Fiber Optics Illustrated Dictionary - Part 28 ppsx

Fiber
Optics
Illustrated
Dictionary
authentication,
and/or
signer authentication
services
for
creating
independent,
interoperable
implementa-
tions.
The
XML
Signature
is
a
method
of
associat-
ing
a
key
with
referenced
data.
The
XML
namespace

Uniform
Resource
Identifier (URI)
and
prefix
for
other
sub-URIs
for
the
W3C
specification
is
xmlns=/I />See
encryption,
JEPI,
Pretty
Good
Privacy,
signature.
Digital Signature Standard
DSS.
A
draft
standard
to
permit
the
creation
and

transmission
ofa
secure
digital
signature
through
a
Digital
Signature
Algo-
rithm
(DSA)
to
provide
authentication of
documents
and
transactions.
Web
commerce
is
eagerly
seeking
means
by
which
documents
can
be
electronically

se-
cured
in
order
to
use
them
for
trade,
banking,
stock
transactions,
contract
negotiations,
etc.
and
will
prob-
ably
quickly
adopt
this
or
another
scheme
when
suf-
ficient
confidence
in

its
efficacy
is
attained.
See
Elec-
tronic
Certification.
Digital Steppingstones project A project
~f
t~e
Tomas
Rivera
Policy
Institute
(TRPI)
that
studies
IS-
sues
of
access
to
technology
and
telecommunications
networks
and
examines
exemplary

practical
imple-
mentations
in
diverse
environments,
including
librar-
ies,
schools,
and
community
centers
across
the
U.S.
The
Digital
Steppingstones
project
focuses
on
~der
served
communities
such
as
low-income
and
minor-

ity
communities.
An
important
aspect
of
this.
project
is
assessing
access
to
the
Internet
and
making
rec-
ommendations
for
its
broad
and
practical
implemen-
tation
in
publicly
accessible
facilities.
http://www.

trpi.org/dss/
Digital Subscriber Line
DSL,
xDSL.
A
data
trans-
mission
service
operating
over
existing
copper
pub-
lic
phone
lines.
Imagine
turning
o~
your
co~put.er,
connecting
to
the
Internet,
and
findIng
somethIng
In-

teresting
that
you
want
to
explain
to
a
busin~ss
col-
league
or
friend.
If
you
have
only
one
phone
h~e,
~~
you're
using
a
modem
to
change
the
computer
s

dIgi-
tal
signals
into
analog
signals
that
can
be
sent
over
the
phone
line,
you
would
historically
have
to
hang
up
the
modem,
wait
for
a
dial
tone,
and
then

call
your
colleague
or
friend.
Digital
Subscriber
Line
is
a
fam-
ily
of
two-way
communications
services
that
makes
it
possible
for
you
to
talk
to
your
friend
without
hang-
ing

up
the
computer
connection
first.
You
can
do
both
at
the
same
time,
which
means
you
can
talk
through
the
phone
while
you
navigate
the
N
et
toge~h~r,
dis-
cussing

the
things
that
you
both
can
see.
This
IS
how
it
is
done.
Phone
services
historically
have
been
analog
sys-
tems,
and
there
are
millions
of
miles
of
copper
wires

installed
around
the
world
to
provide
these
services.
With
the
development
of
computers,
phone switching
centers
began,
in
the
late
1980s,
to
convert
to
digital
equipment
and
software.
This
enabled
vo!ce

and
d~ta
to
be
carried
on
one
line
at
the
same
tzme,
and
zn-
stead
of
using
a
modem
to
change
the
computer
sig-
nal
to
analog,
and
leaving
the

voice
as
an
analog
sig-
nal,
it
could
be
done
the
other
way
around.
In
other
words,
now
the
voice
call
is
changed
to
digital
and
262
the
computersignal
remains

digital.
This
opens
up
a
world
of
possibilitiesforfaster
transmission,
better
compression
and
security,
and simultaneous datal
voice communications, without having
to
replace
those
millions
of
miles
of
copper
wires.
That
sounds
very
practical,
yet
relatively

