SDTP specifies a transport protocol and an associated control protocol PPP-SDTP and PPP-SDCP to be used in conjunction with PPP protocols.. A B-ISDN signaling ATM adaptation layer SAAL m
Trang 1Fiber Optics Illustrated Dictionary
lasers and dopants (deliberate impurities such as
se-lenium) may be added to promote the movement of
electrons through the material With the application
of current, the component lases with a slightly wider
range offrequencies and a slightly wider beam spread
than traditional lasers See gallium arsenide;
Kazarinov, Rudolf; laser diode
semiconductor optical amplifier SOA Asolid state
component for increasing gain inan optical
transmis-sion This is important for achieving power and
dis-tance of sufficient magnitude for effective
transmis-sions See amplifier
SEQUEL See SQL
sequential Anonoverlapping succession or series, in
chronological or data order, with no significant
in-tervening time or data See concurrent, consecutive,
parallel, serial
serial clock transmit external SCTE Adata stream
common to serial cables used to connect computer
modems For example, on 25-pin serial
communica-tions conneccommunica-tions, one of the pins may be assigned
to carry the external clock signal from the data
ter-minal equipment (DTE)
serial communication A means of transferring data
one element at a time, often through a single wire or
trace in a circuit While it may not seem very fast or
efficient, serial communication is easily implemented
and very commonly used in computing systems The
RS-232 standard is the most common specification
for the physical/pin connections for serial
communi-cations See modem, parallel, RS-232
Serial Data Transport Interface SDTI An
emerg-ing packet data standard for the transport of audio,
video, and data among various multimedia systems,
including video servers, cameras, VCRs, editing
sys-tems, etc., especially in professional broadcast video
environments SDTI provides a network environment
for video data exchange, without a lot of network
overhead.Itsupports single-direction, point-to-point,
compressed-data connections snTI has been well
received due to the many advantages of digital
sys-tems, including the reduction ofgeneration loss
com-mon to analog editing and transmission loss
associ-ated with analog broadcasting
SDTI (SMPTE 305M) evolved from SDI (SMPTE
259M), which is used for transporting uncompressed
audio/visual signals between digital broadcast and
post-production devices The two formats are
me-chanically and electrically compatible to support
co-existence in the same facility
SDTI,developedbytheSMPTE PT20.04Workgroup
on Packetized Television Interconnections, extended
this concept by enabling compressed video to be
ex-changed without the need for frequent compression!
decompression processes that could slow or degrade
the transmission SDTI has been recommended by the
EBU/SMPTE Task Force for Harmonized Standards
for the Exchange of Programme Material as
Bitstreams
Serial Data Transport Protocol SDTP A network
data protocol which provides a means of transporting
serial data streams over PPP links SnTP arose out
of the work of the TR30.1 ad hoc committee in the mid-1990s to provide a standard means for synchro-nous data compression SDTP specifies a transport protocol and an associated control protocol (PPP-SDTP and PPP-SDCP) to be used in conjunction with PPP protocols See Point-to-Point Protocol, RFC 1963
serial interface card Aprinted circuit card which fits into a slot in a computer or other computerized de-vice or piggybacks on a motherboard to provide stan-dardized electrical connections for the synchronous serial transmission ofdigital data The connection on the card is typically a 25-pin D connector On con-sumer desktop computer systems, most serial inter-face cards support data rates up to about 28,800 or 38,400 bps.Aserial interface card is a common way
to connect remote computer terminals and data mo-dems to a computer See RS-232
Serial Line Interface Protocol, Serial LineIPSLIP Originating with an early 1980s 3COM UNET TCP/
IP implementation, SLIP became a defacto standard
encapsulation protocol for serial lines, used for point-to-point communications with TCPlIP SLIP has now been superseded by Point-to-Point Protocol (PPP) See Point-to-Point Protocol, RFC 1055
SERN See Software Engineering Research Network server A system which provides services to other computers connected to it through a network Aserver may store and administer software applications, se-curity measures, access to peripherals or external sys-tems, etc The server does not necessarily have to be
an enhanced system, as servers can be specialized as print servers, mail servers, etc (and several servers may be on a system), but servers performing the bulk
of centralized or generalized tasks often have more memory, processing speed, and storage than other systems on the network
The software is probably the most important aspect
of a good server Good network software is robust, configurable, and usually fully multitasking There are many well-tuned network workstation options that are reliable and do not crash, except in the most unusual ofcircumstances Shop around when select-ing network server software; payselect-ing a few hundred
or thousand extra dollars in terms of the initial cost can often be recouped in six months or less through savings on downtime, software reinstallation, and ad-ministrative costs that accrue on unreliable systems server agent In server/client systems, software that handles the major processing or protocols and serves
a request from a client as a Web server,mailserver,
or FTP server
Server Message Block protocol 5MB Aclient and!
