Fiber Optics Illustrated Dictionary - Part 87 potx

Fiber
Optics
Illustrated
Dictionary
lasers
and
dopants
(deliberate
impurities
such
as
se-
lenium)
may
be
added
to
promote
the
movement
of
electrons
through
the
material.
With
the
application
of
current,
the

component
lases
with
a slightly
wider
range
of
frequencies
and
a
slightly
wider
beam
spread
than traditional lasers. See gallium arsenide;
Kazarinov,
Rudolf;
laser
diode.
semiconductor optical amplifier
SOA.
A
solid
state
component
for
increasing
gain
in
an

optical
transmis-
sion.
This
is
important
for
achieving
power
and
dis-
tance
of
sufficient
magnitude
for
effective
transmis-
sions.
See
amplifier.
SEQUEL
See
SQL.
sequentialA
nonoverlapping
succession
or
series,
in

chronological
or
data
order,
with
no
significant
in-
tervening
time
or
data.
See
concurrent,
consecutive,
parallel,
serial.
serial
clock
transmitexternal
SCTE.
A
data
stream
common
to
serial
cables
used
to

connect
computer
modems.
For
example,
on
25-pin
serial
communica-
tions
connections,
one
of
the
pins
may
be
assigned
to
carry
the
external
clock
signal
from
the
data
ter-
minal
equipment

(DTE).
serial communication A
means
oftransferring
data
one
element
at
a
time,
often
through
a
single
wire
or
trace
in
a
circuit.
While
it
may
not
seem
very
fast
or
efficient,
serial

communication
is
easily
implemented
and
very
commonly
used
in
computing
systems.
The
RS-232
standard
is
the
most
common
specification
for
the
physical/pin
connections
for
serial
communi-
cations.
See
modem,
parallel,

RS-232.
Serial Data Transport Interface
SDTI.
An
emerg-
ing
packet
data
standard
for
the
transport
of
audio,
video,
and
data
among
various
multimedia
systems,
including
video
servers,
cameras,
VCRs,
editing
sys-
tems,
etc.,

especially
in
professional
broadcast
video
environments.
SDTI
provides
a
network
environment
for
video
data
exchange,
without a lot of
network
overhead.
It
supports
single-direction,
point-to-point,
compressed-data
connections.
snTI
has
been
well
received
due

to
the
many
advantages
of
digital
sys-
tems,
including
the
reduction
of
generation
loss
com-
mon
to
analog
editing
and
transmission
loss
associ-
ated
with
analog
broadcasting.
SDTI
(SMPTE
305M)

evolved
from
SDI
(SMPTE
259M),
which
is
used
for
transporting
uncompressed
audio/visual
signals
between
digital
broadcast
and
post-production
devices.
The
two
formats
are
me-
chanically
and
electrically
compatible
to
support

co-
existence
in
the
same
facility.
SDTI,
developed
by
the
SMPTEPT20.04
Workgroup
on
Packetized
Television
Interconnections,
extended
this
concept
by
enabling
compressed
video
to
be
ex-
changed
without
the
need

for
frequent
compression!
decompression
processes
that
could
slow
or
degrade
the
transmission.
SDTI
has
been
recommended
by
the
EBU/SMPTE
Task
Force
for
Harmonized
Standards
for the Exchange
of
Programme Material
as
Bitstreams.
Serial Data Transport Protocol

SDTP.
A
network
data
protocol
which
provides
a
means
of
transporting
serial
data
streams
over
PPP
links.
SnTP
arose
out
852
of
the
work
of
the
TR30.1
ad
hoc
committee

in
the
mid-1990s
to
provide
a standard
means
for
synchro-
nous
data
compression.
SDTP
specifies a transport
protocol
and
an
associated control protocol
(PPP-
SDTP
and
PPP-SDCP)
to
be
used
in
conjunction
with
PPP protocols. See Point-to-Point Protocol,
RFC

1963.
serial interface cardA
printed
circuit
card
which
fits
into
a
slot
in
a
computer
or other computerized
de-
vice
or
piggybacks
on
a
motherboard
to
provide
stan-
dardized
electrical
connections
for
the
synchronous

serial
transmission of
digital
data.
The
connection
on
the
card
is
typically a
25-pin
D
connector.
On
con-
sumer
desktop
computer
systems,
most
serial
inter-
face
cards
support
data
rates
up
to

about
28,800
or
38,400
bps.
A serial
interface
card
is
a
common
way
to
connect
remote
computer
terminals
and
data
mo-
dems
to
a
computer.
See
RS-232.
SerialLine Interface Protocol, Serial
Line
IP
SLIP.

Originating
with
an
early
1980s
3COM
UNET
TCP/
IP
implementation,
SLIP
became
a
de
facto standard
encapsulation
protocol
for
serial
lines,
used
for
point-
to-point
communications
with
TCP
lIP.
SLIP
has

now
been
superseded
by
Point-to-Point
Protocol
(PPP).
See
Point-to-Point
Protocol,
RFC
1055.
SERN
See
Software
Engineering
Research
Network.
server A system
which
provides services
to
other
computers
connected
to
it
through
a
network.

A
server
may
store
and
administer
software
applications,
se-
curity
measures,
access
to
peripherals
or
external
sys-
tems,
etc.
The
server
does
not
necessarily
have
to
be
an
enhanced
system,

as
servers
can
be
specialized
as
print
servers,
mail
servers,
etc.
(and
several
servers
may
be
on
a
system),
but
servers
performing
the
bulk
ofcentralized
or
generalized
tasks
often
have

more
memory,
processing
speed,
and
storage
than
other
systems
on
the
network.
The
software
is
probably
the
most
important
aspect
ofa
good
server.
Good
network
software
is
robust,
configurable,
and

usually
fully
multitasking.
There
are
many
well-tuned network workstation options
that
are
reliable
and
do
not
crash,
except
in
the
most
unusual
of
circumstances.
Shop
around
when
select-
ing
network
server
software;
paying

a
few
hundred
or
thousand
extra
dollars
in
terms
of
the
initial
cost
can
often
be
recouped
in
six
months
or
less
through
savings
on
downtime,
software
reinstallation,
and
ad-

ministrative
costs
that
accrue
on
unreliable
systems.
server agent
In
server/client
systems,
software
that
handles
the
major
processing
or protocols
and
serves
a
request
from
a
client
as
a
Web
server,
mail

server,
or
FTP
server.
Server
Message
Block
protocol
5MB.
A
client
and!
or
server
request-response
network
protocol
for
shar-
ing
resources
such
as
files,
ports,
printers,
and
other
useful
services.

5MB
was
defined
by
ffiM
in
the
mid-
1980s
and
is
prevalent
on
Windows-based
systems.
It
works
in
conjunction
with
NetBIOS
over
TCP/IP.
In
conjunction
with
Samba,
it
can
also

be
used
with
Linux
systems.
5MB
is
also
known
as
Common
In-
ternet
File
System
(eIRS).
See
Samba,
5MB
Project,
RFC
1001,
RFC
1002.
© 2003 by CRC Press LLC
Service
Advertising Protocol
See
Service
Location

Protocol.
Service
Agent
A
network
utility
which,
when
que-
ried,
provides
information
about
a
network
service
(printer,
modem,
etc.)
such
as
its
URL.
Service
Location Protocol
SLP.
An intelligent
re-
source
discovery

and
registration
protocol
developed
in
the
mid-1990s.
Described
as
a "quieter"
alterna-
tive
to
Service
Advertising
Protocol
(SAP),
SLP
in-
cludes
extended
attributes
information
to
reduce
net-
work
traffic
queries.
Thus,

a
printer
may
be
described
in
terms
of
its
capabilities
(such
as
duplex
printing,
PostScript-capable,
tabloid
paper)
and
found
trans-
parently,
without
the
user
querying
for
its
IP
address.
See

