Fiber Optics Illustrated Dictionary - Part 85 docx

Data channel, typically used in serial communications, which is an input for DTE devices and an output for DCE devices.. To establish ISDN services, the telephone com-pany typically has

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.

Interms 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

im-age 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 Ifthe

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

be-come 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 ofthe work ofthe Curies and

Philipp Lenard He collaborated with Hans Geiger,

developer ofthe Geiger counter, and influenced Paul

Villard's studies of gamma rays

RVA recorded voice announcement Adigital or

ana-log recorded or synthesized voice announcement, as

on an answering machine The phone company uses

RVAsto communicate with callers using touchtone

menus and to alert the user to problems (such as an

off-hook phone) See Barbe, Jane

RW1 read/write 2 see real world 2 remote

work-station

RWhois, rwhois Referral Whois RWhois is a

pro-gram for looking up information on the Internet.Itis

a primarily hierarchical, client/server distributed

sys-tem for the discovery, retrieval, and maintenance of

directory information on computer networks RWhois

facilitates deterministic routing ofqueries based upon

tags, referring the user closer to the source of the

in-formation The RWhois specification defines a

direc-tory architecture and a direcdirec-tory access protocol

RWhois extends and enhances its predecessor, Whois

(based upon Whols Protocol) in a hierarchical,

scal-able way in order to meet the increased demands on

the Internet The protocol and its architecture are

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 main registry services For example, a VeriSign do-main 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

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

RZreturn to zero Return to a value ofzero for a vari-able, file, or other entity This is often used in binary signaling system contexts See RZL

RZ-code Avariety ofsignaling 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 trans-mission schemes for a variety of data encoding and timing synchronization applications Some theorists make a distinction between RZ and AMI codes on the basis offrequency components, with reference to modulation, with RZ code streams having two com-ponents and AMI code streams having one This dis-tinction has practical implications in terms of filter-ing and resultfilter-ing distortion in the RZ signal See Al-ternate Mark Inversion, RZ

RZLreturn 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 phe-nomenon, but it may also be a reference point such

asOaF.RZL is often used in analog contexts See RZ

trical tennino1ogy Cords, wires, and optical cables

are protected by sleeves The designation is

some-times used to describe plug configurations, for

ex-ample, TRS refers totip/sleeve or tip/ring/sleeve or

tip/ring/ground shield Acommon example ofa TRS

configuration is a balanced stereo plug on stereo

headphones 2.symb sulfur See sulfur.

Sbus In ISDN networks the data transmission path

interconnecting network tenninating equipment (NT1

or NT2) to addressable devices The S bus can

sup-ply up to 8 W nonnal 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 S interface, SBus

S interfaceInISDN networks, a number ofreference

points have been specified as R, S, T, and U

inter-faces To establish ISDN services, the telephone

com-pany 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 ofEurope, a combinedSIT interface provides

a four-wire electrical extension interface between

network tenninating equipment(NT1) and up to eight

addressable devices The use of four wires enables a

pair ofwires to be used for each direction to and from

theNTxequipment The attached devices are

typi-cally telephones or computer interfaces See ISDN

interfaces for a diagram

S Series Recommendations A series ofITU-T

rec-ommended guidelines for telegraph services

tenni-nal equipment The publications are available from

the ITU-T for purchase from the Net Since ITU-T

specifications and recommendations are widely

fol-lowed by vendors in the telecommunications

indus-try,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

trum ranging from about 1700 to 2360 MHz S-band

is typically used for radar, space communications, and some types ofmobile 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 communicationsforward links

(up-links) are modulated on a center 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 transmit-ting simultaneously Similarly thereturn links

(down-links) are modulated on a center carrier frequency of 2287.5 MHz and 2217.5 MHz The Department of Defense(000)forward links are at lower S-band quencies than the NASA systems (the return link fre-quencies 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 ofthe characteristics ofbod-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 calcu-lated by observing the dynamic motion of the space-craft through S-band radio waves transmitted be-tween 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 of geographical 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 wave-length bands are considered promising for increas-ing capacity in dense wavelength division multi-plexed transmission systems carrying C- and L-band