or server request-response network protocol for shar-ing resources such as files, ports, printers, and other useful services.5MB was defined by ffiM in the mid-1980s and is prevalent on Windows-based systems
Itworks in conjunction with NetBIOS over TCP/IP
In conjunction with Samba, it can also be used with Linux systems.5MBis also known as Common In-ternet File System(eIRS).See Samba,5MB Project, RFC 1001, RFC 1002
Trang 2Service Agent A network utility which, when
que-ried, provides information about a network service
(printer, modem, etc.) such as its URL
Service Location Protocol SLP.Anintelligent
re-source discovery and registration protocol developed
in the mid-1990s Described as a "quieter"
alterna-tive to Service Advertising Protocol (SAP), SLP
in-cludes extended attributes information to reduce
net-work traffic queries Thus, a printer may be described
in terms of its capabilities (such as duplex printing,
PostScript-capable, tabloid paper) and found
trans-parently, without the user querying for its IP address
See Service Advertising Protocol, Service Agent,
SLIP, RFC 2165
service quality Standards of service established by
businesses that include such things as service
with-out with-outages, available lines withwith-out lag or busy
sig-nals, technical support availability, good data
integ-rity, etc.· This is not the same as quality of service
(QoS), which has a more specific meaning
Service-Specific Connection-Oriented Protocol
SSCOP A B-ISDN signaling ATM adaptation layer
(SAAL) mechanism for managing the establishment,
monitoring, and release of data exchanged between
signaling peers.Inthe context ofQ.2931, SSCOP
pro-vides error and flow control signaling services
some-what analogous to those provided by TCP for
Inter-net Protocol (IP)
Service-Specific Convergence Sublayer SSCS A
component of the ATM adaptation layer (AAL) that
coordinates protocols of the next higher layer with
the requirements of the next lower layer, the
Com-mon Part Convergence Sublayer (CPCS) See
asyn-chronous transfer mode
services-oo-demand SoD Services provided to an
audience on a request basis, rather than on a
sched-uled broadcast basis The concept is not new; in fact,
it has been available for media services for over 100
years, but new digital technologies are providing
au-tomated services, thus making available
cost-effec-tive SoD delivery options which were not previously
possible See audio-on-demand, video-on-demand
Session Announcement Protocol SAP A network
protocol for sending announcements to users that is
common to broadcast communications It is
consid-ered distinct from broadcast content and data triggers
SAP version 2 was submitted as an Experimental
RFC by Handley et a1 in October 2000 A SAP
an-nouncer periodically multicasts an announcement
packet to a known multicast address and port The
SAP recipient "listens in" on a SAP address and port
for multicast scopes and thus learns ofall the sessions
being announced so that the sessions may be joined
See enhanced TV, RFC 2974
Session Control Protocol SCPo Asimple client/server
network protocol to facilitate multiple conversations
over a singlerepconnection With SCP, parties can
establish (or reject) a virtual session over a single
transport connection
Session Description Protocol SDP Anetwork protocol
multimedia broadcasts The protocol was described
by the Multiparty Multimedia Session Control (MMUSIC) working group ofthe IETF and was sub-mitted as a Standards Track RFC by Handley and Jacobson in April 1998 SDP provides session sup-port for Internet multicast backbone (Mbone) services and more general realtime multimedia services The protocol is not intended for the negotiation of media encodings
SDP is designed to convey session directory infor-mation to recipients that can be used in conjunction with a variety of transport protocols, such as Session Announcement Protocol, Hypertext Transport Proto-col, and others See Session Initiation ProtoProto-col, RFC 2327
Session Initiation Protocol SIP.Anapplication-layer signaling protocol for creating, changing, and termi-nating Internet-based telephony, conferencing, mes-saging, and events notification involving one or more
SIP was originally developed with the IETF Multi-party Multimedia Session Control (MMUSIC) work-ing group and continued, as of September 1999, by the IETF SIP working group Internet Drafts were submitted by the SIP working group in May and Oc-tober 2001
SIP invitations provide a means to convey session descriptions so that compatible media types can be agreed upon by participants Proxy servers are used
to help route requests to users and to assist in fire-wall traversal SIP runs on top of a number of differ-ent transport protocols See Session Description Pro-tocol See RFC 2543, RFC 3050, RFC 3087
Session Initiation Protocol Forum SIP Forum A
E!f:~;i~~~~~~r~~E;~r~::::J~::~~~1I
SET secure electronic transaction A phrase used in electronic commerce to signify a transaction which
is protected by various network security measures such as authentication and verification procedures, digital certificates and signatures, secure servers, etc
set-top box colloq Amedia device which sits on top
of a TV set or within a home entertainment compo-nent cabinet to hook into the system in some way Set-top boxes provide a variety ofcapabilities, including conversion ofcable TV signals, provision ofWebTV services, etc Some set-top boxes are proprietary units offered through lease or purchase by a service pro-vider or vendor
SETI Search for ExtraTerrestrial Intelligence.An interesting, federally funded scientific project in which arrays ofradiotelescopes are used to search for signs ofintelligence in other parts ofthe universe The movieContact (Warner Home Video) provides an
idea of a SETI-like project
The rationale of SETI is that signals can be sent far-ther and faster using radiowaves than by sending spacecraft (SETI is also in favor of spacecraft mis-sions and radio signals sent from spacecraft) and that it's worthwhile to send out signals in the hope that
Trang 3Fiber Optics Illustrated Dictionary
other life fOnTIS may intercept them or that humans
may intercept the transmissions of other life forms.
Unfortunately, to date, no signs of sentient
commu-nications have been detected, but SETI concepts have
resulted in the discovery of interesting radiowave
sig-nals from distant celestial objects.
The Columbus Optical (COSET!) Observatory, a
pio-neering observatory located near Columbus, Ohio,
conducts searches for extraterrestrial intelligence in
the optical spectrum http://www.seti.org/
http://www.coseti.org/
SF I single frequency 2 See SuperFrame.
SF signaling See single-frequency signaling.
SFOCS SeeSubmarine Fiber Optics
Communica-tions Systems.
SFfP See Simple File Transfer Protocol.
SGML Standard Generalized Markup Language A
markup standard adapted by the International
Orga-nization on Standardization (ISO) in 1986 which is
not a language, but is designed for specifying the
con-tent and structure of a document or document
language, with the assumption that the actual output
or display of the document may vary according to the output device SGML allows the development of cross-platform applications and documents, and a document can be processed by an SGML compiler
by referencing a document tag definition (DTD) HyperText Markup Language, widely used on the World Wide Web, is a descendant of SGML that in-corporates some of its capabilities.
