Fiber Optics Illustrated Dictionary - Part 85 docx

Fiber Optics Illustrated Dictionary example, is located. However, there are situations where an absolute URL is not the best solution. Imagine establishing an extensive Web site at www.4-sights.eom that has many document and graphics files hierarchically contained in subdirectories below the main directory. Assume the subdirectories are called does and pies and that you are building a page in the docs directory (absolute URL - http://www.4-sights.comldocs/mydoc.html) that has many images linked from the pies directory. In terms of building the page in HTML, it is tedious to type http://www.4-sights.eom/pies/filenamejpg each time a new image is added to the page with a link. Instead, a relative URL can be used and the image referenced as /pies/filenamejpg. In the context of the current location of the site, it means the same thing. The dot-dot-slash tells the system to go back up a directory and down into the pies subdirectory. Not only is this shorter and easier to type, but it saves a huge amount of time if the entire Web site must be moved en masse to another domain name or Web host (assuming you're not renaming directories or rear- ranging at the same time). With relative URLs, which reference the current location, the links will still point to the same locations in the directory structures as they did on the previous domain location. If the Web- master didn't use relative URLs to build a site for which the domain name is likely to change, rap his or her knuckles. Relative URLs were described in Standards Track RFC 1808 by Fielding in June 1995 and have since become an intrinsic part ofHTML on the World Wide Web. RUS See Rural Utilities Service. Rutherford, Ernest (1871-1937) A New Zealand- born British physicist who contributed substantially to knowledge about atomic physics. Rutherford re- searched at the Cavendish lab studying ionizing gases and following up much of the work of the Curies and Philipp Lenard. He collaborated with Hans Geiger, developer of the Geiger counter, and influenced Paul Villard's studies of gamma rays. RVA recorded voice announcement. A digital or analog recorded or synthesized voice announcement, as on an answering machine. The phone company uses RV As to communicate with callers using touch tone menus and to alert the user to problems (such as an off-hook phone). See Barbe, Jane. RW 1. read/write. 2. see real world. 2. remote work- station. RWhois, rwhois Referral Whois. RWhois is a program for looking up information on the Internet. It is a primarily hierarchical, client/server distributed system for the discovery, retrieval, and maintenance of directory information on computer networks. RWhois facilitates deterministic routing of queries based upon tags, referring the user closer to the source of the information. The RWhois specification defines a directory architecture and a directory access protocol. RWhois extends and enhances its predecessor, Whois (based upon Whols Protocol) in a hierarchical, scalable way in order to meet the increased demands on the Internet. The protocol and its architecture are 832 structurally derived from the Domain Name System (DNS), and concepts from the X.500 Protocol and Simple Mail Transport Protocol (SMTP) have been incorporated into the specification. To use RWhois from a command line shell that supports the utility, type "twhois" in lowercase. Whois is useful for querying about IP numbers, NIC handles, and domain name registrants through domain registry services. For example, a VeriSign domain registry search for crcpress.com yields: Registrant: CRC Press, Inc. (CRCPRESS-DOM) 2000 Corporate Blvd., NW Boca Raton, FL 33431 US Domain Name: CRCPRESS.COM Administrative Contact, Billing Contact: Record last updated on 14-Nov-2001. Record expires on 19-Nov-2003. Record created on 18-Nov-1993. Database last updated on 28-Mar-2002 05:03:00 EST. Domain servers in listed order: NS1.DATARETURN.COM 216.46.236.253 NS2.DATARETURN.COM 63.251.95.25 The RWhois Operational Development Working Group is an IETF group chartered with coordinating, engineering, and operating the RWhois Protocol. See InterNIC, Whois, RFC 2167. RWW See read-while-write. Rx abbrev. receive. Often used in conjunction with Tx (transmit). RxD receive data. Data channel, typically used in serial communications, which is an input for DTE devices and an output for DCE devices. See TxD, RTS, DSR, DTR, RS-232. RZ return to zero. Return to a value of zero for a vari- able, file, or other entity. This is often used in binary signaling system contexts. See RZL. RZ-codeAvariety of signaling codes that are able to return to a zero level to indicate a data value. RZ-AMI (alternate mark inversion), RZ-bipolar, and RZ-uni- polar are examples. RZ codes are used in many transmission schemes for a variety of data encoding and timing synchronization applications. Some theorists make a distinction between RZ and AMI codes on the basis of frequency components, with reference to modulation, with RZ code streams having two components and AMI code streams having one. This distinction has practical implications in terms of filter- ing and resulting distortion in the RZ signal. See Al- ternate Mark Inversion, RZ. RZL return to zero level. Return to a level of zero, which may be zero voltage, zero output (of radio waves, for example), or other varying phenomenon. Zero in this case, usually implies an absence ofa phenomenon, but it may also be a reference point such as OaF. RZL is often used in analog contexts. See RZ. © 2003 by CRC Press LLC S 1. abbrev. sleeve or shield (ground shield) in electrical tennino1 ogy. Cords, wires, and optical cables are protected by sleeves. The designation is sometimes used to describe plug configurations, for example, TRS refers to tip/sleeve or tip/ring/sleeve or tip/ring/ground shield. A common example ofa TRS configuration is a balanced stereo plug on stereo headphones. 2. symb. sulfur. See sulfur. S bus In ISDN networks the data transmission path interconnecting network tenninating equipment (NT 1 or NT2) to addressable devices. The S bus can supply up to 8Wnonnal power or up to 420 mW in re- stricted power situations. Voltages vary from around 31 volts for nonnal and from around 37 volts for re- stricted power. See Sinterface, SBus. Sinterface In ISDN networks, a number of reference points have been specified as R, S, T, and U interfaces. To establish ISDN services, the telephone company typically has to install a number of devices to create the all-digital circuit connection necessary to send and receive digital voice and data transmissions. The S interface is typically used in the u.S. to con- nect an NT1/NT2 network tennination device at the customer's premises to terminal equipment (TE) or terminal adaptors (TA). In parts of Europe, a combined SIT interface provides a four-wire electrical extension interface between network tenninating equipment (NT 1) and up to eight addressable devices. The use of four wires enables a pair of wires to be used for each direction to and from the NTx equipment. The attached devices are typically