Service
Advertising Protocol, Service
Agent,
SLIP,
RFC
2165.
service
quality
Standards
of
service
established
by
businesses
that
include
such
things
as
service
with-
out
outages,
available
lines
without
lag
or
busy
sig-

nals,
technical
support
availability,
good
data
integ-
rity,
etc.·
This
is
not
the
same
as
quality
of
service
(QoS),
which
has
a
more
specific
meaning.
Service-Specific Connection-Oriented Protocol
SSCOP.
A
B-ISDN
signaling

ATM
adaptation
layer
(SAAL)
mechanism
for
managing
the
establishment,
monitoring,
and
release
of
data
exchanged
between
signaling
peers.
In
the
context
ofQ.2931,
SSCOP
pro-
vides
error
and
flow
control
signaling

services
some-
what
analogous
to
those
provided
by
TCP
for
Inter-
net
Protocol
(IP).
Service-Specific
Convergence Sublayer
SSCS.
A
component
of
the
ATM
adaptation
layer
(AAL)
that
coordinates
protocols
of
the

next
higher
layer
with
the
requirements
of
the
next
lower
layer,
the
Com-
mon
Part
Convergence
Sublayer
(CPCS).
See
asyn-
chronous
transfer
mode.
services-oo-demand
SoD.
Services
provided
to
an
audience

on
a
request
basis,
rather
than
on
a
sched-
uled
broadcast
basis.
The
concept
is
not
new;
in
fact,
it
has
been
available
for
media
services
for
over
100
years,

but
new
digital
technologies
are
providing
au-
tomated
services,
thus
making
available
cost-effec-
tive
SoD
delivery
options
which
were
not
previously
possible.
See
audio-on-demand,
video-on-demand.
Session
Announcement Protocol
SAP.
A
network

protocol
for
sending
announcements
to
users
that
is
common
to
broadcast
communications.
It
is
consid-
ered
distinct
from
broadcast
content
and
data
triggers.
SAP
version
2
was
submitted
as
an

Experimental
RFC
by
Handley
et
a1.
in
October
2000.
A
SAP
an-
nouncer periodically
multi
casts
an
announcement
packet
to
a
known
multicast
address
and
port.
The
SAP
recipient
"listens
in"

on
a
SAP
address
and
port
for
multicast
scopes
and
thus
learns
of
all
the
sessions
being
announced
so
that
the
sessions
may
be
joined.
See
enhanced
TV,
RFC
2974.

Session
Control
Protocol
SCPo
A
simple
client/server
network
protocol
to
facilitate
multiple
conversations
over
a
single
rep
connection.
With
SCP,
parties
can
establish
(or
reject)
a virtual
session
over a
single
transport

connection.
Session
Description
Protocol
SDP.
A
network
protocol
used
for
announcements
and
other
notifications
for
multimedia
broadcasts.
The
protocol
was
described
by the Multiparty Multimedia Session Control
(MMUSIC)
working
group
of
the
IETF
and
was

sub-
mitted
as
a
Standards
Track
RFC
by
Handley
and
Jacobson
in
April
1998.
SDP
provides
session
sup-
port
for
Internet
multicast
backbone
(Mbone)
services
and
more
general
realtime
multimedia

services.
The
protocol
is
not
intended
for
the
negotiation
of
media
encodings.
SDP
is
designed
to
convey
session
directory
infor-
mation
to
recipients
that
can
be
used
in
conjunction
with

avariety of
transport
protocols,
such
as
Session
Announcement
Protocol,
Hypertext
Transport
Proto-
col, and others.
See
Session Initiation Protocol,
RFC
2327
Session
Initiation Protocol
SIP.
An
application-layer
signaling
protocol
for
creating,
changing,
and
termi-
nating
Internet-based

telephony,
conferencing,
mes-
saging,
and
events
notification
involving
one
or
more
participants.
I
SIP
was
originally
developed
with
the
IETF
Multi-
party
Multimedia
Session
Control
(MMUSIC)
work-
ing
group
and

continued,
as
of
September
1999,
by
the
IETF
SIP
working
group.
Internet
Drafts
were
submitted
by
the
SIP
working
group
in
May
and
Oc-
tober
2001.
SIP
invitations
provide
a

means
to
convey
session
descriptions
so
that
compatible
media
types
can
be
agreed
upon
by
participants.
Proxy
servers
are
used
to
help
route
requests
to
users
and
to
assist
in

fire-
wall
traversal.
SIP
runs
on
top
ofa
number
of
differ-
ent
transport
protocols.
See
Session
Description
Pro-
tocol.
See
RFC
2543,
RFC
3050,
RFC
3087.
Session
Initiation Protocol Forum
SIP
Forum.

A
E!f:~;i~~~~~~r~~E;~r~::::J~::~~~1I
SET
secure
electronic
transaction.
A
phrase
used
in
electronic
commerce
to
signify a
transaction
which
is
protected
by
various
network security
measures
such
as
authentication
and
verification
procedures,
digital
certificates

and
signatures,
secure
servers,
etc.
set-top
box
colloq.
A
media
device
which
sits
on
top
ofa
TV
set
or
within
a
home
entertainment
compo-
nent
cabinet
to
hook
into
the

system
in
some
way.
Set-
top
boxes
provide
a
variety
ofcapabilities,
including
conversion
of
cable
TV
signals,
provision of
Web
TV
services,
etc.
Some
set-top
boxes
are
proprietary
units
offered
through

lease
or
purchase
by
a
service
pro-
vider
or
vendor.
SETI
Search
for
ExtraTerrestrial Intelligence. An
interesting, federally funded scientific project
in
which
arrays
of
radiotelescopes
are
used
to
search
for
signs
of
intelligence
in
other

parts
of
the
universe.
The
movie
Contact
(Warner
Home
Video)
provides
an
idea
ofa
SETI-like
project.
The
rationale
of
SETI
is
that
signals
can
be
sent
far-
ther
and
faster using radiowaves

than
by
sending
spacecraft
(SETI
is
also
in
favor
of
spacecraft
mis-
sions
and
radio
signals
sent
from
spacecraft)
and
that
it's
worthwhile
to
send
out
signals
in
the
hope

that
853
© 2003 by CRC Press LLC
Fiber
Optics
Illustrated
Dictionary
other life
fOnTIS
may intercept them or that humans
may intercept the transmissions
of
other life forms.
Unfortunately, to date, no signs
of
sentient commu-
nications have been detected, but SETIconcepts have
resulted
in
the discovery
of
interesting radiowave sig-
nals from distant celestial objects.
The Columbus Optical (COSET!) Observatory, a pio-
neering observatory located near Columbus, Ohio,
conducts searches for extraterrestrial intelligence
in
the optical spectrum. /> />SF
I. single frequency.
2.

See SuperFrame.
SF
signaling See single-frequency signaling.
SFOCS
See Submarine Fiber Optics Communica-
tions Systems.
SFfP
See Simple File Transfer Protocol.
SGML
Standard Generalized Markup Language. A
markup standard adapted by the International Orga-
nization on Standardization (ISO)
in
1986 which
is
not a language, but
is
designed for specifying the con-
tent
and
structure
of
a
document
or
document
language, with the assumption that the actual output
or display
of
the document

may
vary according
to
the
output device.
SGML
allows the development
of
cross-platform applications and documents, and a
document can be processed by an
SGML
compiler
by
referencing a document tag definition (DTD).
HyperText Markup Language, widely used
on
the
World Wide Web,
is
a descendant
of
SGML that in-
corporates some
of
its capabilities.
Yuri Rubinsky
(1952-1996) was one
of
the pioneers
who enthusiastically did much to promote the use

of
SGML through educational programs.
SGMP
See Simple Gateway Monitoring Protocol.
SGRAM
synchronous
graphics
random
access
memory. A type
of
memory
optimized for
use
in
memory-hungry graphics applications, particularly
3D rendering and ray tracing.
shadow
mask
A type
of
cathode-ray tube (CRT) color
display technology which incorporates a thin, perfo-
rated metal plate mounted close to the front
of
the
inside
of
the tube to create a mask through which red,
green, or blue (RGB) phosphors can be selectively