Yuri Rubinsky (1952-1996) was one of the pioneers who enthusiastically did much to promote the use of SGML through educational programs.
SGMP See Simple Gateway Monitoring Protocol SGRAM synchronous graphics random access memory A type of memory optimized for use in memory-hungry graphics applications, particularly 3D rendering and ray tracing.
shadow mask A type ofcathode-ray tube (CRT) color display technology which incorporates a thin, perfo-rated metal plate mounted close to the front of the inside of the tube to create a mask through which red, green, or blue (RGB) phosphors can be selectively excited See cathode-ray tube.
Service Environments and Systems The following definitions are related to Service
ar-chitectures that can be used in conjunction with
Intelligent Networks (INs) See also Intelligent
Network.
Service Access Code SAC.
A specific telephone prefix access code (e.g., 800)
for a specific category of service such as toll free
numbers See Service Management System.
Service Control Point SCPo
A point that provides access to an Intelligent
Network (IN) database, willch is connected to a
Service Management System (SMS), and which
accesses Internet Protocol (IP) as needed SCPs
are a mechanism for providing advanced services
by processing the format or content oftransmitted
information Information contained in the SCP
may be downloaded by phone service carriers.
Architecturally, the Service Control Point is
somewhat self-contained, providing services to
Intelligent Networks (e.g., those based upon
Signaling System No.7) that architecturally
separate switch and service functions.
Service Switching Points (SSPs) can relay
value-added service calls to the SCP which can
appropriately complete the call The ITU- T
describes the SSP in Recommendation Q.1205.
Service Creation Environment SCE.
In the context of Service Control Points
interfacing with Intelligent Networks (INs), the
SCE enables new telecommunications services
to be quickly designed and implemented without
necessitating changes in the IN switching system.
Service Creation Environment Point SCEP.
A protocol used for defining and developing a service, for example, for implementing security features and validating services in networks (often in conjunction with LDAP) The SCEP creates service in conjunction with Service Independent Building-blocks (SIBs) and interfaces with the Service Management Point (SMP) The ITU- T describes the SSP in Recommendation Q.1205.
Service Data Point SDP.
A standard database designed for use with Intelligent Networks (INs) Contemporary phone services are complex compared to historic analog residential/business telephone lines Users now have different services from different carriers and different variations of those services offered through bundles, specials, prepaid cards, and carrier offerings in specific regions These services are stored, configured, and managed through software that must be associated with specific subscribers or paid calling card access The SDP works in conjunction with Service Control Points (SCPs) to manage this enormous task.
In SDP, the customer information is separated from the logic that manages the services Tills compartmentalization is important for the privacy of subscribers and also enables updates, maintenance, and new services to be easily integrated into the system without side effects
in other areas Fast processing and large data storage facilities are needed to make the system viable The ITU- T describes the SDP in Recommendation Q.1205.
Trang 4American theorist who contributed significantly to
the study and understanding of information theory
The history of communications emphasizes the
in-ventors, programmers, and hobbyists who have
de-veloped the mechanisms and operations of
informa-tion systems, but few people at the time had taken a
broad look at what information is, how it relates to
the technology (e.g., channel capacity), and what the
process ofconveying information entails from a more
abstract, theoretical, statistical, and broadly practical
viewpoint
Shannon, while working at Bell Laboratories, is
credited with bringing together and clearly stating
Shannon is often credited as wholly developing in-formation theory, and his work may have been done independently ofErlang and Kotel'nikov, who devel-oped similar theories many thousands ofmiles away, but it is derived at least in part from the work ofHarry Nyquist, who is cited in Shannon's writings Nyquist developed principles of communications rates and digital sampling in the 1920s
Shannon has received many awards for his work, in-cluding the National Medal ofScience (1966) and the John Fritz medal (1983) See Erlang, Agner; Hagelbarger, David; Kotel 'nikov, Vladimir; Nyquist, Harry; sampling theorum
Service Environments and Systems, cont
Service Discovery ProtocolSDP
A wireless network resource discovery protocol
from Bluetooth that enables applications/devices
to seek out and find compatible devices with
services that may be useful Thus, using SDP,
Bluetooth devices can connect wireless to nearby
services such as printing, digital cameras, etc
SDP was specified by the Bluetooth SIG in 1999
SDP defines how a Bluetooth client application
shell discovers available services and their
defined characteristics, without prior knowledge
ofthe services, as the device enters the accessible
"region ofaccess" ofthe services SDP also makes
it possible to detect when a service is no longer
available
In March 2001, RidgeRun, Inc., announced
support for the Bluetooth technology by releasing
an Open Source SDP enabling Bluetooth users to
get broader access to on-demand wireless
services
Service Management Access PointSMAP
In a network architecture, the SMAP interfaces
with the Service Management Point and assists
in managing user access and services The
ITV-T describes the SSP in Recommendation Q.1205
See Intelligent Network
Service Management PointSMP
In an Intelligent Network (IN) architecture (e.g.,
SS7), a centralized manager interfacing with a
number of other elements such as the Service
Creation Environment Point (SeEP), a Service
Management Access Point (SMAP), and various
Service Control Points (SCPs) and other elements
to satisfy value-added service calls from the
Service Control Point (SCP) The ITD-T
describes SSP in Recommendation Q.l205 See
Intelligent Network
Service Management SystemSMS
A centralized interactive computer system
dedicated to coordinating network service-related
information The national 800 numbers are
managed in a central database computer from
ffiM This is one type of Service Control Point
providing value-added services to Intelligent
Networks (INs) known as SMS/800 See SMS/800
Service Profile IdentifierSPID