telephones or computer interfaces. See ISDN interfaces for a diagram. S Series Recommendations A series ofITU-T rec- ommended guidelines for telegraph services tenninal equipment. The publications are available from the ITU- T for purchase from the Net. Since ITU- T specifications and recommendations are widely followed by vendors in the telecommunications industry, those wanting to maximize interoperability with other systems should consult the infonnation dissemi- nated by the ITU-T. A full list of general categories is listed in Appendix C and specific series topics are listed individually in this dictionary, e.g., R. Series Recommendations. See the S Series Recommen- dations chart. S-band A radio wave broadcasting frequency spec- trum ranging from about 1700 to 2360 MHz. S-band is typically used for radar, space communications, and some types of mobile services. It is terrain-sensitive. NASA uses S-band phase modulation to transmit and receive infonnation between orbiting systems and the ground either directly or through relay satellites. NASA's orbiter communications forward links (uplinks) are modulated on acenter carrier frequency of 2106.4 MHz for the primary system and 2041.9 MHz for the secondary system. Two frequencies are used to prevent interference if two orbiters are transmitting simultaneously. Similarly the return links (downlinks) are modulated on acenter carrier frequency of 2287.5 MHz and 2217.5 MHz. The Department of Defense (000) forward links are at lower S-band frequencies than the NASA systems (the return link frequencies are the same as NASA) and are channeled through its own ground stations. Radio waves are not used only for communication. They are also used for navigational tracking and the scientific determination of the characteristics of bod- ies in our solar system. For example, S-band Dop- pler effect experiments were conducted on Apollo 15 missions 14 to 17. These experiments enabled gravi- tational fields and other measurements to be calculated by observing the dynamic motion of the spacecraft through S-band radio waves transmitted between the craft and the Earth. One of the limitations of S-band is that it is terrain- sensitive, which is why it has largely been used in space applications. However, if you are measuring terrain, this limitation becomes an asset. NASA has applied L-band and S-band frequency sensors to the remote sensing ofgeographical characteristics such as soil moisture and ocean salinity. The PALS (Pas- sive/ Active L/S-band Sensor) was first flown in July 1999 for these purposes. See band allocations for a chart. See S-band, optical. S-band, optical For optical communications, an ITU- specified transmission band in the 1450- to 1530-nm range. The portion in the 1450- to 1490-nm range has been called the S+-band. The shorter optical wavelength bands are considered promising for increasing capacity in dense wavelength division multi- plexed transmission systems carrying C- and L-band 833 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary frequencies. However, novel means ofamplification are required to fully realize the potential ofS-band frequencies for this use as traditional erbium-doped amplifiers do not provide the needed gain. Bromage et al. at Lucent Technologies have suggested that Raman amplification could fill this need. See C-band, L-band. ITU-T SSeriesRecommendations Recom. Description S.l Intematiol18.1 Telegraph Alphabet No.2 S.2Coding scheme using International l'elegraphAlphabet No.2 (ITAl) to ~llow the transmission. of capital and sma111etters S.3Transmission characteristics of the local end with its termination (ITA2) S.4 Special use of certain characters of the Intemational Telegraph Alphabet No. 2 8.5 Standardization of page-printing start-stop equipment and ~90pera,tio.n betwe~ pa.ge-printing and tape~printing. start-stop equipment (ITA2) 8.6 Chara~nstics of answerback units (ITA2) S.7 Control of teleprinter motors 8.8 Intercontinental standardization of the .modulation rate of start-stop apparatus and of the use of combination No.4 in figure-shift S.9 Switching equipment of start-stop apparatus S.10 Transmission at reduced-character traDsferrate.over astandardized so- baud telegraph channel S.11 Use of start-stop reperforating ~quipmt:Jltforperforated tap~ retransmission S.12 Conditions that must be satisfied by synchronous systems operating in connection with standard SO-baud teleprinter circuits 8.13 Use on radio circuits of7-unit synchronous systems giving error correction by automatic repetition 8 14 Suppression of unwanted reception ipradi()~~l~sraPh multidestination teleprinter systems S.15 Use of the telex network for data transmission at SO bauds 834 S-band Linear Collider Test Facility A facility at DESY developed to serve as a test bed for the technical aspects of a large-scale 500GeV e+ e- collider to enable S-band technologies to be used more effec- tively with linear colliders for fundamental research in physics. See S-band. S-band Single-Access Transmitter SSAT. A trans- Recom. Description 8.16 Connection to the telex network of an automatic tenninal using aV.24 DeE! DTE interface S.17 Answer-back unitsimulators S.18 Conversion between International Telegraph Alphabet No 2 and InternatioDalAlphabet Ng.S 8.19 Calling and answering in the telex network with automatic terminal equipment 8.20 Automatic clearing procedure for a telex terminal 8.21 Use of display screens in telex machines 8.22 "Conversation impossible"· or prerecorded message in response to JIB:eLLsignals from atelex termiIlal 8.23 Automatic request of the answerback of the terminal of the calling party, by the telex. terminal oCtile calledparty9rby the intemationalnetwork. 8.30 Standardization of basic model page- printing machine using International Alphabet No.5 8.31 Transmission characteristics for start stop data terminalequipment using International Alphabet No. 5 8.32 Answer-back units for 200- and 300- baud start-stop machines. in accordance with Recommendation S.30 8.33 Alphabets·· and·presentation characteristics for the intex service Int~x tenninals-llequirements to effect interworking with the international telex service At1swerbackcodingfor the Intex service S.36 INTEX and similar services - Terminal requirements to effect interworking between terminals operating at different speeds 8.140 Definitions· of essential technical· tenns relating to apparatus for alphabetic telegraphy Supplements S.Supl Minimal specifications for the bilingual (ArabiclLatin) teleprinter © 2003 by CRC Press LLC to set a menu setting or flip a switch to direct the device to use the S- Video port as many devices default to a composite port. See I signal, Q signal. 3 1 4 2 3 1 Two male mini-DINS- Video connectors. On the left, the standard 4-pin cable compatible with a wide variety of camcorders, video monitors, VCRs, etc. On the right, a 7-pin mini-DIN connector for Macintosh computers (e.g., PowerMac AV) and some video capture cards with standard camera interfaces. Both cable ends are keyed to prevent incorrect insertion. Pinouts - standard S- Video: 1 GND ground (y) 2 GND ground (C) 3 Y luminance (light intensity) 4 C chrominance (color) Pinouts- PowerMac AV S- Video (capture cards with female connections may assign pins 6 and 7oppo- site to the Mac pinouts shown here): 1 A GND ground 2 A GND ground 3 Y luminance (light intensity 4 C chrominance (color) 5 PC clock composite video 6 + 12 V no connection 7 j2C data no connection i~!;:=i~~r.