few
people
have
switched
to
DSL
services.
One
of
the
reasons
is
distance. While most subscribers are within the
12
000
feet
or
so
in
which
DSL
services
can
operate
at
their
best
speeds,
about
20%

of
the
population
is
not.
Crosstalk
and
other types
of
interference
are
problematic
as
well
and
are
still
being
resolved.
For
example,
traditional
phone
lines
have
loading
coils
installed
at
intervals,

to
extend
the
signals
on
voice
grade
communications.
Unfortunately,
at
higher
digi-
tal
data
rates,
these
coils
cause
interference.
Perhaps
more
important
is
the
way
in
which
DSL
ser-
vices

were
deployed.
Originally,
subscribing
to
DSL
involved
having
the
phone
company
install
a
special
voice/data splitter
on
the
subscriber
premises
and,
further,
installing a
special
peripheral
device
in
the
subscriber's
computer.
This

method
was
costly
and
not
very
practical,
and
most
consumers
are
resistant
to
having
proprietary peripheral
cards
installed
in
their
computers.
Most
prefer
the
option
of
choosing
a
vendor
and
interface,

and
also
of
installing
the
hard-
ware external
to
the
computer,
so
things can be
changed
around
as
needed.
For
this
reason,
a
num-
ber ofcommercial
vendors
have
proposed several
variations
of
DSL
services,
such

as
DSL
Lite.
DSL
was
first
developed
by
Bell
Communications
Research
Inc.
in
1987
to
provide
a
means
to
deliver
interactive
TV
and
video-on-demand
over
copper
wires.
The
name
is

somewhat
confusing,
since
it
is
not
the
line
that
is
installed,
but
rather
the
interfaces
at
each
end
of
the
line.
The
point
of
DSL
was
to
cre-
ate
technology

that
would
make
use
of
existing
lines.
In
fact,
a
DSL
line
typically
consists
of
two
telepho~e
lines.
Since
the
introduction of
DSL,
further
vana-
tions
have
been
adapted,
as
shown

in
the
Digital
Sub-
scriber
Line
Services
chart.
Digital Subscriber Line coding
and
variations
Since
DSL
is
a
multichannel
service,
it
is
necessary
to
split
the
available
bandwidth
to
utilize
it
efficiently.
This

bandwidth
splitting
is
typically
done
with
echo
cancellation
(EC)
or
frequency
division
multiplexing
(FDM).
There
are
two
predominant
schemes
for
subdividing
available
bandwidth
into
smaller
units
to
individu-
ally
evaluate their transmission suitability.

This
is
useful
over
twisted-pair
copper
lines,
which
can
vary
widely
in
their
characteristics.
The
two
most
common
are discrete multitone (DMT) and carrierless
amplitude
and
phase
modulation
(CAP),
~d
others
are
being
developed.
Some

of
these
modulatIon
tec~
niques
have
descended
from
the
telegraph
and
radIO
broadcast
industries,
and
some,
such
as
wavelet
en-
coding,
are
relatively
new
and
still
being
explored.
Each
of

these
has
various
trade-offs
in
terms
of
avail-
ability,
cost,
speed,
and
susceptibility
to
interference,
as
shown
in
the
Common
Modulation
Schemes
chart.
© 2003 by CRC Press LLC
Digital Subscriber Signaling System 1
DSS
1.
A
telephone signaling system standardized
as

ITU-
T
Q.931
that
is
implemented over Layer 3 of
the
ISO
communication
model.
It
defines protocol
for
estab-
lishing,
maintaining,
and
tearing
down
calls.
DSS
1
is
commonly
used
in
local
loops
providing
Integrated

Services
Digital
Network
(ISDN)
services
for
trans-
mission
over
the
D
channel.
Once
the
DSS
1
signal
reaches
the
local
telephone
switching
office,
it
is
usu-
ally transmitted
to
external nodes using Signaling
System