When hooking up ISDN BRI services, the carrier provides the user with a SPill for each number installed, typically two The SPill points to a memory location in the carrier's central office where ISDN parameters, including which services are enabled for a particular subscriber, are stored As not all phone carriers have automatic SPill detection, some newer modems can determine what type of ISDN service is connected and configure the SPills accordingly When connected to carriers with automatic SPill detection, they can configure themselves whether or not a computer is attached to the modem Modems with these capabilities help compensate for some of the problems traditionally associated with the installation of ISDN services
It is recommended that the subscriber keep a record ofSPID numbers filed away somewhere,
as it's easier to look up a lost SPID than to get it again from the phone carrier
Service Switching and Control PointSSCP
An entity in the physical plane that controls network resources and sessions, including the Service Control function, Service Data function, and Service Switching/Call Control functions
Service Switching PointSSP
Anentity in the physical plane, the SSP is a point providing local access and an ISDN interface for a Signaling Transfer Point (STP), which, in
message-based signaling protocols in an Intelligent Network (IN)
The SSP can be implemented as a central office switching system capable of communicating with a Service Control Point (SCP) in order to enable switched calls to be routed through a somewhat separate service environment that will then complete the call The ITV-T describes the SSP in Recommendation Q.1205 See Intelligent Network, Service Control Point
Trang 5Fiber Optics Illustrated Dictionary
that facilitates calculations for assessing an
environ-ment with additive white Gaussian noise and its
im-plications for relative capacities in communications
channels.Inother words, capacity has been
math-ematically defined in relation to bandwidth, energy
per bit, and noise power density, to yield useful
in-formation about total signal and noise power, and
bandwidth efficiency (in bits per second per Hertz)
in communication technologies
similar to a branch office, but co-owned or partially
community- or freelance professionals-funded,
situ-ated in a residential or rural village area, and made
commercially viable by the implementation of new
communications technologies See ADVANCE
Project, telework
commu-nications services applying to residents of multiple
family dwelling units In these types of residences,
cable services, antennas and other communications
facilities are often shared among a group ofresidents
within a complex, section, or building The
distinc-tion is important because, historically, many ofthese
residents have been served by independent private
carriers considered distinct from local exchange
car-riers (LECs) and thus are not bound by the same
regu-latory framework With the advance of technology,
private service provider services are becoming more
like LEC services and thus are debated and evaluated
in the context of changing technologies
videoconfer-encing system from Creative Labs which supports
audio, video, whiteboarding, application and
docu-ment sharing, and file transfers over analog phone
lines An IBM-licensed PC-version called
ShareVision PC3000 is also available See Cameo
Personal Video System, Connect 918, MacMICA,
IRIS, VISIT Video
sheathn.Aclose-fitting protective covering, usually
tubular, often made of plastic Sheaths can be used
to bundle wires, to insulate, to protect from moisture
or wear, or to provide identifying colors or symbols
They are c-ommonly used on conducting wires and
fiber cables See conduit
sheaveThe round, usually rotating track in a
string-ing blockused to direct a line conductor that is being
installed, removed, or temporarily redirected for
maintenance or repair See stringing roller
shell, command shellA computer user interface
in-put and display environment which translates user
commands into operating system instructions
ShenzhenBordering Hong Kong, this is a major
sup-plier of optical cross-connect, multiplexing, and
single- and multimode optical fiber products In
ad-dition to supplying components, the Chinese city puts
major emphasis on linking its populace through
fi-ber optic backbone network systems
passed to prevent the establishment of monopolies
that could hinder U.S trade and competition based
upon free enterprise
SHF super high frequency About 3 to 30 Ghz, used for satellite transmissions
ship to shore telephoneSee marine telephone shock, electricA sudden, often hazardous, electri-cal stimulation to a living body which may greatly affect nerves and cause convulsive contractions through muscles, possibly endangering the heart muscle It may also cause severe burning, confusion, and unconsciousness
Light electric shocks are uncomfortable, but not al-ways dangerous and are sometimes used as perim-eter boundaries for livestock or secure areas Light electric shocks are also used in animal experiments for studying the nervous system and are occasionally used in riot control and law enforcement
It is unwise to open up or attempt to repair cathode-ray tubes (CRTs), which may store a considerable charge, without careful preparation and knowledge
of safety procedures
Electric shocks must be taken seriously and, ifsevere, may require contacting emergency services or the application ofcardiopulmonary resuscitation (CPR) Never touch someone who is experiencing shock from an electrical source until the electricity is turned off or the source of the contact knocked away with a nonconducting material Consult emergency first aid sources for information
English-born American physicist who worked in the Bell Tele-phone Laboratories from 1936 He discovered the rectifying properties of impure germanium crystals
at a time when vacuum tube rectifiers had replaced the old galena and carborundum crystal detectors This led Shockley to explore the various impurities
in germanium and he found electron drift toward the positive or negative pole under controlled conditions When these solid-state rectifiers were combined, the transistor was born and vacuum tubes superseded One of the most significant consequences of transis-tors at the time was miniaturization of communica-tions devices and room-sized computing machines Shoemaker detectorA type of electrolytic detector that incorporates a battery and, consequently, requires
no outside power source It consists of a glass tube with a platinum-sealed point, with a zinc strip rather than the platinum point coming in contact with a mild sulphuric acid solution Shoemaker detectors were used commercially in wireless telephone receiversin the early 1900s See electrolytic detector