~:'~~:e~~:l~:~¥~~~~ ii. MAIL is a developer software kit for building S/ MIME-enabled applications. It is a high-level toolkit providing a plugin engine for secure email mes- saging that enables S/MIME functionality to be integrated into a variety of application types, including EDI software, online service clients, and email clients. The S/MAIL toolkit includes core crypto- graphic components, message formatting, and a security user interface. See Multipurpose Internet Mail Extension, S/MIME. SIMIME A commercial product from RSA Data Se- curity, Inc. for providing interoperable, secure email. S/MIME facilitates the development of interoperable RSA-based security products for electronic messag- ing so that an S/MIME message can be composed and encrypted with one application and decrypted with another. It is based on standard MIME specifications integrated with the Public Key Cryptography Stan- dards (PKCS). See Multipurpose Internet Mail Ex- tension, SIMAIL. SIN See signal-to-noise. SA 1. See Service Agent. 2. source address. In net- working, an address to identify the physical or virtual location of the system initiating a transmission. This mitter installed in the Hubble Space Telescope that sends data gathered by the Hubble to astronomers on Earth using S-band radio frequency signals. Hubble has two of these transmitters and two large communications dishes that direct the data transmissions to orbiting NASA satellites with single-access antennas, where they are collected and relayed to aground station in New Mexico. From there the data are for- warded to the Hubble Space Telescope Science In- stitute in Baltimore. When one of the SSATs failed in 1998, the other was rotated to shoulder some of the load of the failed transmitter. Since the Hubble was designed to be maintained and repaired during space missions, an S-band Single-Access Transmitter was one of the components stored in the Contingency ORU Protec- tive Enclosure (COPE) in the Space Shuttle Discovery's cargo bay within the Orbital Replace- ment Unit Carrier. The Discovery crew was sched- uled to replace the failed transmitter on Day 3of Mis- sion 3A See S-band. The S-band Single-Access Transmitter unit used in the space-based Hubble Space Telescope for transmitting data via orbiting satellites to data processing centers and astronomers on Earth. [NASA image.] S-HTTP See Secure HTTP. S- Video, Super Video A video transmissions standard in which information is carried in two separate signals: chrominance (color) and luminance (bright- ness). It is also known as S-VHS and as Y/C video with Y representing luminance and C representing chrominance (which in turn carries I and Q signals). S- Video provides a higher quality, sharper image than traditional composite video in which chrominance and luminance are transmitted as a combined signal. S- Video is commonly supported on newer monitors, camcorders, and other consumer and professional video devices. Beware of bargain basement S- Video cables; inadequate shielding can result in a type of interference called crosstalk and cancels out the ben- efits ofS- Video. Most S- Video cables are 4-pin mini- DINs but some manufacturers have created custom cables/pinouts for S- Video (e.g., for game boxes). Note that it may not be enough to hook up an S- Video cable to get S- Video output/input; you may also have 835 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary is important for various reasons, depending on the device and the topology of the network, but is used for establishing point-to-point communications, for routing return information, establishing an efficient communications path, auditing secured communications, tracing a path, and many other functions. SAA 1. Standards Association of Australia. See Stan- dards Australia International, Ltd. 2. Supplemental Alert Adapter. A connection device for interfacing alerting devices to analog multiline phones. 3. See Systems Application Architecture. Saco River Telegraph & Telephone Company SRTT. Maine's oldest independent phone company, established in 1889. The company has gone through many changes in its more than 1 DO-year history. By fall 2001, it had published the Saco River Yellow Pages on its Web site and announced the coming availability of DSL services for its subscribers. SAFE 1. Security and Freedom through Encryption. SAFE Act See Security and Freedom through En- cryption Act. See Clipper Chip, Pretty Good Privacy. SAFENET The U.S. Department of Defense's (DoD's) military standard for a Survivable Adaptable Fiber Optic Embedded Network intended to provide mission-critical, networked communications. See Xpress Transport Protocol. safety saddle A wide sling-shaped seat that fits around the buttocks of a worker suspended from a pole, line, building, crane, or other high prominence for the purposes of installing, testing, repairing, or maintaining equipment, especially telecommunications lines. The saddle is usually attached to a line or support hook by a hitch arrangement on the top of the saddle, above the user's belly, although those designed for use when climbing poles may wrap around to the other side of the pole to be hitched up as the lineworker climbs. Safety saddles are typically made of thick, tough, resilient materials (e.g., leather) as the worker's life depends upon their reliability. sag, cable The characteristic downward caternary curve occurring in the center of horizontally hung cables due to the effect of gravity. The curvature is based upon a variety of factors, including the diam- eter of the cable, the "straightline" distance (the distance between two adjacent cable supporting structures such as utility poles), and the flexibility of the materials surrounding the conductive materials, if present. Wind, temperature, and humidity sometimes influence the degree of sag in lighter cables. Cable sag estimates are important, since extra cable must be calculated and ordered to compensate for the effects of sag (the cable displacement), especially over long cable segments. Sag estimates must also be calculated for cables that are fragile and more likely to break and for fiber optic cables in which the angle of the light beam should be as straight as possible to prevent signal loss into the cladding. The sag estimate can help determine the maximum recom- mended distance between attachment points. There are sites on the Web that enable users to calcu- late displacement cable sag errors by inputting cable tension, distance, weight, and gravity force data. 836 sag, voltage A short dip or decrease in voltage from a power source. In "mains" alternating current (AC) power sources, voltage sag can result from lightning storms, fallen trees, malfunctions, and other causes. Loss of power may result in a voltage