#7
(SS
7).
See
D
channel,
Integrated
Services
Digital
Network.
Digital Subtitle Encoder
DSE.
A PoliStream
sec-
ond-generation
multimedia titling encoder
that
is
ca-
pable of transmitting ideographic languages (e.g.,
Chinese)
in
varied
font
styles.
It
is
a Digital
Video
Broadcasting

(DVB)
standards-compliant bitmap
imaging
system
designed
to
transmit multilanguage
subtitles.
It
accepts
subtitle
data
from
traditional
con-
trollers
and
generates
realtime output ofcompressed
bitmaps.
The
data
may
be
sent
through
regular
broad-
cast
channels,

satellite broadcast
relays,
or
computer
network
links
(e.g.,
Ethernet).
Digital SupervisoryAudioTone
DSAT.
A
means
of
signaling
using
audio
tones
on
cellular
data
networks.
In
AMPS
cellular
signaling,
there
are
three
designated
supervisory audio tones (SATs). However, in

NAMPS,
there
are
instead
seven
subaudible digital
vectors
called
DSATs.
SATs
and
DSATs
are
used
to
verify
the
correct
channel
tuning
after
the
channel
has
been
assigned.
The
central office
(CO)
notifies

as
to
the
new
voice
channel
and
vector.
digital switching matrix
DSM.
Amatrix
format
for
digital
switching
that
facilitates
signal
routing
in
high
bandwidth
applications.
The
device
can
be
concep-
tualized
as

a
grid
of
channels
(e.g.,
64
¥
64
data
chan-
nels),
which
may
be
individually selected
to
input
or
output
to
any
other selected
channel,
or
the
channels
may
be
grouped
into

frames
for
combined
process-
ing.
In
commercial applications,
the
DSM
may
ac-
cept
both
digital
and
analog
signals,
but
the
signals
will
be
converted
to
digital signals before being
switched
(routed). Pulse
code
modulation
(PCM)

is
typically used
for
digitization
in
telephony
applica-
tions.
A commercial
DSM
for
high-capacity tele-
phonic
switching
is
microprocessor controlled
and
may
also
include
memory,
time
base,
speech,
and
control
components.
DSM
is
used

in
data
switchers
for
realtime
switch-
ing
between synchronous video sources.
It
is
also
used
in
high-capacity telephone
networks
for
rout-
ing
calls
to
various application processing
units.
In
a
Datapath architecture digital telephone
network,
it
is
the
switching unit

between
the
data
line
card
and
the
digital carrier module or trunk controller. See
Datapath
Loop
Extension.
Digital Telephony and Communications Privacy
ImprovementsAct
Also
known
as
the
Digital
Tele-
phony
Bill,
this
law
was
passed
by
the
U.S.
Congress
in

October
1994.
The
terms
of
the
bill,
drafted
to
be
supportive
of
the
efforts
of
law
enforcement
agen-
cies,
would
require
common
telecommunications
car-
riers
to
design
networks
in
such

a
way
that
law
en-
forcement
agencies
could
access,
in
realtime,
the
con-
tents
of
communications
on
their
networks
and
trans-
actional signaling.
The
Electronic Frontier
Founda-
tion
came
out
strongly against
the

bill
at
the
time,
but
after
the
events
of
11
Sept.
2001,
the
same
issues
have
been
raised again
and
proposed
as
changes
to
the
Se-
curity
and
Freedom through Encryption
Act
devel-

oped
in
the
late
1990s.
When
the
Digital
Telephony
Bill
was
first
introduced,
Varieties
of
Digital Subscriber Line Services
Type
Abbreviation
Speed
Notes
asymmetric
DSL
ADSL
6
Mbps
+
Twisted-pair
copper
phone
wires.

