signaling System 7 in North America but is still popular in Europe. Both SS6 and SS7 owe their origins to Common Channel Interoffice Signaling (CCIS). SS6 was the first sys- tem to incorporate packet switching into public switched telephone networks (PSTNs). It supports a message-based protocol for requesting services, simi- lar to the widespread X.25 standard. Signal units of 28 bits each were assembled into data blocks for transmission. See Signaling System 7. Signaling System 7, Signaling System No.7 SS7. SS7 is a common channel network signaling system, descended from Signaling System 6 and its precur- sor Common Channel Interoffice Signaling (CCIS). CCIS was an out-of-band that was inherently more secure than in-band multi frequency signaling systems prevalent at the time. It is sometimes referred to as Common Channel Signaling System 7 (CCS7). SS7, introduced in the 1980s, is more flexible and powerful than earlier versions, making it possible to implement broadband digital services far in advance of basic voice circuits. One of the ways in which SS 7 © 2003 by CRC Press LLC differs from SS6 is that it supports variable-length sig- nal units (up to a defined maximum), while SS6 was constrained to fixed-length units. Unlike earlier phone signaling systems, which oper- ated through many semi-independent switching cen- ters, SS1 brings the communications channels into a more integrated whole. The international data rate for SS7 networks is 64 kbps, although faster data rates are being studied and implemented, such as 1.544 and 2.048 Mbps (inter- national). There are many factors influencing the adoption of SS7, including its flexibility and applicability to both wire line and wireless communications. But one of the fIrSt motivations for switching to SS7 was the devel- opment of phone services that could be dialed using a common area code, regardless of the geographical location of the subscriber (e.g., 800 numbers). Since local databases and switches could not be used to route calls that could be placed to any region, a cen- tral cross-reference registry was needed to associate the 800 number with a regional location and switch- ing office. With SS7's message-passing capabilities, it became straightfolWard to associate a virtual area code with a geographical routing code. This opened the door to other types of virtual area code services, such as emergency 911, pay 900, as well as custom calling services (e.g., Caller ill) and enhanced ser- vices (e.g., number portability). SS7 is now an important aspect of digital telecom- munications services. Users have come to expect the many features that were difficult or impossible to implement over older analog tone-based signaling systems. With its out-of-band architecture, it is also inherently much more secure than historic in-band signaling systems. SS7 is being gradually integrated into ATM/Tl and PCSIUPT networks. See Common Channel Interof- fice Signaling. Signaling System Rl SS Rl. An international ana- log telephony signaling standard, equivalent to Bell's out-of-band 2600 Hz tone in DOD trunks in North America. Signaling System R2 SS R2. An international ana- log telephony signaling standard, equivalent to Bell's out-of-band 3825 Hz tone inserted between voice channels in the carrier system. signature 1. An identifying mark, usually a name, intended to relate a document or other transactional device to the individual associated with the document. A signature is often used as a means to acknowledge understanding or agreement to the terms ofa trans- action, e.g., a contract. See digital signature. 2. In a more general sense, a mark, style, method of doing things, or musical sequence that is identifiable as coming from a specific source or strongly associated with a certain person or group. The signing ofa name in a person's handwriting, the creation ofa painting with a distinctive and uniquely recognizable origin, or even a specific way oftuming a phrase are all ex- amples of signatures in the broader sense. On the In- ternet, individuals often come up with names, signa- ture files, or icons to uniquely and quickly identify themselves to others in much the same way a corpo- ration identifies itself with a trademark or logo. 2. In printing, a grouping of pages that is created in order to organize the pages for binding. Common sizes for signature groups are 8 or 16 pages. With many people now doing their own desktop publishing and submit- ting them to printers over the Internet for publication, it helps to understand some of the basic terms and procedures used to create documents in the printing industry. See fascicle, imposition. SUA See Software & Information Industry Associa- tion. silence compressionA technique used in voice over data network applications which involves removing the pauses and spaces that typically occur in many conversations. This reduces transmission time. Two common techniques typically used together include voice activity detection (VAD), which distinguishes speech from the surrounding background noise, and comfort noise generation (CNG), which creates a low type of static that gives humans a certain comfort level and trust that the line is still active and the call hasn't been cut off. silent discard In packet networking, the discard ofa packet without further processing. The system may log the event and may even store the contents of the discarded packet for later evaluation. silica Silicon dioxide. See silicon. silicon An abundant nonmetallic, tetravalent element (AN 14), widely used in semiconductor technology. Silicon comprises about 26% of the Earth's crust (by weight). Silica occurs in many common forms, in- cluding sand, quartz, flint, and opals. Silica is a main ingredient in glass manufacture. Highly pure silicon can be doped with a number of other elements for use in solid-state devices. silicon detector An early type of radio wave detec- tor similar in some aspects to electrolytic detectors. Silicon is used in place of the electrolyte, making contact with a platinum wire, and the thumbscrew contact with the silicon can be finely adjusted by fil- ing the end of the thumbscrew to a fine point, using a spring with the thumbscrew to assure even pressure. The interaction of the thumbscrew and the silicon sets up a thermoelectric reaction which can be translated into audible waves in the receiver. See detector, elec- trolytic detector. Silicon Graphics Incorporated SGI. A computer company known for innovative software and hard- ware workstation-level computers, especially those with good graphics and sound. SGI was founded in 1981 by James Clark, who later became affiliated with Netscape Communications Corporation. Silicon Valley A region of California with a high den- sity of high':technology companies, many of which pioneered computer technology. The economy, edu- cational institutions, research labs, and climate were all factors that contributed to the growth of technol- ogy companies in Silicon Valley. silicone rubber insulatorA more recent type of in- sulator for installing conducting lines found on utility 861 © 2003 by CRC Press LLC . Sublayer SSCS. A component of the ATM adaptation layer (AAL) that coordinates protocols of the next higher layer with the requirements of the next lower layer, the Com- mon Part Convergence Sublayer (CPCS). See asyn- chronous transfer mode. services-oo-demand SoD. Services provided to an audience on a request basis, rather than on a sched- uled broadcast basis. The concept is not new; in fact, it has been available for media services for over 100 years, but new digital technologies are providing au- tomated services, thus making available cost-effec- tive SoD delivery options which were not previously possible. See audio-on-demand, video-on-demand. Session Announcement. LLC Fiber Optics Illustrated Dictionary communicate with astronauts in space and for con- ducting communications experiments with ground- based amateur radio operators. SAREX was later in- tegrated into the International Space Station project and has even been used for emergency messaging. Amateur radio has a long history of cooperation with U.S standard encapsulation protocol for serial lines, used for point- to-point communications with TCP lIP. SLIP has now been superseded by Point-to-Point Protocol (PPP). See Point-to-Point Protocol, RFC 1055. SERN See Software Engineering Research Network. server