excited. See cathode-ray tube.
Service
Environments
and
Systems
The following definitions are related to Service ar-
chitectures that can be used
in
conjunction with
Intelligent Networks (INs). See also Intelligent
Network.
ServiceAccess Code SAC.
A specific telephone prefix access code (e.g., 800)
for aspecific category
of
service such as toll free
numbers. See Service Management System.
Service
Control
Point
SCPo
A point that provides access to an Intelligent
Network (IN) database, willch
is
connected
to
a
Service Management System (SMS), and which
accesses Internet Protocol (IP)
as

needed. SCPs
are a mechanism for providing advanced services
by processing the format or content
of
transmitted
information. Information contained
in
the SCP
may
be
downloaded by phone service carriers.
Architecturally, the Service Control Point is
somewhat self-contained, providing services
to
Intelligent Networks (e.g., those based upon
Signaling System
No.7)
that architecturally
separate switch and service functions.
Service Switching Points (SSPs) can relay value-
added
service
calls
to the
SCP
which
can
appropriately complete the call. The ITU-T
describes the SSP
in

Recommendation Q.1205.
Service
Creation
Environment
SCE.
In the
context
of
Service
Control
Points
interfacing with Intelligent Networks (INs), the
SCE enables new telecommunications services
to
be
quickly designed and implemented without
necessitating changes
in
the IN switchingsystem.
854
Service
Creation
Environment
Point
SCEP.
A protocol used for defining and developing a
service, for example, for implementing security
features and validating services in networks
(often
in

conjunction with LDAP). The SCEP
creates service
in
conjunction with Service
Independent
Building-blocks
(SIBs)
and
interfaces with the Service Management Point
(SMP).
The
ITU-
T
describes
the
SSP
in
Recommendation Q.1205.
Service
Data
Point
SDP.
A standard database designed for use with
Intelligent
Networks
(INs).
Contemporary
phone services are complex compared to historic
analog residential/business telephone lines.
Users now have different services from different

carriers and different variations
of
those services
offered through bundles, specials, prepaid cards,
and carrier offerings
in
specific regions. These
services are stored, configured, and managed
through software that must be associated with
specific subscribers or paid calling card access.
The SDP works
in
conjunction with Service
Control Points (SCPs)
to
manage this enormous
task.
In SDP, the customer information
is
separated
from the logic that manages the services. Tills
compartmentalization
is
important
for the
privacy
of
subscribers and also enables updates,
maintenance, and new services to be easily
integrated into the system without side effects

in
other areas. Fast processing and large data
storage facilities are needed
to
make the system
viable.
The
ITU-T
describes
the
SDP
in
Recommendation Q.1205.
© 2003 by CRC Press LLC
Shannon,
Daude
Elwood
(1916-2001)Acelebrated
American theorist who contributed significantly
to
the study
and
understanding
of
information theory.
The
history
of
communications emphasizes the
in-

ventors, programmers, and hobbyists who have de-
veloped the mechanisms and operations
of
informa-
tion systems, but
few
people at the time had taken a
broad look
at
what information
is,
how it relates
to
the technology (e.g., channel capacity), and what the
process ofconveying information entails
from
amore
abstract, theoretical, statistical,
and
broadly practical
viewpoint.
Shannon, while working at Bell Laboratories,
is
credited with bringing together and clearly stating
fundamental theories
of
information
in
1948.
Shannon

is
often credited
as
wholly developing in-
formation theory, and his work may have been done
independently ofErlang and Kotel'nikov,
who
devel-
oped similartheories many thousands
of
miles
away,
but it
is
derived
at
least in part
from
the work
of
Harry
Nyquist, who
is
cited
in
Shannon's writings. Nyquist
developed principles
of
communications rates and
digital sampling in the 1920s.

Shannon
has
received many awards for his work, in-
cluding the National Medal
of
Science (1966) and
the
John Fritz medal (1983). See Erlang, Agner;
Hagelbarger, David; Kotel'nikov, Vladimir; Nyquist,
Harry; sampling theorum.
Service Environments
and
Systems, cont.
Service Discovery Protocol
SDP.
A wireless network resource discovery protocol
from
Bluetooth that enables applications/devices
to
seek out and find compatible devices with
services that
may
be
useful. Thus, using
SDP,
Bluetooth devices
can
connect wireless
to
nearby

services
such
as
printing, digital cameras,
etc.
SDP
was
specified
by
the
Bluetooth
SIG
in
1999.
SDP
defines
how
a Bluetooth client application
shell discovers available services and their
defined characteristics, withoutprior knowledge
of
the
services,
as
the
device enters
the
accessible
"regionof
access"

of
the
services.
SDP
also
makes
it
possible
to
detect when a service
is
no
longer
available.
In March 2001, RidgeRun, Inc., announced
support
for
the
Bluetooth technology
by
releasing
an
Open
Source
SDP
enabling Bluetooth users
to
get broader access to on-demand wireless
services.
Service ManagementAccess Point

SMAP.
In
a network architecture,
the
SMAP
interfaces
with the Service Management Point and assists
in
managing
user access and services.
The
ITV-
Tdescribes
the
SSP
in
Recommendation
Q.1205.
See
Intelligent Network.
Service Management Point
SMP.
In
an
Intelligent Network (IN) architecture (e.g.,
SS7),
a centralized manager interfacing with a
number of other elements such
as
the Service

Creation Environment Point (SeEP), a Service
Management Access Point
(SMAP),
and
various
Service
Control
Points
(SCPs)
and
other elements
to
satisfy value-added service calls from the
Service Control Point (SCP). The ITD-T
describes
SSP
in
Recommendation Q.l205.
See
Intelligent Network.
Service Management System
SMS.
A centralized interactive computer system
dedicated
to
coordinating network service-related
information. The national 800 numbers are
managed
in
a central database computer

from
ffiM.
This
is
one
type
of
Service Control Point
providing value-added services
to
Intelligent
Networks
(INs)
known
as
SMS/800.
See
SMS/800.
Service Profile Identifier
SPID.
When
hooking
up
ISDN
BRI
services,
the
carrier
provides
the

user
with
a SPill
for
each number
installed, typically
two.
The
SPill points
to
a
memory location
in
the carrier's central office
where ISDN parameters, including which
services
are
enabled
for
a particular subscriber,
are stored. As not all phone carriers have
automatic SPill detection,
some
newer
modems
can determine what type
of
ISDN service
is
connected

and
configure
the
SPills
accordingly.
When
connected
to
carriers
with
automatic
SPill
detection, they can configure themselves
whether or not a computer
is
attached
to
the
modem.
Modems with these capabilities help
compensate for some
of
the problems
traditionally associated with
the
installation
of
ISDN
services.
It

is
recommended that the subscriber
keep
a
record ofSPID numbers filed
away
somewhere,
as
it's easier
to
look
up
a lost
SPID
than
to
get
it
again
from
the
phone carrier.
Service Switching
and
Control
Point
SSCP.
An
entity
in

the physical plane that controls
network resources
and
sessions, including the
Service Control
function,
Service Data
function,
and
Service Switching/Call Control functions.
Service Switching Point
SSP.
An entity
in
the
physical plane,
the
SSP
is
apoint
providing local access and
an
ISDN
interface
for
a Signaling Transfer Point (STP),
which,
in
tum, provides packet switching
for

message-
based signaling protocols in
an
Intelligent
Network
(IN).
The
SSP
can
be
implemented
as
a central office
switching system capable
of
communicating
with a Service Control Point (SCP)
in
order
to
enable switched calls
to
be
routed through a
somewhat
separate
service
environment that
will
then complete

the
call.
The
ITV-
T describes
the
SSP
in
Recommendation Q
.1205.
See
Intelligent
Network, Service Control Point.
855
© 2003 by CRC Press LLC
Fiber
Optics
Illustrated
Dictionary
Shannon-Hartley Capacity Theorem A
theorem
that
facilitates
calculations
for
assessing
an
environ-
ment
with

additive
white
Gaussian
noise
and
its
im-
plications
for
relative
capacities
in
communications
channels.
In
other
words,
capacity
has
been
math-
ematically
defined
in
relation
to
bandwidth,
energy
per
bit,

and
noise
power
density,
to
yield
useful
in-
formation
about
total
signal
and
noise
power,
and
bandwidth
efficiency
(in
bits
per
second
per
Hertz)
in
communication
technologies.
Shared Facility Centers A telework organization
similar
to

a
branch
office,
but
co-owned
or
partially
community-
or
freelance
professionals-funded,
situ-
ated
in
a
residential
or
rural
village
area,
and
made
commercially
viable
by
the
implementation
of
new
communications technologies.