The
possible
maximum
rate
of
transmissions
is
inversely
proportional
to
distance.
Typically
uses
discrete
multitone
(DMT)
line
coding
for
data;
frequency
division
multiplexing
(PDM)
or
echo
cancellation
is
used
to

subdivide
the
bandwidth.
high
bit-rate
DSL
HDSL
TIlE
1
speeds
Symmetric.
Longer
distances
can
be
supported
through
the
use
of
repeaters.
See
high
bit-rate
Digital
Subscriber
Line.
single
line
DSL

SDSL
Still
in
development.
Can
be
used
over
a
single
wire
pair.
rate
adaptive
DSL
RADSL
up
to
8.7
Mbps
Bandwidth
can
be
tuned
to
subscriber
needs.
It
works
over

longer
transmission
lines.
Rate
and
speed
adjust
to
the
line
length
and
quality.
very
high
rate
DSL
VDSL
13-60
Mbps
Used
in
conjunction
with
FTTC
or
FTTB.
Different
downstream
and

upstream
speeds.
(Upstream
speed
is
1.5
to
2.3
Mbps.)
Shorter
maximum
distance.
Still
in
development.
263
© 2003 by CRC Press LLC
Fiber
Optics
Illustrated
Dictionary
wiretapping proponents suggested that electronic
wiretapping
was
just
an
extension
of
current
wiretap-

ping
practices
as
applied
to
electronic
communica-
tions,
but
technologists
and
privacy
advocates
argue
that
the
analogy
is
not
tenable
-
the
capabilities
in-
herent
in
tapping
into
the
National

Information
In-
frastructure
through
electronics
opens
up
far-reach-
ing
potentialities
that
are
not
safeguarded
by
current
or
proposed
legislation
in
a
way
that
is
comparable
to
the
safeguarding
oftraditional
wiretaps.

As
important
as
it
is
for
law
enforcement
to
find
ways
to
keep
up
with
crimes
that
are
increasingly
con-
ducted
through
new
electronic
technologies,
it
is
also
important
to

examine
the
implications
of
expanding
law
enforcement
tapping
capabilities.
How
is
elec-
tronic
wiretapping
different
from
traditional
wiretap-
ping?
Here
is
a
summary
ofjust a
few
of
the
many
complex
issues

involved:
Visibility.
The
establishment
of
an
electronic
wiretap
can
be
remotely
executed
and
is
thus
invisible,
in
contrast
to
traditional
wiretaps
where
a
telecommu-
nications
technician
has
to
physically install
and

deinstall
a
tap.
In
a
physical
connection
system,
ac-
countability
is
more
readily
detennined
and
traced.
Electronic
wiretapping,
on
the
other
hand,
is
hard
to
monitor.
A
tap
can
potentially

be
engaged
and
disengaged
through
software
that
cannot
be
seen
by
anyone
other
than
the
user.
Duration
of
a
Tap.
There
are
laws
and
warrants
de-
termining
the
physical location
and

effective
du-
ration
of
traditional
wiretaps.
The
physical
removal
ofa
tap
or
changing
ofa
switch
at
a
phone
com-
pany
ends
the
transaction
when
the
court-assigned
permit
expires.
Even
with

the
same
court
order
re-
quirements
in
place,
there
are
currently
few
safe-
guards
consistently
integrated
into
the
network
sys-
tems
of
Internet
Services
Providers
or
the
Internet
as
a

whole
that
can
ensure
that
the
process
has
been
ended
as
required.
Due
to
the
nature
of
software
and
network
communications,
an
electronic
tap
could
be
continued
beyond
the
stated

deadline
with
little
chance
of
discovery.
Some
proponents
feel
that
ac-
countability
needs
to
be
ensured
by
agents
outside
of
the
system
requesting
the
tap.
ISPs
might
be
a
natural

choice,
but
then
the
time
and
economic
bur-
den
of
compliance
fall
on
vendors
rather
than
on
law
enforcement
agencies
and
vendors
would,
in
essence,
be
entrusted
with
watching
the

police,
a
situation
which
has
historically
led
to
problems
such
as
deal-making
and,
in
some
cases,
corrupt alli-
ances.
Before
electronic
wiretapping
can
be
fully
endorsed
and
implemented,
a
new
accountability

technology
needs
to
be
built
into
global
networks.
Perhaps
commercial
vendors
looking
for
new
mar-
kets
can
help
resolve
these
issues
with
innovative
products.
Eavesdropping
on
Computer
Users
Outside
the