short circuit, shortAnunintended or harmful cross connection, of low resistance, of electrical circuits Short circuits can occur from an excess of solder, in-correctly connected wires, conductive debris (such
as a screw falling into a circuit box), worn-out insu-lation in bundled wire, water, or physical bumping
of electric conductors, etc The result is often a sud-den flow of current in the wrong direction or of the wrong magnitude, which can potentially damage components Some systems are configured to shut down or blow a fuse or breaker in the event of an ex-cess of current or other abnormal electrical activity See burst, spike
Trang 6actual length depends upon the situation or medium
employed A short haul for a SCSI cable is about 3
feet or less; above 6 feet, serious signal degradation
occurs and special hardware is needed for distances
over 12 feet (see Fibre Channel) A short haul for
other media may be several yards or thousands of
miles
combina-tion used for short distance communicacombina-tions up to a
couple of dozen miles, usually over a copper
single-channel line See baseband modem
Short Message ServiceSMS Aglobal, wireless, low
bandwidth, two-way service first distributed in
Eu-rope in the early 1990s and later in North America
SMS provides the capability of transmitting
alpha-numeric messages between mobile systems and
ex-ternal systems that support paging, email, and voice
mail The handsets used in these services can send
or receive at any time, regardless of whether a data
or voice call is in progress SMS is appropriate for
applications like stock quotes, paging, short fax and
email messages, online quick banking, etc
administrative center for Short Message Service
(SMS) which provides store and forward services
This is somewhat like an enhanced alphanumeric
paging system with two-way service and guaranteed
delivery See Short Message Service
EIA/ANSI-785.Anindustry standard for the
trans-mission of data at 100 Mbps over 850-nm optical
links approved in June 2001 This technology is also
known as 100BASE-SX and provides a
cost-effec-tive upgrade path from 10 to 100 Mbps through the
use of short wavelength opto-electronic devices
Overall this standard can lower the cost
ofimplement-ing Fiber to the Desktop (FTTD) and Fiber to the
Home (FTTH) services
transmis-sion frequencies in approximately the 1.6 to 30 MHz
range, above the commercial broadcast bands
Short-wave signals are easier to apprehend at night, due to
lowered atmospheric noise and the fact that many
shortwave broadcasters prefer to send in the evening
hours Coordinated Universal Time (UTC) is often
used as the reference time for broadcasts The
Inter-net has sites that list broadcast times and frequencies
for various shortwave stations around the world See
microwave, ionospheric wave, radio
SHT Short Hold Time
Shugart floppy disk drives were used on some ofthe
earliest microcomputers
shuntn.A switch, pipe, detour sign, or other
divert-ing mechanism
shunt, electricalIn electrical circuits, a means to
di-vert some or all of the current A shunt is sometimes
used to divert part of a current in order to prevent
damage to sensitive measuring instruments
Tempo-rary shunts are sometimes established with jumper
wires or alligator clip connections
cuit configuration through which a specific portion ofthe current is redirected or subdivided Often used for diagnostic purposes, temporary arrangements, or circuits in which variable conditions are accommo-dated or where the original current can be more ef-fectively used by dividing it Shunts are sometimes incorporated into the internal workings of diagnos-tic instruments
shunting errorAcondition in which current divided through two circuits, as through a component and a measuring instrument, will vary depending upon the frequency See reversing error
Amateur Radio TechnologyinSpace
Two members of the Spacelab Life Sciencesc~
McArthur and Searfoss, training with amateur radio equipment for Space Shuttle mission STS-58 in Sep-tember 1993 {NASA/JSC image detail.}
EndeavourAstronaut Linda M Godwin uses the Shuttle Amateur Radio Experiment, as did several of the other STS-59 crew members, to communicatefrom space with ham radio operators and students on Earth {NASA/JSC image, April1994.}
SAREX-2, SAREX-II Aseries of amateur radio ex-periments consisting of equipment and procedures carried as payload aboard a number of U.S space shuttles SAREX was designed to provide an op-portunity for school and community groups to
Trang 7Fiber Optics Illustrated Dictionary
communicate with astronauts in space and for
con-ducting communications experiments with
ground-based amateur radio operators SAREX was later
in-tegrated into the International Space Station project
and has even been used for emergency messaging
Amateur radio has a long history ofcooperation with
U.S aeronautical/space programs leading up to
SAREX In November, 1983, Astronaut Owen
Garriott (W5LFL) took his amateur (ham) radio into
orbit on the Space ShuttleColumbia STS-9 mission
and communicated with Earth amateur radio stations
This tightened the relationship between ham
opera-tors and shuttle crewmembers and the experiments
continued on future shuttle missions ofColumbia,
Challenger, Atlantis, Discovery, and Endeavor Many
astronauts are licensed amateur radio operators In
several cases, the entire crew of a space shuttle
mis-sion comprised ham operators (e.g., STS-74)
Asexamples ofspecific missions,aSAREX payload
was carried aboard the Space ShuttleColumbia in
1993 and the Space ShuttleEndeavour in 1993 and
1994 The SAREX-2 amateur radio system was used
to contact elementary and middle school children
from space on October 22, 1993 SAREX-II was used
in 1994 to contact school and Boy Scout groups and
also to communicate with Russian cosmonauts
aboard the MIR space station Amateur radio
communications were monitored in realtime by
ama-teur radio stations via rebroadcasts and the telebridge
system SAREX-II was further used to communicate
with the Star City training center outside Moscow,
Russia, on 16 April 1994 SAREX evolved into
Ama-teur Radio on the International Space Station program
(ARISS)
Slow Scan Video Converter
~ ~
;~~~~:~; ::' i:::~t",'i ;
\,~'~I 'f"(~~ l~Aia' i ~ J
The major unit in an early SAREXcontaining a slow
scan video converter, control circuits andpower
sup-plies The small tape recorder enables recording of
video image data on tape for retransmission through
the SAREXequipment [NASA/JSC image June 1985.J
The American Relay Radio League (ARRL)
chronicles the full history of the interesting SAREX
and pioneer amateur radio experiments in their
publications and on their Web sites NASA provides instructional support materials for SAREX partici-pants on their Division ofEducation Educational Ser-vices Web site See Amateur Radio on the interna-tional Space Station, America Relay Radio League, AMSAT
Sisymb silicon See silicon.