Fiber Optics Illustrated Dictionary

frequencies However, novel means of amplification

are required to fully realize the potential of S-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'elegraphAlphabetNo.2 (ITAl)to

~llowthetransmission.ofcapital

andsma111etters

S.3Transmission characteristics ofthe

local endwithits termination

(ITA2)

S.4 Special use of certain charactersof

theIntemational Telegraph

Alphabet No 2

8.5 Standardizationofpage-printing

start-stop equipment and

~90pera,tio.n betwe~pa.ge-printing

and tape~printing.start-stop

equipment (ITA2)

8.6 Chara~nsticsofanswerbackunits

(ITA2)

S.7 Controlofteleprintermotors

8.8 Intercontinental standardization of

the modulation rateofstart-stop

apparatusandof the useof

combination No.4infigure-shift

S.9 Switchingequipmentofstart-stop

apparatus

S.10 Transmission at reduced-character

traDsferrate over a standardized

so-baud telegraph channel

S.11 Useof start-stopreperforating

~quipmt:Jltforperforated tap~

retransmission

S.12 Conditions that must be satisfiedby

synchronous systems operatingin

connectionwithstandard SO-baud

teleprinter circuits

8.13 Useon radiocircuitsof7-unit

synchronoussystemsgivingerror

correction by automatic repetition

8 14 Suppression ofunwanted reception

ipradi()~~l~sraPh multidestination

teleprinter systems

S.15 Use ofthetelex network for data

transmission atSObauds

S-band Linear Collider Test Facility Afacility at DESY developed to serve as a test bed for the tech-nical aspects of a large-scale 500GeVe+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 TransmitterSSAT.A

trans-Recom Description 8.16 Connectiontothetelexnetwork of an

automatic tenninal usingaV.24 DeE! DTE interface

S.17 Answer-back unitsimulators

S.18 Conversion between International

Telegraph Alphabet No 2and

InternatioDalAlphabetNg.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 ofdisplay screens in telex

machines

8.22 "Conversation impossible"· or

prerecorded messageinresponseto JIB:eLLsignals from a telex termiIlal

8.23 Automaticrequest of theanswerbackof

the terminalof thecalling party, bythe

telex.terminal oCtile calledparty9r by the intemationalnetwork

8.30 Standardizationofbasic model

page-printing machine using International Alphabet No.5

8.31 Transmission characteristics for start

stopdataterminalequipment using International Alphabet No 5 8.32 Answer-back units for 200- and

300-baud start-stop machines.inaccordance

with Recommendation S.30 8.33 Alphabets·· and·presentation

characteristics for the intex service Int~x tenninals-llequirementsto effect interworking with the international telex service At1swerbackcodingfor the Intex service

S.36 INTEX and similar services - Terminal

requirementsto effectinterworking between terminals operating at different speeds

8.140 Definitions·of essential technical· tenns

relating to apparatusforalphabetic telegraphy

Supplements S.Supl Minimal specifications for the bilingual

(ArabiclLatin) teleprinter

vice 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 vari-ety ofcamcorders, video monitors, VCRs, etc On the right, a 7-pin mini-DIN connectorfor Macintosh com-puters (e.g., PowerMac AV) and some video capture cards with standard camera interfaces Both cable ends are keyed to prevent incorrect insertion.

Pinouts - standardS-Video:

3 Y luminance (light intensity)

4 C chrominance (color) Pinouts - PowerMac AVS-Video (capture cards with female connections may assign pins6and7 oppo-site to the Mac pinouts shown here):

3 Y luminance (light intensity

4 C chrominance (color)

5 PC clock composite video

6 +12V no connection

7 j2C data no connection

MAIL is a developer software kit for building S/

MIME-enabled applications It is a high-level tool-kit providing a plugin engine for secure email mes-saging that enables S/MIME functionality to be in-tegrated into a variety of application types, includ-ing EDI software, online service clients, and email clients The S/MAIL toolkit includes core crypto-graphic components, message formatting, and a se-curity 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 ofinteroperable 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