sag while a backup power supply comes online or while a transformer feeds a load during the electrical fault. In public power distribution systems, those closer to the fault are usually the most affected; those farther from the fault may be buffered by intervening transformer stations. Voltage sags may damage many types of sensitive electronic components, especially in their manufac- ture. As a consequence, the Semiconductor Industry Association (SIA) has developed voltage sag immu- ni ty standards for semiconductor manufacturing equipment. Certification according to the SEMI F47 standard indicates that equipment complies with certain voltage sag duration tolerances. Similarly, the IEEE P 1564 Task Force on Voltage Sag Indices met in 2001 to review a proposed five-step process for developing Sag Indices based upon a draft IEC document (61000-4-30), as well as alternative means of obtaining voltage sag indices. Voltage sag susceptibility testing is a process for iden- tifying weak links in a system through simulated production modes to identify problem areas and to test possible solution scenarios. A power conditioner is a system installed in conjunction with the electrical distribution system to prevent sags. In electrically sensitive fabrications plants, the cost of a conditioner or other sag-prevention product may be less than the cost of loss or productivity or sag-caused damage. saganA tongue- in-cheek tribute to Carl Sagan, indi- catin& a very, very large amount. "Billions and bil- . lions, ' as he would say with infectious enthusiasm in his popular TV series when referring to the many stars in the cosmos. Sagan's premature death was mourned by many amateur astronomers who got their first taste of the wonders of the galaxy and beyond through Sagan's show. Sagan, Carl E. (1934-1996) An American astrono- mer, writer, educator, and inspirational host of the popular "Cosmos" television series on the U.S. Pub- lic Broadcast System (PBS). Sagan was the director of the Laboratory for Planetary Studies and the David Duncan Professor of Astronomy and Space Sciences at Cornell University. See sagan. SAGE Semi-Automatic Ground Enviromnent. A U.S. government security digital communications, detec- tion, and craft control network. SAIL See Stanford Artificial Intelligence Laboratory. Salva i Campillo, Francese (1751-1828) One of the genuine pioneers of telegraphic technology, Salva was a prodigy who received a degree in medicine at the age of 20 and went on to study communicable diseases, promoting the use of the smallpox vaccine. He was a prolific researcher in many fields and developed a type ofunderwater craft, a historic sub- marine, as well as an aeronautic balloon that was demonstrated in flight in 1784. © 2003 by CRC Press LLC Salva was already describing ideas for telegraphs by 1795, before the invention of the voltaic pile, and he designed an electrochemical telegraph signaling system around 1804 in Barcelona, Spain, more than three decades earlier than the Wheatstone/Cooke and Morse telegraphs. Each character was assigned to an electrical wire that produced gas bubbles in an acid bath at the receiving end when current was applied to the cable on the transmitting end. This system provided inspiration for Samuel Thomas von Sommering a few years later. See Sommering, Samuel Thomas; telegraph history. SAM security accounts manager. samarium Sm. A silver rare earth metallic element (AN 62) discovered spectroscopically in samarskite in the late 1800s. Samarium is used for doping cal- cium fluoride crystals for use in lasers. It is one of the rare earth metals used in carbon arc lights. See doping. SambaAn open source client/server system running on Linux systems, Samba facilitates peaceful coex- istence between Unix and Windows platforms. Samba communicates with Windows clients transpar- ently, enabling a Unix system to join a Windows- based ''Network Neighborhood." In the other direction, Windows users can access file and print services and other resources on the Unix system. Communi- cation is facilitated by the Common Internet File Sys- tem (CIFS), the heir to Server Message Block (SMB) protocol. Samba has been ported to other operating systems as well, including VMS, NetWare, and AmigaOS. See Server Message Block protocol. Samba is freely downloadable from the Samba site. http://samba.org/ sampling Recording a signal by quantizing it at in- tervals in order to capture its basic properties, usually also accompanied by saving the samples in a digital or abbreviated form. It is a form of analog to digital conversion. In digital sound sampling, for example, the sound ofa musical instrument or a voice can be digitally sampled a certain number of times per second in order to be able to play back the sound so that it retains and conveys the character of the original, although not necessarily all the information or format of the original. Music synthesizers use sound samples to recreate the sounds of various traditional instruments. Comput- ers use sound samples to alert or amuse users or to enhance video games or other applications. Telephone systems use sound samples to send voice mail an- nouncements or instructions to users or to transmit voice conversations over digital systems. In general, more frequent samples result in better fidelity to the original during playback. However, there are practical and perceptual limits. More frequent sampling makes higher demands on the equipment, requires more memory to store, and more bandwidth to transmit. Since humans can't distinguish the sample from the original above certain parameters, it is not practical to commit extra resources to rec- reating a signal above these limits. In commercial applications, a sampling rate of 44.1 kHz is used on audio CDs. Audio-only DVDs are higher, but some of this is related more to marketing and technical compatibilities than to increased perceptual enjoy- ment. Some sounds, like the sound ofaconcerto played on a violin, are more complex than others (e.g., a door- bell), and require more frequent samples and a greater frequency range to retain the perceptual quality of the original. In some cases, different sampling techniques must be used at different pitch ranges in order to recreate the sound as it is heard by humans. Sampling algorithms are often applied to sound transmissions, but they are also relevant to video transmissions or multimedia transmissions. In videocon- ferencing over slow transmission lines, the image is usually sampled rather than played in realtime, that is, a new still image is grabbed or digitized and transmitted every few seconds or every few minutes. Gen- erally, as in sound sampling, more frequent samples provide greater fidelity to the original. In video sampling, rates of at least 20 to 30 frames per second are perceived by humans as natural motion. See anima- tion, audiographics, pulse code modulation. sampling rate The number ofcaptures of an input, such as light or sound waves, per unit of time. Sam- pling rates for images are generally expressed in frames per second, with 24 or more appearing natural to the viewer. Sampling