SIA 1 Securities Industries Association 2 See Semi-conductor Industry Association
York and Mississippi Valley Printing Telegraph Com-pany which took on the name ofWestern Union, sug-gested by Ezra Cornell in the mid-1850s, when it be-gan westward expansion Sibley remained president during the expansion and Western Union installed the first transcontinental cable in 1861 After the failure
of Western Union's first Atlantic cable, Sibley trav-eled to Russia to investigate the installation of a Si-berian-Alaskan communications line, and the Rus-sians offered to sell Alaska to Western Union Sibley turned down the offer, but alerted the U.S govern-ment to the opportunity Along with his colleague, Ezra Cornell, Sibley helped to found Cornell Univer-sity See Western Union
side circuitIn telephone installations and other cir-cuits where additional endstations are desired, but where resources are limited, a side circuit is a means
to build an additional circuit using the resources of two adjacent circuits See phantom circuit
side lobeSL In a directional antenna, any segment generating a lobe in the antenna pattern other than the primary/main lobe Antenna patterns are often diagramed with a radiating region around the seg-ment, a region ofinfluence that appears lobe-shaped
In a Cartesian coordinate system, the lobe shows up
as a pronounced hump or "shoulder." Side lobe peaks are often describedin terms oftheir ratio to the main lobe peak and will vary, depending up the shape of the antenna, its electrical characteristics, and the fre-quencies transmitted
Depending upon the configuration, an antenna may have several side lobes In general, low side lobes are desired Antennas that are likely to be physically ad-jacent to one another (toll booths, cell phones, even satellites, etc.) are often designed to deliberately suppress side lobes
In radar sensing, side lobe signals can muddy the ra-dar signal and cause confusion Sometimes an omni-directional antenna separate from the main antenna
is used to send out a reference pulse that is lower than the signal from the main antenna, but stronger than the signal from its side lobes This enables the tran-sponder to make a few calculations and determine whether a signal came from the main antenna or the side lobe It can then respond or not respond as ap-propriate
Antenna pattern modeling programs are used for gen-erating and assessing the configuration and influence
of side lobes (including thermal emissions) Side lobe interference in antennas can be a problem, but it is not restricted to larger physical structures; side lobe issues are also important to small-scale
Trang 8In optical network links, side lobes can result from
various structures integrated into the optical fiber to
enhance its transmission qualities For example,
Bragg gratings suffer undesired effects from side
lobes in their spectral response This can reduce the
usable bandwidth
which a directional antenna loses power due to
dis-persion of the signals or wave patterns out of the
"sides" of the antenna (e.g., portions that are not part
of the main lobe)
self-illumi-nating (through microwaves) electronic
image-cre-ation system derived from a radar beam transmitted
perpendicular to the ground track during acquisition
from an aircraft Thus, the signal hits the terrain at a
rather flattened angle and the view of the terrain is
vertical, revealing fine surface features useful in
in-terpretation of the data The imaging is provided in
strips or mosaics, as is true for many satellite
imag-ing systems SLAR imagery is used by geologists,
Earth resource scientists, cartographers, engineers,
and others SLAR encompasses real-aperture and
synthetic-aperture radar (SAR) SLAR is not used for
very precise topographic mapping, as the resolution
is only up to about 30m
sidebandThe frequencies on either side of the main
frequency or carrier band in a communications
sig-nal These frequencies are within themodulation
en-velopeof a transmission wave, but were originally
not used because of problems with noise Later, as
technology improved and the demand for airspace
continued to grow, sideband transmissions became
more interesting, and it was found that one sideband
could be transmitted, sometimes even without the
carrier wave, and the original wave mathematically
"rebuilt" at the receiving end The advantages
in-cluded lower power requirements for the
transmis-sion and a narrower wave overall, leaving more room
for other transmissions
sidetoneIna telephone receiver, transmitting currents
are directed into the receiver to make it possible for
the speaker to hear his or her own voice (somewhat
like an echo) as a form offeedback mechanism This
has to be carefully controlled so that it doesn't
be-come excessive, and various anti-sidetone circuits are
applied to minimize feedback and reduce
transmis-sion of acoustical noise
In early telephones, the sidetone was loud enough to
be distracting.In1920, G.A Campbell designed a
circuit to reduce the excess current from the line and
the local receiver, thus reducing sidetone and
improv-ing electrical efficiency
Sidetones are also used in telegraph keying systems
as well, to provide feedback to the person keying in
the (Morse) code
mail filtering language proposed as a Standards Track
RFC by T Showalter, in Jan 2001 Sieve is a
lan-guage for filtering email messages at time of final
delivery that can be implemented on either the mail
systems, including Internet Message Access Proto-col (IMAP) servers
Sieve uses IMAIL-compliant messages to enable a user to create filters for organizing incoming elec-tronic mail (email) The language is intended to be powerful enough to be useful, while not being so powerful that it can break out of its operating envi-ronment or wreak havoc on a system through uncon-trolled variable, loops, or programming bugs Sieve also facilitates the use of graphical user interfaces (GUIs) for creating and manipulating email filters
See FLAMES, RFC 3028
oftelecom-munications services and network equipment empha-sizing robust, secure technologies to regional Bell operating companies and independent telephone and holding companies
In March 2001, Siemens AG announced that it was cleared by the Federal Trade Commission to merge with Efficient Networks, Inc (Texas-based suppliers ofhigh-speed DSL networks).InDecember 200 1, to-gether with Cingular Wireless, the second largest mobile carrier in the U.S., Siemens announced that the two companies would be working to upgrade their services/systems to support 3G (third generation) mobile communications using Enhanced Data Rates for Global Evolution (EDGE) technology