See
ADVANCE
Project,
telework.
shared tenant services
STS.
A
category
of
commu-
nications
services
applying
to
residents
of
multiple
family
dwelling
units.
In
these
types
of
residences,
cable
services,
antennas
and
other

communications
facilities
are
often
shared
among
a
group
of
residents
within
a
complex,
section,
or
building.
The
distinc-
tion
is
important
because,
historically,
many
of
these
residents
have
been
served

by
independent
private
carriers
considered
distinct
from
local
exchange
car-
riers
(LECs)
and
thus
are
not
bound
by
the
same
regu-
latory
framework.
With
the
advance
of
technology,
private
service

provider
services
are
becoming
more
like
LEC
services
and
thus
are
debated
and
evaluated
in
the
context
of
changing
technologies.
ShareView 3000 A
Macintosh-based
videoconfer-
encing
system
from
Creative
Labs
which
supports

audio,
video,
whiteboarding,
application
and
docu-
ment
sharing,
and
file
transfers
over
analog
phone
lines.
An
IBM-licensed PC-version called
ShareVision
PC3000
is
also
available.
See
Cameo
Personal
Video
System,
Connect
918,
MacMICA,

IRIS,
VISIT
Video.
sheath
n.
A
close-fitting
protective
covering,
usually
tubular,
often
made
of
plastic.
Sheaths
can
be
used
to
bundle
wires,
to
insulate,
to
protect
from
moisture
or
wear,

or
to
provide
identifying
colors
or
symbols.
They
are
c-ommonly
used
on
conducting
wires
and
fiber
cables.
See
conduit.
sheave
The
round,
usually
rotating
track
in
a string-
ing
block
used

to
direct
a
line
conductor
that
is
being
installed,
removed,
or
temporarily redirected
for
maintenance
or
repair.
See
stringing
roller.
shell, command shell A
computer
user
interface
in-
put
and
display
environment
which
translates

user
commands
into
operating
system
instructions.
Shenzhen
Bordering
Hong
Kong,
this
is
a
major
sup-
plier of optical cross-connect, multiplexing,
and
single-
and
multimode
optical
fiber
products.
In
ad-
dition
to
supplying
components,
the

Chinese
city
puts
major
emphasis
on
linking
its
populace
through
fi-
ber
optic
backbone
network
systems.
ShermanAntitrustActAn
important
1890
U.S.
act
passed
to
prevent
the
establishment
of
monopolies
that
could

hinder
U.S.
trade
and
competition
based
upon
free
enterprise.
856
SHF
super
high
frequency.
About
3
to
30
Ghz,
used
for
satellite
transmissions.
ship to shore telephone
See
marine
telephone.
shock, electric A
sudden,
often

hazardous,
electri-
cal
stimulation
to
a
living
body
which
may
greatly
affect nerves
and
cause convulsive contractions
through muscles, possibly endangering
the
heart
muscle.
It
may
also
cause
severe
burning,
confusion,
and
unconsciousness.
Light
electric
shocks

are
uncomfortable,
but
not
al-
ways
dangerous
and
are
sometimes
used
as
perim-
eter
boundaries
for
livestock
or
secure
areas.
Light
electric
shocks
are
also
used
in
animal
experiments
for

studying
the
nervous
system
and
are
occasionally
used
in
riot
control
and
law
enforcement.
It
is
unwise
to
open
up
or
attempt
to
repair
cathode-
ray
tubes
(CRTs),
which
may

store
a considerable
charge,
without
careful
preparation
and
knowledge
of
safety
procedures.
Electric
shocks
must
be
taken
seriously
and,
if
severe,
may
require contacting
emergency
services
or
the
application
of
cardiopulmonary
resuscitation

(CPR).
Never touch
someone
who
is
experiencing shock
from
an
electrical
source
until
the
electricity
is
turned
off
or
the
source
of
the
contact
knocked
away
with
a
nonconducting
material.
Consult
emergency

first
aid
sources
for
information.
Shockley, William Bradford (1910- )An
English-
born
American
physicist
who
worked
in
the
Bell
Tele-
phone
Laboratories
from
1936.
He
discovered
the
rectifying
properties
of
impure
germanium
crystals
at

a
time
when
vacuum
tube
rectifiers
had
replaced
the
old
galena
and
carborundum
crystal
detectors.
This
led
Shockley
to
explore
the
various
impurities
in
germanium
and
he
found
electron
drift

toward
the
positive
or negative
pole
under
controlled
conditions.
When
these
solid-state
rectifiers
were
combined,
the
transistor
was
born
and
vacuum
tubes
superseded.
One
of
the
most
significant
consequences
of
transis-

tors
at
the
time
was
miniaturization
of
communica-
tions
devices
and
room-sized
computing
machines.
Shoemaker detector A
type
of
electrolytic
detector
that
incorporates
a
battery
and,
consequently,
requires
no
outside
power
source.

It
consists
ofa
glass
tube
with
a
platinum-sealed
point,
with
a
zinc
strip
rather
than
the
platinum
point
coming
in
contact
with
a
mild
sulphuric
acid
solution.
Shoemaker
detectors
were

used
commercially
in
wireless
telephone
receivers
in
the
early
1900s.
See
electrolytic
detector.
short circuit, short An
unintended
or
harmful
cross
connection,
of
low
resistance,
of
electrical
circuits.
Short
circuits
can
occur
from

an
excess
of
solder,
in-
correctly
connected
wires,
conductive
debris
(such
as
a
screw
falling
into
a
circuit
box),
worn-out
insu-
lation
in
bundled
wire,
water,
or
physical
bumping
of

electric
conductors,
etc.
The
result
is
often
a
sud-
den
flow
of
current
in
the
wrong
direction
or
of
the
wrong
magnitude, which
can
potentially damage
components.
Some
systems
are
configured
to

shut
down
or
blow
a
fuse
or
breaker
in
the
event
of
an
ex-
cess
of
current
or
other
abnormal
electrical
activity.
See
burst,
spike.
© 2003 by CRC Press LLC
shorthaulA
short
travel
or

installation
distance.
The
actual
length
depends
upon
the
situation
or
medium
employed.
A
short
haul
for
a
SCSI
cable
is
about
3
feet
or
less;
above
6
feet,
serious
signal

degradation
occurs
and
special
hardware
is
needed
for
distances
over
12
feet
(see
Fibre
Channel).
A
short
haul
for
other
media
may
be
several
yards
or
thousands
of
miles.
short haul modem A

softwarelhardware
combina-
tion
used
for
short
distance
communications
up
to
a
couple
of
dozen
miles,
usually
over
a
copper
single-
channel
line.
See
baseband
modem.
ShortMessage Service
SMS.
A
global,
wireless,

low
bandwidth,
two-way
service
first
distributed
in
Eu-
rope
in
the
early
1990s
and
later
in
North
America.
SMS
provides
the
capability of
transmitting
alpha-
numeric
messages
between
mobile
systems
and