rates for audio are generally expressed in kilohertz (kHz); an instrument might be sampled at 50 kHz, that is 50,000 bits per second. Higher sampling rates generally require more sophis- ticated equipment, higher processing speeds, and faster transmission speeds, especially if the signals are sent over a network. See sampling, sampling theorem. sampling rate, Nyquist Atheoretical sampling frequency (in terms of rate not wavelength) at which the rate is the minimum separation of samples in a F ou- rier plane that enables a complete reconstruction of the original sampled data. The sampling process it- self may introduce errors into the quantization, so the actual sampling rate needed to reconstruct a signal may be higher than the theoretical Nyquist rate. De- pending upon the application, half the Nyquist rate may be referred to as the Nyquist frequency. The mathematical and theoretical groundwork for defining the Nyquist rate is based on work by Nyquist in 1928 and Shannon in the late 1940s. The theorem has variously been called the Nyquist or Shannon or Nyquist-Shannon sampling theorem. Sampling at rates below the Nyquist rate is called undersampling and results in less-than-perfect reconstruction of the original sampled data, but may be expedient in terms ofresources. Humans are very good at perceptually filling in the blanks when pre- sented with incomplete data. Apicture ofa familiar face can have most of the face removed with scis- sors or blotted out by amarking pen and still be recognizable to many. In the same sense, information can be left out of an image or sound sample and still be recognizable (if not optimal) for many purposes. Sampling beyond the Nyquist level is termed over- 837 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary sampling and is unnecessary in terms ofreconstruc- tion but may be useful in creating redundancy for error correction or transmission purposes. The application ofNyquist rate theories is as much art as science. Since filters are introduced into quantization methods to create high and low cutoff values, some assumptions are made about the signal even before it is sampled. It cannot always be known in advance what these values should be and some adjustments may have to be made based on trial and error. The "complete" reconstruction of a signal is very hard to judge in advance as well. When choos- ing a sampling rate for audio, for example, and play- ing it back to a general audience, it might be accept- able to the audience and seem to them to be identical to the original analog signal. If, on the other hand, the same sample is played to a trained musician, such as a concert performer, that individual might notice significant differences between the original and the reconstruction. In fact, this situation happened in the production of early audio CDs, with many music lov- ers complaining that CD music sounded flat, asitua- tion that has since been improved with experience, technological adjustments, and a better understand- ing of human auditory perception. See sampling, sampling rate, sampling theorem. sampling theorem A theoretical basis for relating discrete representations, or samples, to continuous functions with the implication that the continuous function can be recreated from the discrete representations to a lesser or greater degree, depending upon the characteristics of the continuous function. In sim- pler terms, a sampling theorem is a theoretical context within which analog-to-digital and digital-to- analog mathematical conversion formulas can be developed. Sampling theorems form a theoretical basis for developing formulas and practical applications such as analog sound digitization and reconstruction. These formulas are still evolving and it has been suggested that the Poisson Summation Formulas can be used to prove corresponding sampling theorems and, in turn, give rise to new uniform sampling formulas (Benedetto and Zimmermann, 1997). The Interpola- tion Identity has been proposed as a means to develop a new class of sampling theorems for obtaining efficient discrete-time (DT) systems and the efficient interpolation and reconstruction of samples (Eldar and Oppenheim, 2000). Sampling theorems are often named after the scien- tists who developed them or who set the mathematical groundwork for subsequent sampling theories. There are sampling theorems named for the work of Whittaker in 1915 and for Shannon in the late 1940s. The theories ofWhittaker and Shannon led to important advances in pulse code modulation (PCM). The Shannon sampling theorem is used in uniform sampling, for example, and is especially practical for certain types of transmissions, e.g., FM broadcasts. A variant on this is the Whittaker-Shannon-Kotel'nikov sampling theorem which may be used, for example, for sampling stochastic signals. A. Papoulis developed 838 a generalized expansion theorem for uniform sampling and M. Unser suggested an extension of this for nonband-limited functions. The Papoulis-Gerchberg theorem can be used to recover missing samples in finite-length records of band-limited data and has also been applied to wavelet subspaces (Xia et aI., 1995). More recent applications of sampling theorems in wavelet technologies have resulted in some intrigu- ing quantization and compression algorithms. See Kotel'nikov, Vladimir. In practical applications, most samples are uniform (periodic) and Fourier transforms are often used in the context of sampling theorems. However, not all sampling environments are uniform; multichannel sets may be used or samples may vary according to time and traditional Fourier-based methods of han- dling the data may be impractical. Consequently, re- searchers have been developing general theories to encompass nonuniform sampling in shift-invariant spaces (e.g., Aldroubi et al.). See Fourier transform, pulse code modulation, sampling, sampling rate, wavelet theory. SAN 1. See satellite access node. 2. See storage area network. Sandbox A Sun Microsystems Java security block. Since Java applets are freely shared through public sites on the Internet, there is always reason to ques- tion whether they contain hidden viruses or other de- structive or annoying capabilities. The Sandbox is a means of restricting doubtful applets to a confined area, that is, quarantining them so they cannot affect other data on the disk or other Sandboxes. SANS Institute The System Administration, Net- working, and Security Institute, founded in 1989, is a cooperative research and education organization serving security professionals and systems adminis- trators worldwide. The Institute helps to promote research, incident awareness, and security certification programs related to global network security. http://www.sans.org/ SANZ Standards Association of New Zealand. SAP I. Scientific Advisory Panel. 2. See Second Audio Program. 3. Service Access Point. An interface point in a network, often associated with a specific layer. 4. See Service Advertising Protocol. 5. See Session Announcement Protocol. SAPI Service Access Point Identifier. SAR I. See segmentation and reassembly. 