Pur-
view
of
the
Tap.
A
physical
wiretap
is
limited
in
scope.
You
must
install
the
tap
at
or
near
the
pre-
mises
being
tapped.
The
tap
may
inadvertently
pick

up
conversations
of
innocent
individuals
in
the
vi-
cinity,
but
this
is
usually
a
local
rather
than
a
large-
264
scale
problem.
With
an
electronic
wiretap,
the
lo-
cal
physical

limitation
is
gone.
Theoretically,
com-
munications
and
activities
of
hundreds
of
millions
of
individuals
could
be
monitored
with
the
same
mechanism
used
to
monitor
the
alleged
criminal,
without
any
obvious

sign
that
this
is
happening.
This
aspect
must
be
addressed
before
broad-rang-
ing
powers
are
given
to
officials
to
tap
a
medium
like
the
Internet
with
technology
that
is
exception-

ally
powerful
and
generalized
in
its
capabilities.
See
American
Civil
Liberties
Association,
Electronic
Freedom
Foundation,
Security
and
Freedom
through
Encryption
Act.
Digital
Television
Standard
DTS.
See
ATSC
Digi-
tal
Television

Standard.
Digital
Television
Station Project, Inc.
DTSP.
A
collaborative
project
of
the
broadcast
industry,
includ-
ing
almost
300
broadcasters,
computer
manufactur-
ers,
content
creators,
and
service
providers.
Based
in
Washington,
D.C.,
the

DTSP
originated
as
the
Model
HDTV
Station
Project
(WHD-TV),
established
by
the
Consumer
Electronics Manufacturers Association
(CEMA)
and
the
Association
for
Maximum
Service
Television
(MSTV)
in
1996,
to
provide
a
hands-on
educational

facility
for
the
implementation
of
digital
television
(DTV)
technologies.
DTSP
is
the
January
2000
follow-up
to
this
project,
supported
by
the
origi-
nal
sponsors
and
also
by
the
National
Association

of
Broadcasters
(NAB).
DTSP
was
established
with
an
I8-month mandate
to
operate
WHD-
TV,
a
model
digital
television
station,
and
provides
continued
op-
portunities
to
develop
and
test
interactive
television,
data

broadcasting,
and
overlay
services
such
as
closed
captioning, lip syncing,
etc.
See
Association
for
Maximum
Service
Television.
digital trunked radio
system
DTRS.
A
radio
com-
munications
system
in
which
communications
chan-
nels
are
shared

over
trunks,
as
opposed
to
individual
channels
being
shared
by
users
as
they
become
avail-
able.
DTRS
systems
can
be
set
up
as
individual
or
group
channels
(called
talk
groups)

or
combined
as
groups
of
groups,
making
it
a
useful
system
for
gov-
ernment,
emergency/safety,
and
industrial
communi-
cations.
Priority
and
encryption
can
be
incorporated
into
digital
trunked
systems
and

they
can
be
config-
ured
to
intelligently
manage
traffic
over
the
trunks
to
optimize
efficiency of
the
system.
digital
to
analog conversion
The
conversion
of
data
stored
as
discrete
units,
usually
in

ones
and
zeros
on
computer
systems,
to
modulated
analog
wave
pat-
terns.
A
modem
is
a
common
device
which
performs
digital
to
analog
conversion
when
changing
computer
signals
to
modulated

analog
signals
that
can
be
car-
ried
electrically
through
a
phone
line
connection.
The
process
is
reversed
at
the
receiving
end.
See
D/
A
con-
version,
modem.
Digital
Versatile
Disc