ex-
ternal
systems
that
support
paging,
email,
and
voice
mail.
The
handsets
used
in
these
services
can
send
or
receive
at
any
time,
regardless of
whether
a
data
or
voice
call

is
in
progress.
SMS
is
appropriate
for
applications
like
stock
quotes,
paging,
short
fax
and
email
messages,
online
quick
banking,
etc.
Short Message Service Center
SMSC.
A
relay
and
administrative center
for
Short Message Service
(SMS)

which
provides
store
and
forward
services.
This
is
somewhat
like
an
enhanced alphanumeric
paging
system
with
two-way
service
and
guaranteed
delivery.
See
Short
Message
Service.
Short Wavelength Fast Ethernet Standard
TW
EIA/ANSI-785.
An
industry
standard

for
the
trans-
mission
of
data
at
100
Mbps
over
850-nm
optical
links
approved
in
June
2001.
This
technology
is
also
known
as
100BASE-SX
and
provides
a
cost-effec-
tive
upgrade

path
from
10
to
100
Mbps
through
the
use
of short wavelength opto-electronic devices.
Overall
this
standard
can
lower
the
cost
of
implement-
ing
Fiber
to
the
Desktop
(FTTD)
and
Fiber
to
the
Home

(FTTH)
services.
shortwave, shortwave
Long-range
radio
transmis-
sion
frequencies
in
approximately
the
1.6
to
30
MHz
range,
above
the
commercial
broadcast
bands.
Short-
wave
signals
are
easier
to
apprehend
at
night,

due
to
lowered
atmospheric
noise
and
the
fact
that
many
shortwave
broadcasters
prefer
to
send
in
the
evening
hours.
Coordinated
Universal
Time
(UTC)
is
often
used
as
the
reference
time

for
broadcasts.
The
Inter-
net
has
sites
that
list
broadcast
times
and
frequencies
for
various
shortwave
stations
around
the
world.
See
microwave,
ionospheric
wave,
radio.
SHT
Short
Hold
Time.
Shugart

A historic disk development company.
Shugart
floppy
disk
drives
were
used
on
some
of
the
earliest
microcomputers.
shunt
n.
A
switch,
pipe,
detour
sign,
or
other
divert-
ing
mechanism.
shunt, electrical
In
electrical
circuits,
a

means
to
di-
vert
some
or
all
of
the
current.
A
shunt
is
sometimes
used
to
divert
part ofa current
in
order
to
prevent
damage
to
sensitive
measuring
instruments.
Tempo-
rary
shunts

are
sometimes
established
with
jumper
wires
or
alligator
clip
connections.
shunt circuit, bypass circuit, detour circuit A
cir-
cuit
configuration
through
which
a
specific
portion
of
the
current
is
redirected
or
subdivided.
Often
used
for
diagnostic

purposes,
temporary
arrangements,
or
circuits
in
which
variable
conditions
are
accommo-
dated
or
where
the
original
current
can
be
more
ef-
fectively
used
by
dividing
it.
Shunts
are
sometimes
incorporated

into
the
internal
workings
of
diagnos-
tic
instruments.
shunting
error
A
condition
in
which
current
divided
through
two
circuits,
as
through
a
component
and
a
measuring
instrument,
will
vary
depending

upon
the
frequency.
See
reversing
error.
Amateur
Radio
Technology
in
Space
Two
members
of
the Spacelab Life Sciences
c~
McArthur
and
Searfoss, training with amateur radio
equipment
for
Space Shuttle mission STS-58 in Sep-
tember
1993. {NASA/JSC image detail.}
Endeavour Astronaut Linda M Godwin uses the
Shuttle AmateurRadio Experiment, as did several
of
the otherSTS-59 crew members, to communicatefrom
spacewith ham radio operators
and

students on Earth.
{NASA/JSC image, April
1994.}
Shuttle
Amateur
Radio
Experiment
SAREX,
SAREX-2,
SAREX-II.
A
series
of
amateur
radio
ex-
periments
consisting of
equipment
and
procedures
carried
as
payload
aboard
a
number
of
U.S.
space

shuttles.
SAREX
was
designed
to
provide
an
op-
portunity for school and community groups
to
857
© 2003 by CRC Press LLC
Fiber
Optics
Illustrated
Dictionary
communicate
with
astronauts
in
space
and
for
con-
ducting
communications
experiments
with
ground-
based

amateur
radio
operators.
SAREX
was
later
in-
tegrated
into
the
International
Space
Station
project
and
has
even
been
used
for
emergency
messaging.
Amateur
radio
has
a
long
history
of
cooperation

with
U.S. aeronautical/space programs leading
up
to
SAREX.
In
November, 1983, Astronaut Owen
Garriott
(W5LFL)
took
his
amateur
(ham)
radio
into
orbit
on
the
Space
Shuttle
Columbia
STS-9
mission
and
communicated
with
Earth
amateur
radio
stations.

This
tightened
the
relationship
between
ham
opera-
tors
and
shuttle
crewmembers
and
the
experiments
continued
on
future
shuttle
missions
of
Columbia,
Challenger,
Atlantis,
Discovery,
and
Endeavor.
Many
astronauts
are
licensed

amateur
radio
operators.
In
several
cases,
the
entire
crew
ofa
space
shuttle
mis-
sion
comprised
ham
operators
(e.g.,
STS-74).
As
examples
of
specific
missions,
a
SAREX
payload
was
carried
aboard

the
Space
Shuttle
Columbia
in
1993
and
the
Space
Shuttle
Endeavour
in
1993
and
1994.
The
SAREX-2
amateur
radio
system
was
used
to
contact elementary
and
middle
school children
from
space
on

October
22,
1993.
SAREX
-
II
was
used
in
1994
to
contact
school
and
Boy
Scout
groups
and
also
to
communicate with Russian cosmonauts
aboard the MIR space station. Amateur radio
communications
were
monitored
in
realtime
by
ama-
teur

radio
stations
via
rebroadcasts
and
the
telebridge
system.
SAREX-II
was
further
used
to
communicate
with
the
Star
City
training
center
outside
Moscow,
Russia,
on
16
April
1994.
SAREX
evolved
into

Ama-
teur
Radio
on
the
International
Space
Station
program
(ARISS).
Slow
Scan
Video
Converter
~

~

;~~~~:~;
::'
i:::~t",'i
;
\,~'~I
'f"(~~

l~Aia'
i
~
J
The

major
unit
in
an
early SAREXcontaining aslow
scan
video
converter,
control
circuits.
andpowersup-
plies.
The
small
tape
recorder enables recording
of
video
image
data
on
tape
for retransmission
through
the
SAREX
equipment.
[NASA/JSC
image.
June

1985.J
The American Relay Radio League (ARRL)
chronicles
the
full
history
of
the
interesting
SAREX
and
pioneer amateur radio experiments
in
their
858
publications
and
on
their
Web
sites.
NASA
provides
instructional
support
materials
for
SAREX
partici-
pants

on
their
Division
of
Education
Educational
Ser-
vices
Web
site.
See
Amateur
Radio
on
the
interna-
tional
Space
Station,
America
Relay
Radio
League,
AM
SAT.
Si
symb.
silicon.
See
silicon.