SINSee 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

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

commu-nications dishes that direct the data transmissions to

orbiting NASA satellites with single-access antennas,

where they are collected and relayed to a ground

sta-tion in New Mexico From there the data are

for-warded to the Hubble Space Telescope Science

In-stitute in Baltimore

When one ofthe 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 3

ofMis-sion 3A See S-band

The S-band Single-Access Transmitter unit used in

the space-based Hubble Space Telescope for

trans-mitting 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

stan-dard 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 inturncarries I and Q signals)

S-Video provides a higher quality, sharper image than

traditionalcompositevideo 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 ofbargain 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

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

communica-tions, tracing a path, and many other functions

SAA 1 Standards Association ofAustralia 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 1DO-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

SAFEActSee 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 saddleA 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

telecommunica-tions 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

de-signed 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, cableThe 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

dis-tance between two adjacent cable supporting

struc-tures 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

pos-sible 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

sag, voltageA 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 trans-former feeds a load during the electrical fault In pub-lic 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.Asa consequence, the Semiconductor Industry Association (SIA) has developed voltage sag immu-nity standards for semiconductor manufacturing equipment Certification according to the SEMI F47 standard indicates that equipment complies with cer-tain voltage sag duration tolerances

Similarly, the IEEE P1564 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 pro-duction modes to identify problem areas and to test possible solution scenarios

Apower conditioner is a system installed in conjunc-tion with the electrical distribuconjunc-tion system to prevent sags In electrically sensitive fabrications plants, the cost of a conditioner or other sag-prevention prod-uct may be less than the cost of loss or prodprod-uctivity

or sag-caused damage

saganAtongue-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, CarlE (1934-1996)AnAmerican 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 ofthe Laboratory for Planetary Studies and the David Duncan Professor ofAstronomy and Space Sciences

at Cornell University See sagan

SAGESemi-Automatic Ground Enviromnent AU.S government security digital communications, detec-tion, and craft control network

SAIL See StanfordArtificial Intelligence Laboratory Salva i Campillo, Francese(1751-1828) One ofthe 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 de-veloped a type of underwater craft, a historic sub-marine, as well as an aeronautic balloon that was demonstrated in flight in 1784

1795, before the invention of the voltaic pile, and he

designed an electrochemical telegraph signaling

sys-tem 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

pro-vided inspiration for Samuel Thomas von Sommering

a few years later See Sommering, Samuel Thomas;

telegraph history

SAM security accounts manager

samariumSm 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

SambaAnopen 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

direc-tion, 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/

samplingRecording a signal by quantizing it at

in-tervals in order to capture its basic properties,

usu-ally also accompanied by saving the samples in a

tal or abbreviated form It is a form ofanalog to

digi-tal conversion In digidigi-tal sound sampling, for

ex-ample, the sound of a 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 thatitretains 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 usComput-ers 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

Ingeneral, more frequent samples result in better

fi-delity 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

ap-plications, a sampling rate of 44.1 kHz is used on

of this is related more to marketing and technical compatibilities than to increased perceptual enjoy-ment

Some sounds, like the sound ofa concerto 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 ofthe original In some cases, different sampling techniques must be used at different pitch ranges in order to re-create the sound as it is heard by humans

Sampling algorithms are often applied to sound missions, but they are also relevant to video trans-missions or multimedia transtrans-missions 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 trans-mitted every few seconds or every few minutes Gen-erally, as in sound sampling, more frequent samples provide greater fidelity to the original In video sam-pling, 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 rateThe number of captures 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 natu-ral to the viewer Sampling rates for audio are gener-ally 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 theo-rem

sampling rate, NyquistA theoretical sampling fre-quency (in terms ofrate not wavelength) at which the rate is the minimum separation of samples in a Fou-rier plane that enables a complete reconstruction of the original sampled data The sampling process it-selfmay 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

undersamplingand results in less-than-perfect recon-struction of the original sampled data, but may be expedient in terms of resources Humans are very good at perceptually filling in the blanks when pre-sented with incomplete data A picture of a familiar face can have most of the face removed with scis-sors or blotted out by a marking pen and still be rec-ognizable 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-Fiber Optics Illustrated Dictionary