inven-tor who, along with his brother, William, developed the dynamo, a device to convert mechanical energy into electrical energy without the use of permanent magnets In the 1870s, he demonstrated that the ve-locity ofelectrical conductivity through a wire could approximately equal that of light
rather than the amplitude of the signal itself by first integrating and then delta modulating the signal It
is sometimes called delta-sigma modulation, depend-ing upon the sequence of the processes.Itis a form ofoversampling and noise-shaping signal conversion scheme
The system came into practical use about two decades after delta modulation was developed in the 1940s, when VLSI technology began to emerge, making it possible to develop practical embodiments ofthe sys-tem SDM helped overcome some of the limitations associated with delta modulation, a simple scheme for sampling an analog signal and encoding the samples based upon the previous state
SDM is useful for quantizing and encoding audio sig-nals for transmission over telephone circuits and is also used in image processing
In sigma-delta modulation, an analog signal is quan-tized and the sum of the previous difference and the current signal compared rather than directly compar-ing the current and previous signal This has greater spectrum independence than delta modulation and
"conditions" the shape of the quantized signal in terms of its maximum possible range SDM is also
Trang 9Fiber Optics Illustrated Dictionary
known as pulse density modulation (PDM)
While the resolution of SDM is good, the filtering
introduces latency and it is not as fast as might be
desired for newer, high-speed communications
tech-nologies (e.g., multiplexed systems) Thus, research
into improving the speed has yielded some
deriva-tive schemes For example, reduced-sample-rate
sigma-delta modulation (RSRSD) schemes require
only a fraction ofthe speed necessary for sigma-delta
encoding without trading offbandwidth RSRSD may
be applicable toAIDand D/A conversion for
appli-cations such as Extended Digital Subscriber Line
(xDSL)
There are hobby kits available for demonstrating delta
and sigma-delta modulation/demodulation processes
SeeAIDconversion, delta modulation, pulse width
modulator
facil-ity established at Fort Monmouth in March 1918 as
Camp Alfred Vail The lab was active in classified
research including the development of radio
direc-tion finding equipment and aviadirec-tion radio
cations Following World War I, aviation
communi-cation research was moved to the Signal Corps
Air-craft Radio Laboratory in Ohio A few years later, a
number of labs in Washington, D.C and New York
state were moved to the Fort Monmouth location and
it was redesignated as the Signal Corps Laboratories
The lab was used for radio communications
devel-opment and adjunct meteorological services In the
early 1930s, marine and aerial sensing were added
and radar studies stepped up in the late 1930s
point which provides access to a database and packet
switching for message-based signaling protocols for
the Service Control Point (SCP) inanIntelligent
Net-work(IN).STPs are widely used in Common
Chan-nel Signaling systems
To provide redundancy in a telephone network,
Ser-vice Control Points (SCPs) and STPs are usually
paired up Depending on the implementation, the
STPs may be arranged hierarchically, with the lower
layer serving the smaller or local region and the
higher layer serving the larger region The Intelligent
Network is based around Signaling System 7 See
Intelligent Network, Service Switching Point
electronics and communications to quantify the
proportion ofa signal (or communication) that is
de-sired and useful to the proportion of a signal that is
undesired and distracting or destructive to the desired
signal It is preferable to have a high signal-to-noise
ratio
In electronics the signal can be an electrical or
opti-cal signal or a data stream and the noise can be
vari-ous types ofinterference such as crosstalk, echo,
spu-rious signals, etc
When Internet Relay Chat (IRC) was first developed,
it had a pretty good signal-to-noise ratio in terms of
the content of conversations As the Net has grown,
this has changed to the point that public online
dis-cussions are often avoided in deference to private
chats and moderated email discussions See noise Signaling System 1SS 1 Historically, an interna-tional CCITT-standardized supervision single-tone signaling protocol to signal call requests between tele-phone switchboards.A500-Hz signal tone at 20 Hz was used Bell had a similar standard for manual ringdown signaling These days Signaling System 1
is more often associated with newer digital signal-ing systems, i.e., DSS 1 See Digital Subscriber Sig-naling 1
Signaling System 2SS2 Historically, a CCIIT-stan-dardized supervision dual-tone signaling protocol to handle pulse-dial selection Dual 600/750 Hz signal tones were used It was somewhat similar to historic Bell radiotelephone dial signaling systems
Signaling System 3SS3 Historically, a CCIIT-stan-dardized supervision single-tone signaling protocol used in one-way telephone circuits and not intended for use for multinational transit connections This system was still in use until about the late 1970s Signaling System 4SS4 Historically, an interna-tional CCIIT-standardized supervision dual-tone sig-naling protocol for international and terminal tran-sit SS4 was the first to fully support global direct dialing signaling SS4 is not entirely obsolete; it is sometimes used for signaling for Voice over Packet (VoP) devices
Signaling System 5SS5 Historically, an interna-tional CCIIT-standardized supervision dual-tone sig-naling protocol used in T1 and E1 telephone trunks
to Bell DDD trunks with supervision used in North America SS5 is not entirely obsolete Although it has generally been superseded by SS6 and SS7 for sub-scriber systems, testing and maintenance devices for central office equipment sometimes use SS5 signal-ing on digital trunk interfaces for testsignal-ing gateways and SS5 is still used on analog international gateways Signaling System 6SS6 Historically, a CCIIT out-of-band signaling system developed in the 1960s, which is being superseded by Signaling System 7 in North America but is still popular in Europe Both SS6 and SS7 owe their origins to Common Channel Interoffice Signaling (CCIS) SS6 was the first sys-tem to incorporate packet switching into public switched telephone networks (PSTNs) It supports a message-based protocol for requesting services, simi-lar to the widespread X.25 standard Signal units of