SIA
1.
Securities
Industries
Association.
2.
See
Semi-
conductor
Industry
Association.
Sibley,
Hiram (1807-1888)
Sibley
founded
the
New
York
and
Mississippi
Valley
Printing
Telegraph
Com-
pany
which
took
on
the
name

of
Western
Union,
sug-
gested
by
Ezra
Cornell
in
the
mid-1850s,
when
it
be-
gan
westward
expansion.
Sibley
remained
president
during
the
expansion
and
Western
Union
installed
the
first
transcontinental

cable
in
1861.
After
the
failure
of
Western
Union's
first
Atlantic
cable,
Sibley
trav-
eled
to
Russia
to
investigate
the
installation
of
a
Si-
berian-Alaskan
communications
line,
and
the
Rus-

sians
offered
to
sell
Alaska
to
Western
Union.
Sibley
turned
down
the
offer,
but
alerted
the
U.S.
govern-
ment
to
the
opportunity.
Along
with
his
colleague,
Ezra
Cornell,
Sibley
helped

to
found
Cornell
Univer-
sity.
See
Western
Union.
side circuit
In
telephone
installations
and
other
cir-
cuits
where
additional endstations
are
desired,
but
where
resources
are
limited,
a
side
circuit
is
a

means
to
build
an
additional circuit
using
the
resources
of
two
adjacent circuits.
See
phantom
circuit.
side lobe
SL.
In
a directional
antenna,
any
segment
generating
a
lobe
in
the
antenna
pattern
other
than

the
primary/main
lobe.
Antenna
patterns
are
often
diagramed
with a radiating
region
around
the
seg-
ment,
a
region
of
influence
that
appears
lobe-shaped.
In
a
Cartesian
coordinate
system,
the
lobe
shows
up

as
a
pronounced
hump
or
"shoulder."
Side
lobe
peaks
are
often
described
in
terms
oftheir
ratio
to
the
main
lobe
peak
and
will
vary,
depending
up
the
shape
of
the

antenna,
its
electrical characteristics,
and
the
fre-
quencies
transmitted.
Depending
upon
the
configuration,
an
antenna
may
have
several
side
lobes.
In
general,
low
side
lobes
are
desired.
Antennas
that
are
likely

to
be
physically
ad-
jacent
to
one
another
(toll
booths,
cell
phones,
even
satellites, etc.)
are
often designed
to
deliberately
suppress
side
lobes.
In
radar
sensing,
side
lobe
signals
can
muddy
the

ra-
dar
signal
and
cause
confusion.
Sometimes
an
omni-
directional
antenna
separate
from
the
main
antenna
is
used
to
send
out
a
reference
pulse
that
is
lower
than
the
signal

from
the
main
antenna,
but
stronger
than
the
signal
from
its
side
lobes.
This
enables
the
tran-
sponder
to
make
a
few
calculations
and
determine
whether
a
signal
came
from

the
main
antenna
or
the
side
lobe.
It
can
then
respond
or
not
respond
as
ap-
propriate.
Antenna
pattern
modeling
programs
are
used
for
gen-
erating
and
assessing
the
configuration

and
influence
of
side
lobes
(including
thermal
emissions).
Side
lobe
interference
in
antennas
can
be
a
problem,
but
it
is
not
restricted
to
larger
physical
structures;
side
lobe
issues
are

also
important
to
small-scale
© 2003 by CRC Press LLC
transmissions
phenomena
such
as
laser
light
paths.
In
optical
network
links,
side
lobes
can
result
from
various
structures
integrated
into
the
optical
fiber
to
enhance

its
transmission qualities. For example,
Bragg
gratings suffer undesired effects
from
side
lobes
in
their
spectral response.
This
can
reduce
the
usable
bandwidth.
side lobe dispersion
An
undesirable condition
in
which
a
directional
antenna
loses
power
due
to
dis-
persion of

the
signals or wave patterns out of
the
"sides" of
the
antenna
(e.g.,
portions that
are
not
part
of
the
main
lobe).
side-looking airborne
radar
SLAR.
A self-illumi-
nating
(through
microwaves)
electronic image-cre-
ation
system
derived
from
a radar
beam
transmitted

perpendicular
to
the
ground
track
during
acquisition
from
an
aircraft.
Thus,
the
signal
hits
the
terrain
at
a
rather
flattened
angle
and
the
view
of
the
terrain
is
vertical,
revealing

fine
surface
features
useful
in
in-
terpretation of
the
data.
The
imaging
is
provided
in
strips
or
mosaics,
as
is
true
for
many
satellite
imag-
ing
systems.
SLAR
imagery
is
used

by
geologists,
Earth
resource
scientists, cartographers, engineers,
and
others.
SLAR
encompasses real-aperture
and
synthetic-aperture
radar
(SAR).
SLAR
is
not
used
for
very
precise
topographic
mapping,
as
the
resolution
is
only
up
to
about

30m.
sideband
The
frequencies
on
either
side
of
the
main
frequency
or
carrier
band
in
a communications
sig-
nal.
These
frequencies
are
within
the
modulation
en-
velope
ofa transmission
wave,
but
were

originally
not
used
because
ofproblems
with
noise.
Later,
as
technology
improved
and
the
demand
for
airspace
continued
to
grow,
sideband transmissions
became
more
interesting,
and
it
was
found
that
one
sideband

could
be
transmitted, sometimes
even
without
the
carrier
wave,
and
the
original
wave
mathematically
"rebuilt"
at
the
receiving
end.
The
advantages
in-
cluded
lower
power requirements
for
the
transmis-
sion
and
a

narrower
wave
overall,
leaving
more
room
for
other
transmissions.
sidetoneIna
telephone
receiver,
transmitting
currents
are
directed
into
the
receiver
to
make
it
possible
for
the
speaker
to
hear
his
or

her
own
voice
(somewhat
like
an
echo)
as
a
form
offeedback
mechanism.
This
has
to
be
carefully controlled
so
that
it
doesn't
be-
come
excessive,
and
various
anti-sidetone
circuits
are
applied

to
minimize
feedback
and
reduce
transmis-
sion
of
acoustical
noise.
In
early
telephones,
the
sidetone
was
loud
enough
to
be
distracting.
In
1920,
G
.A.
Campbell
designed
a
circui
t

to
reduce
the
excess
current
from
the
line
and
the
local
receiver,
thus
reducing
sidetone
and
improv-
ing
electrical
efficiency.
Sidetones
are
also
used
in
telegraph keying
systems
as
well,
to

provide
feedback
to
the
person
keying
in
the
(Morse)
code.
Sieve A protocol-
and
OS-independent, extensible
mail
filtering
language
proposed
as
a
Standards
Track
RFC
by
T.
Showalter,
in
Jan.
2001.
Sieve
is

a
lan-
guage
for
filtering
email
messages
at
time
of
final
delivery
that
can
be
implemented
on
either
the
mail
server
or
a
mail
client.
It
is
suitable
for
a variety of

systems,
including Internet
Message
Access
Proto-
col
(IMAP)
servers.
Sieve
uses
IMAIL-compliant
messages
to
enable
a
user
to
create filters
for
organizing
incoming
elec-
tronic
mail
(email).
The
language
is
intended
to

be
powerful enough
to
be
useful, while
not
being
so
powerful that
it
can
break
out
of
its
operating
envi-
ronment
or wreak
havoc
on
asystem
through
uncon-
trolled variable,
loops,
or
programming
bugs.
Sieve

also
facilitates
the
use
of graphical user interfaces
(GUIs)
for
creating
and
manipulating
email
filters.
See
FLAMES,
RFC
3028.
Siemens Telecom Networks Aprovider oftelecom-
munications
services
and
network
equipment
empha-
sizing robust, secure technologies
to
regional Bell
operating
companies
and
independent

telephone
and
holding
companies.
In
March
2001,
Siemens
AG
announced
that
it
was
cleared
by
the
Federal
Trade
Commission
to
merge
with
Efficient
Networks,
Inc.
(Texas-based
suppliers
of
high-speed
DSL

networks).
In
December
200
1,
to-
gether with Cingular Wireless,
the
second largest
mobile
carrier
in
the
U.S.,
Siemens
announced
that
the
two
companies
would
be
working
to
upgrade
their
services/systems
to
support
3G