samplingand is unnecessary in terms of

reconstruc-tion but may be useful in creating redundancy for

er-ror correction or transmission purposes

The application of Nyquist rate theories is as much

art as science Since filters are introduced into

quan-tization methods to create high and low cutoff

val-ues, 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

ad-justments 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 samplchoos-ing 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.Infact, this situation happened in the

production ofearly audio CDs, with many music

lov-ers complaining that CD music sounded flat, a

situa-tion that has since been improved with experience,

technological adjustments, and a better

understand-ing ofhuman auditory perception See samplunderstand-ing,

sam-pling rate, samsam-pling 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

represen-tations to a lesser or greater degree, depending upon

the characteristics ofthe continuous function In

sim-pler terms, a sampling theorem is a theoretical

con-text within which analog-to-digital and

digital-to-analog mathematical conversion formulas can be

de-veloped

Sampling theorems form a theoretical basis for

de-veloping 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

effi-cient discrete-time (DT) systems and the effieffi-cient

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

mathemati-cal 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

impor-tant advances in pulse code modulation (PCM) The

Shannon sampling theorem is used in uniform

sam-pling, for example, and is especially practical for

cer-tain 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

a generalized expansion theorem for uniform sam-pling and M Unser suggested an extension ofthis 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 re-search, 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.Aninterface 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 of Sarnoff at the console, which lends some doubt to the claim that Sarnoffhandled the post

Sarnoff became the general manager of the Radio

Corporation ofAmerican 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

Armstrongin the late 1920s which ended abruptly in

1935 with the removal ofArmstrong's test equipment

from the premises and the development oftelevision

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 ofAmerica

SARTSee search and rescue radar transponder

SAS I single address space 2 simple attachment

scheme 3 Survivable Adaptive Systems

SASLSee Simple Security and Authentication Layer

SASMOSyrian Arab Organization for

Standardiza-tion and Metrology

SASOSaudi Arabian Standards Organization

sat-, -satA prefix/suffix often prepended/appended

to satellite-related names and technologies

SATCOM, SatCom Satellite Communications

SATCOM is both a colloquial abbreviation and the

trade name ofquite a number ofsatellite-related firms

and organizations around the world

• North American Treaty Organization Satellite

Communications, known as NATO

SATCOM, is a branch ofthe Communications

Systems Division that supports the Satellite

Communications Project, which intum

pro-vides support to Major NATO Commands

See SATCOM Integrated Test Network

Satcom Resources is a commercial supplier

of satellite 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

electron-ics products for wireless and broadband

sat-ellite 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

communi-cations and navigations opportunities

SATCOM Integrated Test NetworkSATIN

Acen-tral testbed for NATO SATCOM experiments in

com-munications 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, artificialA 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

ground stations and transportation vehicles (cars, trains, boats, planes, etc.) Satellites now provide the main means for wireless long-distance communica-tions The first artificial satellite was Sputnik I in

1957, followed by the first geostationary satellite in

1963 See global positioning systems, satellite antennas satellite, naturalA celestial body in orbit around another

satellite access nodeSAN Aterrestrial satellite link, usually consisting of an Earth station or Earth station hub

satellite antennasSatellite antennas were originally launched into orbit for military monitoring and com-munications, space research, and cable TV broadcast-ing, but increasing numbers serve individual para-bolic home receivers and data communications pro-viders GPS satellites orbit at about 18,000km (11 ,000 miles) and broadcast satellites at about 36,000kIn(22,300 miles) altitude

Inits basic form, a broadcast satellite system consists

of a broadcasting station sending signals through an uplink dish aimed directly at a geostationary satel-lite 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, para-bolic antenna

satellite broadcast frequenciesThe various ranges offrequencies over which satellite antenna transmis-sions take place These are dependent on many fac-tors, including the type of transmission, 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 ofthe 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 closetA centralized wiring closet for inter-connection of cables and equipment.Ina 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 communicationsA wide variety of radio, television, telephone, data, and other broadcast and two-way wireless communications provided by transmission via orbiting satellites to centralized