28 bits each were assembled into data blocks for transmission See Signaling System 7
Signaling System 7, Signaling System No.7SS7 SS7 is a common channel network signaling system, descended from Signaling System 6 and its precur-sor Common Channel Interoffice Signaling (CCIS) CCIS was an out-of-band that was inherently more secure than in-band multifrequency signaling systems prevalent at the time It is sometimes referred to as Common Channel Signaling System 7 (CCS7) SS7, introduced in the 1980s, is more flexible and powerful than earlier versions, makingitpossible to implement broadband digital services far in advance ofbasic voice circuits One ofthe ways in which SS7
Trang 10nal units (up to a defined maximum), while SS6 was
constrained to fixed-length units
Unlike earlier phone signaling systems, which
oper-ated through many semi-independent switching
cen-ters, SS1 brings the communications channels into a
more integrated whole
The international data rate for SS7 networks is 64
kbps, although faster data rates are being studied and
implemented, such as 1.544 and 2.048 Mbps
(inter-national)
There are many factors influencing the adoption of
SS7, including its flexibility and applicability to both
wireline and wireless communications But one ofthe
fIrSt motivations for switching to SS7 was the
devel-opment of phone services that could be dialed using
a common area code, regardless of the geographical
location of the subscriber (e.g., 800 numbers) Since
local databases and switches could not be used to
route calls that could be placed to any region, a
cen-tral cross-reference registry was needed to associate
the 800 number with a regional location and
switch-ing office With SS7's message-passswitch-ing capabilities,
it became straightfolWard to associate a virtual area
code with a geographical routing code This opened
the door to other types of virtual area code services,
such as emergency 911, pay 900, as well as custom
calling services (e.g., Callerill)and enhanced
ser-vices (e.g., number portability)
SS7 is now an important aspect of digital
telecom-munications services Users have come to expect the
many features that were difficult or impossible to
implement over older analog tone-based signaling
systems With its out-of-band architecture, it is also
inherently much more secure than historic in-band
signaling systems
SS7 is being gradually integrated into ATM/Tl and
PCSIUPT networks See Common Channel
Interof-fice Signaling
Signaling SystemRlSS Rl.Aninternational
ana-log telephony signaling standard, equivalent to Bell's
out-of-band 2600 Hz tone in DOD trunks in North
America
Signaling SystemR2 SS R2.Aninternational
ana-log telephony signaling standard, equivalent to Bell's
out-of-band 3825 Hz tone inserted between voice
channels in the carrier system
signature 1.Anidentifying mark, usually a name,
intended to relate a document or other transactional
device to the individual associated with the document
A signature is often used as a means to acknowledge
understanding or agreement to the terms of a
trans-action, e.g., a contract See digital signature.2.In a
more general sense, a mark, style, method of doing
things, or musical sequence that is identifiable as
coming from a specific source or strongly associated
with a certain person or group The signing ofa name
in a person's handwriting, the creation of a painting
with a distinctive and uniquely recognizable origin,
or even a specific way oftuming a phrase are all
ex-amples of signatures in the broader sense On the
In-ternet, individuals often come up with names,
signa-themselves to others in much the same way a corpo-ration identifies itselfwith a trademark or logo 2 In printing, a grouping of pages that is created in order
to organize the pages for binding Common sizes for signature groups are 8or 16 pages With many people now doing their own desktop publishing and submit-ting them to printers over the Internet for publication,
it helps to understand some of the basic terms and procedures used to create documents in the printing industry See fascicle, imposition
SUA See Software&Information Industry Associa-tion
silence compressionAtechnique used in voice over data network applications which involves removing the pauses and spaces that typically occur in many conversations This reduces transmission time Two common techniques typically used together include voice activity detection (VAD), which distinguishes speech from the surrounding background noise, and comfort noise generation (CNG), which creates a low type of static that gives humans a certain comfort level and trust that the line is still active and the call hasn't been cut off
silent discardIn packet networking, the discard of a packet without further processing The system may log the event and may even store the contents of the discarded packet for later evaluation
silicaSilicon dioxide See silicon
silicon Anabundant nonmetallic, tetravalent element (AN 14), widely used in semiconductor technology Silicon comprises about 26% ofthe Earth's crust (by weight) Silica occurs in many common forms, in-cluding sand, quartz, flint, and opals Silica is a main ingredient in glass manufacture Highly pure silicon can be doped with a number ofother elements for use
in solid-state devices
silicon detector Anearly type of radio wave detec-tor similar in some aspects to electrolytic detecdetec-tors Silicon is used in place of the electrolyte, making contact with a platinum wire, and the thumbscrew contact with the silicon can be finely adjusted by fil-ing the end of the thumbscrew to a fine point, usfil-ing
a spring with the thumbscrew to assure even pressure The interaction ofthe thumbscrew and the silicon sets
up a thermoelectric reaction which can be translated into audible waves in the receiver See detector, elec-trolytic detector
Silicon Graphics Incorporated SGI A computer company known for innovative software and hard-ware workstation-level computers, especially those with good graphics and sound SGI was founded in
1981 by James Clark, who later became affiliated with Netscape Communications Corporation
Silicon ValleyAregion ofCalifornia with a high den-sity of high':technology companies, many of which pioneered computer technology The economy, edu-cational institutions, research labs, and climate were all factors that contributed to the growth of technol-ogy companies in Silicon Valley
silicone rubber insulator Amore recent type of in-sulator for installing conducting lines found on utility