(third generation)
mobile
communications using Enhanced
Data
Rates
for
Global
Evolution
(EDGE)
technology.
Siemens, Werner(1816-1892) An
American
inven-
tor
who,
along
with
his
brother,
William,
developed
the
dynamo,
a
device
to
convert mechanical energy
into
electrical energy without
the

use
ofpermanent
magnets.
In
the
1870s,
he
demonstrated
that
the
ve-
locity ofelectrical conductivity
through
a
wire
could
approximately
equal
that
of
light.
~~i~;:::~::~lrEf:!~:
{is:
:;~::~
•••
rather
than
the
amplitude
of

the
signal itself
by
first
integrating
and
then
delta
modulating
the
signal.
It
is
sometimes
called
delta-sigma
modulation,
depend-
ing
upon
the
sequence
of
the
processes.
It
is
a
form
of

oversampling
and
noise-shaping
signal
conversion
scheme.
The
system
came
into
practical
use
about
two
decades
after
delta modulation
was
developed
in
the
1940s,
when
VLSI
technology began
to
emerge,
making
it
possible

to
develop
practical
embodiments
of
the
sys-
tem.
SDM
helped
overcome
some
of
the
limitations
associated
with
delta modulation, a
simple
scheme
for
sampling
an
analog signal
and
encoding the
samples
based
upon
the

previous state.
SDM
is
useful
for
quantizing
and
encoding
audio
sig-
nals
for
transmission
over
telephone circuits
and
is
also
used
in
image
processing.
In
sigma-delta modulation,
an
analog
signal
is
quan-
tized

and
the
sum
of
the
previous difference
and
the
current
signal
compared
rather
than
directly
compar-
ing
the
current
and
previous
signal.
This
has
greater
spectrum independence
than
delta modulation
and
"conditions" the shape
of

the quantized signal
in
terms
of
its
maximum
possible
range.
SDM
is
also
859
© 2003 by CRC Press LLC
Fiber
Optics
Illustrated
Dictionary
known
as
pulse
density
modulation
(PDM).
While
the
resolution of
SDM
is
good,
the

filtering
introduces
latency
and
it
is
not
as
fast
as
might
be
desired
for
newer,
high-speed
communications
tech-
nologies
(e.g.,
multiplexed
systems).
Thus,
research
into
improving
the
speed
has
yielded

some
deriva-
tive schemes. For example, reduced-sample-rate
sigma-delta
modulation
(RSRSD)
schemes
require
only
a
fraction
of
the
speed
necessary
for
sigma-delta
encoding
without
trading
offbandwidth.
RSRSD
may
be
applicable
to
AID
and
D/
Aconversion

for
appli-
cations
such
as
Extended Digital Subscriber
Line
(xDSL).
There
are
hobby
kits
available
for
demonstrating
delta
and
sigma-delta
modulation/demodulation
processes.
See
AID
conversion,
delta
modulation,
pulse
width
modulator.
Signal Corps Radio Laboratories
SCRL.

A
facil-
ity
established
at
Fort
Monmouth
in
March
1918
as
Camp
Alfred
Vail.
The
lab
was
active
in
classified
research
including
the
development
of
radio
direc-
tion
finding
equipment

and
aviation
radio
communi-
cations.
Following
World
War
I,
aviation
communi-
cation
research
was
moved
to
the
Signal
Corps
Air-
craft
Radio
Laboratory
in
Ohio.
A
few
years
later,
a

number
of
labs
in
Washington,
D.C.
and
New
York
state
were
moved
to
the
Fort
Monmouth
location
and
it
was
redesignated
as
the
Signal
Corps
Laboratories.
The
lab
was
used

for
radio
communications
devel-
opment
and
adjunct
meteorological
services.
In
the
early
1930s,
marine
and
aerial
sensing
were
added
and
radar
studies
stepped
up
in
the
late
1930s.
Signal Transfer Point
STP.

A non-terminal
node
point
which
provides
access
to
a
database
and
packet
switching
for
message-based
signaling protocols
for
the
Service
Control
Point
(SCP)
in
an
Intelligent
Net-
work
(IN).
STPs
are
widely

used
in
Common
Chan-
nel
Signaling
systems.
To
provide
redundancy
in
a
telephone
network,
Ser-
vice Control Points (SCPs)
and
STPs
are
usually
paired
up.
Depending
on
the
implementation,
the
STPs
may
be

arranged
hierarchically,
with
the
lower
layer serving
the
smaller
or
local region
and
the
higher
layer
serving
the
larger
region.
The
Intelligent
Network
is
based
around
Signaling
System
7.
See
Intelligent
Network,

Service
Switching
Point.
signal-to-noise
SIN.
A ratio frequently used
in
electronics
and
communications
to
quantify the
proportion ofa
signal
(or
communication)
that
is
de-
sired
and
useful
to
the
proportion ofa
signal
that
is
undesired
and

distracting
or
destructive
to
the
desired
signal.
It
is
preferable
to
have
a
high
signal-to-noise
ratio.
In
electronics
the
signal
can
be
an
electrical
or
opti-
cal
signal
or
a

data
stream
and
the
noise
can
be
vari-
ous
types
of
interference
such
as
crosstalk,
echo,
spu-
rious
signals,
etc.
When
Internet
Relay
Chat
(IRC)
was
first
developed,
it
had

a
pretty
good
signal-to-noise
ratio
in
terms
of
the
content
of
conversations.
As
the
Net
has
grown,
this
has
changed
to
the
point
that
public
online
dis-
cussions
are
often

avoided
in
deference
to
private
860
chats
and
moderated
email
discussions.
See
noise.
Signaling System 1
SS
1.
Historically,
an
interna-
tional
CCITT-standardized supervision single-tone
signaling
protocol
to
signal
call
requests
between
tele-
phone

switchboards. A
500-Hz
signal
tone
at
20
Hz
was
used.
Bell
had
a similar standard for manual
ring
down
signaling.
These
days
Signaling
System
1
is
more
often
associated
with
newer
digital signal-
ing
systems,
i.e.,

DSS
1.
See
Digital Subscriber
Sig-
naling
1.
Signaling System2
SS2.
Historically,
aCCIIT-stan-
dardized
supervision dual-tone signaling protocol
to
handle
pulse-dial selection.
Dual
600/750
Hz
signal
tones
were
used.
It
was
somewhat similar
to
historic
Bell radiotelephone
dial

signaling
systems.
Signaling System 3
SS3.
Historically,
aCCIIT-stan-
dardized supervision single-tone signaling protocol
used
in
one-way
telephone
circuits
and
not
intended
for
use
for
multinational transit connections.
This
system
was
still
in
use
until
about
the
late
1970s.

Signaling System 4
SS4.
Historically,
an
interna-
tional
CCIIT-standardized
supervision
dual-tone
sig-
naling protocol
for
international
and
terminal
tran-
sit.
SS4
was
the
first
to
fully
support
global
direct
dialing signaling.
SS4
is
not

entirely
obsolete;
it
is
sometimes
used
for
signaling
for
Voice
over
Packet
(VoP)
devices.
Signaling System 5
SS5.
Historically,
an
interna-
tional
CCIIT-standardized
supervision
dual-tone
sig-
naling
protocol used
in
T1
and
E1

telephone
trunks.
Dual
2400/2600
Hz
tones
were
used.
SS5
is
similar
to
Bell
DDD
trunks
with supervision
used
in
North
America.
SS5
is
not
entirely
obsolete.
Although
it
has
generally
been

superseded
by
SS6
and
SS7
for
sub-
scriber
systems,
testing
and
maintenance
devices
for
central
office equipment
sometimes
use
SS5
signal-
ing
on
digital trunk interfaces
for
testing
gateways
and
SS5
is
still

used
on
analog
international
gateways.
Signaling System 6
SS6.
Historically, aCCIIT
out-
of-band signaling system developed
in
the
1960s,
which
is
being superseded
by
Signaling
System
7
in
North
America
but
is
still popular
in
Europe.
Both
SS6

and
SS7
owe
their
origins
to
Common
Channel
Interoffice Signaling
(CCIS).
SS6
was
the
first
sys-
tem
to
incorporate packet switching into public
switched telephone
networks
(PSTNs).
It
supports a
message-based
protocol
for
requesting
services,
simi-
lar

to
the
widespread
X.25
standard.
Signal
units of
28
bits each were assembled into data blocks
for
transmission.
See
Signaling
System
7.
Signaling System
7,
Signaling System
No.7
SS7.
SS7
is
a
common
channel
network
signaling
system,
descended
from