2. syn- thetic aperture radar. SAREX See Shuttle Amateur Radio Experiment. Sarnoff, David (1891-1971) A Russian emigrant to America, Sarnoff was an ambitious, energetic radio operator and Marconi station manager at the Radio Corporation of America (RCA) widely reported to have intercepted the messages ofthe Carpathia when the Titanic struck an iceberg, and to have relayed the messages to relatives and friends of the passengers on the sinking ship. While it is likely that Sarnoff did playa part in relaying the messages, the RCA pro- motional information of this event included a doc- tored picture ofSarnoff at the console, which lends some doubt to the claim that Sarnoffhandled the post © 2003 by CRC Press LLC single-handedly for 72 hours, as widely promoted. Sarnoff became the general manager of the Radio Corporation of American in 1921 and went on to play a large part in its history and development. He is also remembered for his association with the inventor E. Armstrong in the late 1920s which ended abruptly in 1935 with the removal ofArmstrong's test equipment from the premises and the development of television. In 1926, Sarnoff was instrumental in founding the National Broadcasting Company (NBC). The IEEE now honors Sarnoffs contributions with the David Sarnoff Award in Electronics. See Radio Corporation of America. SART See search and rescue radar transponder. SAS I. single address space. 2. simple attachment scheme. 3. Survivable Adaptive Systems. SASL See Simple Security and Authentication Layer. SASMO Syrian Arab Organization for Standardiza- tion and Metrology. SASO Saudi Arabian Standards Organization. sat-, -satAprefix/suffix often prep ended/appended to satellite-related names and technologies. SATCOM, SatCom Satellite Communications. SATCOM is both a colloquial abbreviation and the trade name of quite a number of satellite-related firms and organizations around the world. • North American Treaty Organization Satellite Communications, known as NATO SATCOM, is a branch of the Communications Systems Division that supports the Satellite Communications Project, which in tum provides support to Major NATO Commands. See SATCOM Integrated Test Network. Satcom Resources is a commercial supplier ofsatellite communications systems. SatCom Systems, Inc. is a Federal Commu- nications Commission-licensed supplier of u.S. market mobile satellite services through the MSAT-l satellite system. SatCom Electronics, Inc. provides electronics products for wireless and broadband satellite communications, including DBS- TV satellite services. • The British National Space Centre (BNSC) uses a variation on the name, S@TCOM for a program designed to help companies in the United Kingdom exploit satellite communications and navigations opportunities. SATCOM IntegratedTest Network SATIN. A central testbed for NATO SATCOM experiments in communications services and technologies in a SATCOM environment. SATIN can provide communications to other testbeds and can interconnect with other divi- sions and organizations through NC3A Satellite Ex- perimental Terminal (SET) facilities. Examples of SATIN projects include ATM over SATCOM and Maritime over SATCOM. satellite, artificial A manufactured object launched to orbit the Earth, Moon, or other celestial body. There are currently many communications and Global Po- sitioning System (GPS) satellites in orbit around Earth which send and receive signals to and from ground stations and transportation vehicles (cars, trains, boats, planes, etc.). Satellites now provide the main means for wireless long-distance communications. The first artificial satellite was Sputnik I in 1957, followed by the first geostationary satellite in 1963. See global positioning systems, satellite antennas. satellite, natural A celestial body in orbit around another. satellite access node SAN. Aterrestrial satellite link, usually consisting of an Earth station or Earth station hub. satellite antennas Satellite antennas were originally launched into orbit for military monitoring and communications, space research, and cable TV broadcasting, but increasing numbers serve individual parabolic home receivers and data communications providers. GPS satellites orbit at about 18,000 km (11 ,000 miles) and broadcast satellites at about 36,000 kIn (22,300 miles) altitude. . In its basic form, a broadcast satellite system consists ofabroadcasting station sending signals through an uplink dish aimed directly at a geostationary satellite antenna in synchronized orbit with the Earth. The signal subsequently is sent from the satellite to a downlink dish (parabolic antenna) attached variously to business complexes or rebroadcast stations, and subsequently directed to subscribers through coaxial cable. Some stations broadcast directly through scrambled signals to apartment blocks or individual households. The lead from the downlink dish feeds into the user's television or computer system. See antenna, C-band, feed hom, Global Positioning Sys- tem, Ka-band, Ku-band, microwave antenna, parabolic antenna. satellite broadcast frequencies The various ranges of frequencies over which satellite antenna transmissions take place. These are dependent on many factors, including the type oftransmission, the type of satellite, and regulatory guidelines and restrictions. Broadcast stations typically operate in the C-band, with uplinks at about 6000 MHz and downlinks at about 4000 MHz to rebroadcast stations with power- ful antennas. The frequency levels are tied to the size of the receiving dishes, with higher frequencies more difficult to accommodate technologically, but with the advantage of much smaller receiving dishes. Higher frequencies can broadcast to smaller receivers, mak- ing it possible for some frequencies to be broadcast directly to smaller consumer dishes. See C-band, Ka-band, Ku-band. satellite closet Acentralized wiring closet for interconnection of cables and equipment. In a number of satellite installations, the programming is beamed from the satellite to a central service provider with a satellite receiving dish and, from there, delivered by wire or cable to subscribers, necessitating local loop hookups. See distribution frame. satellite communications A wide variety of radio, television, telephone, data, and other broadcast and two-way wireless communications provided by transmission via orbiting satellites to centralized 839 © 2003 by CRC Press LLC Fiber Optics Illustrated Dictionary distribution providers or individual subscriber satellite dishes. The age of satellite communications began in the late 1950s, with the launch of Sputnik I, although it was described with remarkable insight by Arthur C. Clarke in the 1940s and 1950s in various articles and books. The early satellites did not last long (from a few weeks to a few months) and power consumption and radiation problems had to be solved before wide- spread use became practical. In less than four decades from their modest begin- nings, satellite communications have developed rap- idly and now hundreds of satellites of different de- signs orbit the Earth at various distances. Their lifespans now