Fiber Optics Illustrated Dictionary

distribution providers or individual subscriber

satel-lite dishes

The age ofsatellite 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

Inless 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

ra-dio 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 of higher frequencies

became practical and smaller, more convenient

sat-ellite 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

ofuplink 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 constellation Agroup ofsatellites in a

clus-ter A group of satellites is commonly used for

Glo-bal Positioning System (GPS) applications, for

ex-ample, where data from three or more related

satel-lites are mathematically manipulated to yield precise

positioning information of an Earth location

Satellite Home Viewer Improvement Act SHVIA

AnAct signed into law by President Clinton in

No-vember 1999 This significant Act modifies a

num-ber ofexisting communications and intellectual

prop-erty acts, including the Satellite Home Viewer Act of

1988, the Communications Act, and theu.s.

Copy-right Act It permits satellite broadcasting services

companies to retransmit signals within their

desig-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 pro-vide 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 un-derserved areas

The Act further permits satellite companies to pro-vide distant network stations to eligible satellite sub-scribers 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 sat-ellite TV service This is no longer mandatory The Act did not pass without subsequent opposition

A number of satellite providers jointly and individu-ally 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 ofpublic television stations Concerns over rural and other underserved populations resulted in other re-lated 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 be-fore retransmitting on the downlink Uplinks and downlinks are often managed separately For ex-ample, television broadcasts are primarily in one di-rection (though interactive TV applications are in-creasing), while phone and computer data commu-nications are typically in two directions See geosta-tionary, 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 ofthe environment to enhance communicamanipula-tions

Inthe latter, it was discovered that ionization of air-borne particles could open up communications win-dows which otherwise were not available Thus, bums from launched spacecraft or deliberate "seeding" of high regions with elements like barium could provide

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

com-munications, but it's valuable to understand the

na-ture ofthe various phenomena and receive occasional

glimpses into frequencies emanating from space

satellite servicesProfit and not-for-profit

organiza-tions which provide various types of satellite-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 CentersA telework organization

similar to a branch office, placed in a residential or

rural village area by a business entity, and made

mercially viable by the implementation of new

com-munications technologies See ADVANCE Project,

Shared Facility Centers, telework

saturationTo add or adjust such that no more can

be absorbed, or contained In color applications,

satu-ration refers to color purity Undithered colors on a

computer monitor or printing inks made from primary

pigments tend to be highly saturated

SAWSurface acoustic wave See acoustic wave

Sbsymb.antimony See antimony

SBESee Society ofBroadcast Engineers

SBusA Sun Microsystems data bus is used to

sup-port a standardized data format that can be

transmit-ted over a wide variety of computer devices and

ser-vices, 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- connectorA standardized optical connector

de-signed for CATV and data network hookups for use

with hand or machine-polished fiber filaments See

ST- connector

scalableAdjustable, able to increase or decrease in

size, capacity, or other relevant characteristics,

with-out 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-in-dependentimages

to changing conditions In static environments, scal-ability is not a critical factor, and nonscalable sys-tems tend to be less expensive In dynamic environ-ments, such as the Internet or large WAN implemen-tations, 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 ofusers, 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 ac-commodate changing topologies and numbers of workstations

Scalable Coherent InterfaceSCI Ahigh bandwidth, scalable, media-independent network transmission technology developed in the late 1980s that operates

up to about 1Gbps 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 pro-cessing See cropping 2 Adjusting to capacity, or number of members In programming there are of-ten scalable ways of designing algorithms For ex-ample, an operating system may have a fixed num-ber of windows which can be open at one time (e.g., maximum of200) oritmay be scalable, in which the maximum number ofwindows is limited only by the system resources, and becomes greater as greater re-sources 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 pro-gram In network transmissions with a variety ofpro-tocols, scaling may occur to accommodate differ-ences in bandwidth, data, or speed capacities of the various systems through which the transmission may travel

scan converterA device for converting a video sig-nal 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 lineOn a display monitor, a nar-row more-or-Iess continuous line illuminated by the movement ofthe electron beam across the inside sur-face of the tube.Intelevision broadcasting and ras-ter monitors, these are typically horizontal On vec-tor monivec-tors, the scan line can be traced in any direc-tion See raster, vector

scannerA device that samples objects, information,