Signaling
System
6
and
its
precur-
sor
Common
Channel
Interoffice
Signaling
(CCIS).
CCIS
was
an
out-of-band
that
was
inherently
more
secure
than
in-band
multi
frequency
signaling
systems
prevalent
at
the

time.
It
is
sometimes
referred
to
as
Common
Channel
Signaling
System
7
(CCS7).
SS7,
introduced
in
the
1980s,
is
more
flexible
and
powerful
than
earlier versions,
making
it possible
to
implement
broadband digital

services
far
in
advance
of
basic
voice
circuits.
One
of
the
ways
in
which
SS
7
© 2003 by CRC Press LLC
differs
from
SS6
is
that
it
supports
variable-length
sig-
nal
units
(up
to

a
defined
maximum),
while
SS6
was
constrained
to
fixed-length
units.
Unlike
earlier
phone
signaling
systems,
which
oper-
ated
through
many
semi-independent
switching
cen-
ters,
SS1
brings
the
communications
channels
into

a
more
integrated
whole.
The
international
data
rate
for
SS7
networks
is
64
kbps,
although
faster
data
rates
are
being
studied
and
implemented,
such
as
1.544
and
2.048
Mbps
(inter-

national).
There
are
many
factors
influencing
the
adoption
of
SS7,
including
its
flexibility
and
applicability
to
both
wire
line
and
wireless
communications.
But
one
of
the
fIrSt
motivations
for
switching

to
SS7
was
the
devel-
opment
of
phone
services
that
could
be
dialed
using
a
common
area
code,
regardless
of
the
geographical
location
of
the
subscriber
(e.g.,
800
numbers).
Since

local
databases
and
switches
could
not
be
used
to
route
calls
that
could
be
placed
to
any
region,
a
cen-
tral
cross-reference
registry
was
needed
to
associate
the
800
number

with
a
regional
location
and
switch-
ing
office.
With
SS7's
message-passing
capabilities,
it
became
straightfolWard
to
associate
a
virtual
area
code
with
a
geographical
routing
code.
This
opened
the
door

to
other
types
of
virtual
area
code
services,
such
as
emergency
911,
pay
900,
as
well
as
custom
calling
services
(e.g.,
Caller
ill)
and
enhanced
ser-
vices
(e.g.,
number
portability).

SS7
is
now
an
important
aspect
of
digital
telecom-
munications
services.
Users
have
come
to
expect
the
many
features
that
were
difficult
or
impossible
to
implement
over
older
analog
tone-based

signaling
systems.
With
its
out-of-band
architecture,
it
is
also
inherently
much
more
secure
than
historic
in-band
signaling
systems.
SS7
is
being
gradually
integrated
into
ATM/Tl
and
PCSIUPT
networks.
See
Common

Channel
Interof-
fice
Signaling.
Signaling System
Rl
SS
Rl. An
international
ana-
log
telephony
signaling
standard,
equivalent
to
Bell's
out-of-band
2600
Hz
tone
in
DOD
trunks
in
North
America.
Signaling System
R2
SS

R2.
An
international
ana-
log
telephony
signaling
standard,
equivalent
to
Bell's
out-of-band
3825
Hz
tone
inserted
between
voice
channels
in
the
carrier
system.
signature
1.
An
identifying
mark,
usually
a

name,
intended
to
relate
a
document
or
other
transactional
device
to
the
individual
associated
with
the
document.
A
signature
is
often
used
as
a
means
to
acknowledge
understanding
or
agreement

to
the
terms
ofa
trans-
action,
e.g.,
a
contract.
See
digital
signature.
2.
In
a
more
general
sense,
a
mark,
style,
method
of
doing
things,
or
musical
sequence that
is
identifiable

as
coming
from
a
specific
source
or
strongly
associated
with
a
certain
person
or
group.
The
signing
ofa
name
in
a person's
handwriting,
the
creation
ofa
painting
with
a
distinctive
and

uniquely
recognizable
origin,
or
even
a
specific
way
oftuming a
phrase
are
all
ex-
amples
of
signatures
in
the
broader
sense.
On
the
In-
ternet,
individuals
often
come
up
with
names,

signa-
ture
files,
or
icons
to
uniquely
and
quickly
identify
themselves
to
others
in
much
the
same
way
a
corpo-
ration
identifies itself
with
a
trademark
or
logo.
2.
In
printing,

a
grouping
of
pages
that
is
created
in
order
to
organize
the
pages
for
binding.
Common
sizes
for
signature
groups
are
8
or
16
pages.
With
many
people
now
doing

their
own
desktop
publishing
and
submit-
ting
them
to
printers
over
the
Internet
for
publication,
it
helps
to
understand
some
of
the
basic
terms
and
procedures
used
to
create
documents

in
the
printing
industry.
See
fascicle,
imposition.
SUA
See
Software
&
Information
Industry
Associa-
tion.
silence compressionA
technique
used
in
voice
over
data
network
applications
which
involves
removing
the
pauses
and

spaces
that
typically
occur
in
many
conversations.
This
reduces
transmission
time.
Two
common
techniques
typically
used
together
include
voice
activity
detection
(VAD),
which
distinguishes
speech
from
the
surrounding
background
noise,

and
comfort
noise
generation
(CNG),
which
creates
a
low
type
of
static
that
gives
humans
a certain
comfort
level
and
trust
that
the
line
is
still
active
and
the
call
hasn't

been
cut
off.
silent discard
In
packet
networking,
the
discard
ofa
packet
without
further
processing.
The
system
may
log
the
event
and
may
even
store
the
contents
of
the
discarded
packet

for
later
evaluation.
silica
Silicon
dioxide.
See
silicon.
silicon An
abundant
nonmetallic,
tetravalent
element
(AN
14),
widely
used
in
semiconductor
technology.
Silicon
comprises
about
26%
of
the
Earth's
crust
(by
weight).

Silica
occurs
in
many
common
forms,
in-
cluding
sand,
quartz,
flint,
and
opals.
Silica
is
a
main
ingredient
in
glass
manufacture.
Highly
pure
silicon
can
be
doped
with
a
number

of
other
elements
for
use
in
solid-state
devices.
silicon detector An
early
type
of
radio
wave
detec-
tor
similar
in
some
aspects
to
electrolytic
detectors.
Silicon
is
used
in
place
of
the

electrolyte,
making
contact
with
a
platinum
wire,
and
the
thumbscrew
contact
with
the
silicon
can
be
finely
adjusted
by
fil-
ing
the
end
of
the
thumbscrew
to
a
fine
point,

using
a
spring
with
the
thumbscrew
to
assure
even
pressure.
The
interaction
of
the
thumbscrew
and
the
silicon
sets
up
a
thermoelectric
reaction
which
can
be
translated
into
audible
waves

in
the
receiver.
See
detector,
elec-
trolytic
detector.
Silicon Graphics
Incorporated
SGI.
A
computer
company
known
for
innovative
software
and
hard-
ware
workstation-level
computers,
especially
those
with
good
graphics
and
sound.

SGI
was
founded
in
1981
by
James
Clark,
who
later
became
affiliated
with
Netscape
Communications
Corporation.
Silicon Valley A
region
of
California
with
a
high
den-
sity
of
high':technology
companies,
many
of

which
pioneered
computer
technology.
The
economy,
edu-
cational
institutions,
research
labs,
and
climate
were
all
factors
that
contributed
to
the
growth
of
technol-
ogy
companies
in
Silicon
Valley.
silicone
rubber

insulatorA
more
recent
type
of
in-
sulator
for
installing
conducting
lines
found
on
utility
861
© 2003 by CRC Press LLC