range from about 5 to 15 years, and most are powered by solar panels with battery backup. It was not long after the first satellites were launched . that they were used by commercial and amateur radio stations. The first television broadcast station to use satellites was the Canadian Broadcasting Corpo- ration, transmitting through ANIK in 1972. Direct broadcast to consumers, rather than through interme- diary stations, did not really become prevalent until the 1990s, when broadcasts ofhigher frequencies became practical and smaller, more convenient satellite dishes were manufactured. Satellites are launched along with spacecraft or shuttle craft into elliptical or geostationary orbits from about 500 Ian to about 36,000 Ian above the Earth and are able either to passively transmit data back to Earth (these are becoming rare), or actively regener- ate or otherwise amplify the signal and retransmit, usually at a different frequency to avoid interference of uplink and downlink signals. Satellites are general purpose with many transponders or specialized for data, voice, broadcast, etc. Broadcast satellites tend to be unidirectional, while data and voice satellites, as for mobile systems, are bidirectional. See AMSAT; Clarke, Arthur C; Global Positioning System; direct broadcast satellite, OSCAR, Syncom, Telstar, and the many listings under satellite services. satellite communications control SCC. The Earth- based station facilities and equipment which control satellite transmissions, including signaling functions, access control, error correction, signal conditioning and noise reduction, etc. satellite constellationA group of satellites in a clus- ter. A group of satellites is commonly used for Glo- bal Positioning System (GPS) applications, for example, where data from three or more related satellites are mathematically manipulated to yield precise positioning information of an Earth location. Satellite Home Viewer Improvement Act SHVIA. An Act signed into law by President Clinton in No- vember 1999. This significant Act modifies a number of existing communications and intellectual prop- erty acts, including the Satellite Home Viewer Act of 1988, the Communications Act, and the u.s. Copy- right Act. It permits satellite broadcasting services companies to retransmit signals within their desig- 840 nated market area (DMA) without paying a royalty, beginning on January 1, 2002. To offset this privilege, satellite carriers must carry the signals of all full- power TV broadcast stations within that market, upon request. The purpose of the Act is to promote competition among multichannel video programming distributors such as cable television and satellite suppliers, while increasing the programming range for subscribers. Satellite broadcasters would now be permitted to provide local broadcast TV signals to all subscribers within the licensed market (Designated Market Area [DMA]) as defined by Nielsen Media Research. This is known as local-into-Iocal service and would ini- tially include only major network affiliates. Unfor- tunately, as the service is optional, unprofitable mar- kets such as rural areas would potentially be neglected by this Act without further legislative actions and in- centives to promote the delivery of services in underserved areas. The Act further permits satellite companies to provide distant network stations to eligible satellite subscribers. Some exemptions are contained in the Act, including an exemption for those subscribing to C-band services on or before October 31, 1999 and exemptions for certain recreational vehicles and trucks. Prior to SHVIA, consumers who terminated cable service had to wait 90 days before receiving satellite TV service. This is no longer mandatory. The Act did not pass without subsequent opposition. A number ofsatellite providers jointly and individually filed suit against the FCC and the Copyright Of- fice on the grounds that the local-to-Iocal service pro- visions were unconstitutional. A number of public broadcasting organizations intervened in the interests of public television stations. Concerns over rural and other underserved populations resulted in other related Acts, including the LOCAL TV Act of 2000. See Grade B signal, LOCAL TV Act of 2000, Na- tional Rural Telecommunications Cooperative, Ru- ral Local Television Signals Act. satellite link A system of transmitters and receivers communicating with a satellite, usually through an active transponder which will amplify and shift the received communications to another frequency before retransmitting on the downlink. Uplinks and downlinks are often managed separately. For example, television broadcasts are primarily in one direction (though interactive TV applications are increasing), while phone and computer data communications are typically in two directions. See geostationary, satellite. satellite scanner See scanner. satellite scatter This has two opposite meanings, as scatter can be the undesirable diffusion and weaken- ing ofa signal or, conversely, a deliberate manipula- tion of the environment to enhance communications. In the latter, it was discovered that ionization of air- borne particles could open up communications windows which otherwise were not available. Thus, bums from launched spacecraft or deliberate "seeding" of high regions with elements like barium could provide © 2003 by CRC Press LLC possibilities for detecting or sending transmissions through these temporary holes. There have also been experiments with heating the ionosphere with high- powered waves to form a type of"aurora" which can facilitate transmissions. Due to their transient nature and the strength of the signals needed, these are not major sources of communications, but it's valuable to understand the nature of the various phenomena and receive occasional glimpses into frequencies emanating from space. satellite services Profit and not-for-profit organizations which provide various types ofsatellite-based communications services. See American Mobile Sat- ellite Corporation, AMSAT, ARIES, Astrolink, Con- stellation Communications, Inc., CyberStar, ECCO, Ellipso, ICO Global Communications, INMARSAT, Globalstar, OrbLink, Skynet, Spaceway, Teledesic. Satellite Work Centers A telework organization similar to a branch office, placed in a residential or rural village area by a business entity, and made com- mercially viable by the implementation of new communications technologies. See ADVANCE Project, Shared Facility Centers, telework. saturation To add or adjust such that no more can be absorbed, or contained. In color applications, saturation refers to color purity. Undithered colors on a computer monitor or printing inks made from primary pigments tend to be highly saturated. SAW Surface acoustic wave. See acoustic wave. Sb symb. antimony. See antimony. SBE See Society of Broadcast Engineers. SBus A Sun Microsystems data bus is used to support a standardized data format that can be transmitted over a wide variety of computer devices and services, including Fast and Gigabit Ethernet, SCSI, Token-Ring, ISDN, parallel connections, graphics adaptors, and frame buffers. Computers equipped with SBus slots can be extended with SBus-compli- ant peripheral cards in the same basic way that PCI peripheral cards are installed in Intel-based PCs and Macintosh systems. Sbus cards may provide a data connection to another device or may provide conversion capabilities. For example, there are SBus cards that serve as PCMCIA adapters, providing one or more Type-II PCMCIA slots for inserting popular PC cards. The SBus format is specified in IEEE 1496-1993. SC- connectorAstandardized optical connector designed for CATV and data network hookups for use with hand or machine-polished fiber filaments. See S T- connector. scalable Adjustable, able to increase or decrease in size, capacity, or other relevant characteristics, without significant degradation in quality of service or functioning. Scalable fonts and images, usually defined as vec- tors, have the capability to adapt to lower and higher screen and printer resolutions, displaying at the best possible resolution for that particular device due to the internal algorithmic nature of the font definition. Scalable images are sometimes called resolution-independent images. Scalab Ie networks allow the system to accommodate to changing conditions. In static environments, scalability is not a critical factor, and nonscalable systems tend to be less expensive. In dynamic environments, such as the Internet or large WAN implementations, scalability can be a crucial factor, especially over time, contributing to the flexibility and usabil- ity ofa system. Many aspects of networks need to be scalable. The system software should be scalable to adapt to smaller and larger numbers of users, sometimes on a minute- to-minute basis. Physical storage mediums need to be scalable to accommodate less or more storage as needed. Routing protocols need to be scalable to accommodate changing topologies and numbers of workstations. Scalable CoherentInterface SCI. A high bandwidth, scalable, media-independent network transmission technology developed in the late 1980s that operates up to about 1 Gbps. It is an ANSI/ISO/IEEE standard (1596-1992). SCI supports parallel distributed mul- tiprocessing and cache-coherent interconnection and fits into the upper mid-range in throughput. It is faster than ATM, Fibre Channel, and Ethernet, but slower than HIPPI-6400, and does not have HIPPI-6400's retransmission capabilities. Initial implementations of SCI tend to be high-end commercial/industrial and military supercomputing applications. scaling ~ t. 1. Sizing, adjusting to size. May be pro- portional scaling, or selective scaling in one or more axes. Scaling is a common operation in image processing. See cropping. 2. Adjusting to capacity, or number of members. In programming there are often scalable ways of designing algorithms. For example, an operating system may have a fixed number of windows which can be open at one time (e.g., maximum of200) or it may be scalable, in which the maximum number of windows is limited only by the system resources, and becomes greater as greater resources are added (e.g., memory, storage space, CPU speed, etc.). Scalable systems are more flexible and less likely to go out of date, but are often more re- source-intensive and sometimes more difficult to program. In network transmissions with a variety of protocols, scaling may occur to accommodate differences in bandwidth, data, or speed capacities of the various systems through which the transmission may travel. scan converterA device for converting a video signal. With computers it is common to take the RGB signal that normally leads to the computer monitor and feed the signal through a scan converter so it can be recorded on a video tape. scan line,scanning line On a display monitor, a nar- row more-or-Iess continuous line illuminated by the movement of the electron beam across the inside surface of the tube. In television broadcasting and raster monitors, these are typically horizontal. On vector monitors, the scan line can be traced in any direction. See raster, vector. scannerA device that samples objects, information, 841 © 2003 by CRC Press LLC . A New Zealand- born British physicist who contributed substantially to knowledge about atomic physics. Rutherford re- searched at the Cavendish lab studying ionizing gases and following up much of the work of the Curies and Philipp Lenard. He collaborated with Hans Geiger, developer of the Geiger counter, and influenced Paul Villard's studies of gamma rays. RVA recorded voice announcement. A digital or analog recorded or synthesized voice announcement, as on an answering machine. The phone company uses RV As to communicate with callers using touch tone menus and to alert the user to problems (such as an off-hook phone). See Barbe, Jane. RW 1. read/write. 2. see real world. 2. remote work- station. RWhois, rwhois Referral Whois. RWhois is a program for looking up information on the Internet. It is a primarily hierarchical, client/server distributed system for the discovery, retrieval, and maintenance of directory information on computer networks. RWhois facilitates deterministic routing of queries based upon tags, referring the user closer to the source of the information. The RWhois specification defines a directory architecture and a directory access protocol. RWhois extends and enhances its predecessor, Whois (based upon Whols Protocol) in a hierarchical, scalable way in order to meet the increased demands on the Internet. The protocol and its architecture are 832 structurally derived from the Domain Name System (DNS), and concepts from the X.500 Protocol and Simple Mail Transport Protocol (SMTP) have been incorporated into the specification. To use RWhois from a command line shell that supports the utility, type "twhois" in lowercase. Whois is useful for querying about IP numbers, NIC handles, and domain name registrants through domain registry services. For example, a VeriSign domain registry search for crcpress.com yields: Registrant: CRC Press, Inc. (CRCPRESS-DOM) 2000 Corporate Blvd., NW Boca Raton, FL 33431 US Domain Name: CRCPRESS.COM Administrative Contact, Billing Contact: Record last updated on 14-Nov-2001. Record expires on 19-Nov-2003. Record created on 18-Nov-1993. Database last updated on 28-Mar-2002 05:03:00 EST. Domain servers in listed order: NS1.DATARETURN.COM 216.46.236.253 NS2.DATARETURN.COM 63.251.95.25 The RWhois Operational. characteristics such as soil moisture and ocean salinity. The PALS (Pas- sive/ Active L/S-band Sensor) was first flown in July 1999 for these purposes. See band allocations for a chart. See S-band, optical. S-band, optical For optical communications, an ITU- specified transmission band in the 145 0- to 1530-nm range. The portion in the 145 0- to 1490-nm range has been called the S+-band. The shorter optical wavelength bands are considered. Replace- ment Unit Carrier. The Discovery crew was sched- uled to replace the failed transmitter on Day 3of Mis- sion 3A See S-band. The S-band Single-Access Transmitter unit used in the space-based

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