DVD.
Also
frequently
referred
to
as
digital
videodisc,
when
the
content
is
primarily
image-based,
but
the
format
is
not
limited
to
video
images,
and
a
large
proportion
of
digital
videodiscs

also
contain
audio.
DVD
is
a
vendor
consortium-developed
180-9660-
© 2003 by CRC Press LLC
supporting
optical
disc
fonnat specification, similar
to
compact
disc
(CD),
except that
it
is
designed
to
store
a
much
larger quantity
of
data.
Standardization

has
not
been
a
single
process.
The
DVD
Forum,
a
consortium
of
developers,
and
Philips
and
Sony,
in-
dependent commercial developers, have provided
somewhat
different
versions
ofDVD.
The
Philips
and
Sony
technologies
can
be

licensed
on
a royalty
ba-
sis,
while
the
DVD
Forum
specifications
are
shared
technologies.
The
DVD
fonnat
is
gradually
becom-
ing
standardized
so
that
DVD
consumer players
and
DVD-enabled
computers
can
interchangeably

use
the
discs.
DVD
is
quickly
growing
in
support
and
accep-
tance
by
consumers,
especially
for
entertainment
and
computer
data
storage
purposes.
One
of
the
more
re-
cent
developments
is

the
release of
the
DVD+RW
fonnat by
the
DVD+RW
Alliance,
which
is
not
di-
rectly
related
to
the
DVD
Forum.
DVD
physical
discs
are
the
same
diameter
as
the
CDs
popularly
used

for
music
(120
mm),
but
very
slightly
thicker,
bringing
the
recorded
surface
of
the
disc
a
little closer
to
the
laser pickup, pennitting a higher
resolution
or
areal
density,
i.e.,
smaller,
more
precise
pits
can

be
used
to
store
the
information,
depending
upon
whether
the
disc
is
recorded single-
or
double-
layered,
and
single- or double-sided. DVDs use
higher
density
storage
and
different modulation
and
error-correction
schemes
than
CDs.
Further
flexibil-

ity
is
possible
through
the
use
of
dual
lens
apertures
in
the
laser
pickup
to
provide a
dual
CDIDVD
player.
While
a
DVD
player
can
usually
play
CDs
for
back-
ward

compatibility,
a
CD
player
does
not
inherently
have
the
capability
to
play
DVDs.
DVD
specifies
more
than
the
compression
and
play-
back
format,
it
also
provides functionality
to
build
interactivity
into

the
medium
through
menus,
mul-
tiple
languages,
and
other
features.
This
functional-
ity
makes
it
attractive
for
educational software
and
games
programming
and
allows
movie
makers
to
in-
clude
extra
features.

DVD
designates
MPEG-2
as
the
digital
compression
standard
for
video
recorded
on
DVDs.
MPEG-2
is
a
fast
digital
motion
recording
and
playback specifi-
cation.
A
DVD
can
be
recorded
on
both

sides
for
up
to
a
total
of
about
18
Gbytes
of
data.
This
capability
is
very
attractive
to
developers,
as
it
means
a
two-
hour
MPEG-2
encoded
movie
can
fit

on
one
side.
Sound
is
encoded
in
either
MPEG
audio
or
Dolby
AC-3.
DVD
can
be
played
on
a stand-alone
system
similar
to
a
combination
CD/laserdisc
system;
it
works
like
a

laserdisc
player,
but
is
small
like
a
CD
player.
It
can
also
be
played
on
a computer through a
DVD
computer
peripheral
player.
DVD
can
provide
many
types
of
audio
storage
and
playback

but,
because of its
high
storage
capacity,
vendors
are
particularly interested
in
offering
mov-
ies
on
DVD
rather
than
on
cassette
tapes
or
the
larger-
format
videodisc
formats.
The
DVD
medium
is
more

robust
and
convenient
than
tapes
or large
discs,
pro-
vides
better
sound
than
most
tape
technoloogies,
and
can
store motion pictures
of
up
to
133
minutes
in
length,
including subtitles.
This
capacity isn't
suffi-
cient

to
hold
every
type
of
movie,
so
longer
films
may
be
offered
on
two
discs
or
are
compressed
with
a
number
ofinnovative predictive
coding
methods.
It
is
probable that
DVD,
or something
like

it,
will
eventually supersede traditional
video
cassette
tape
movies.
DVD
is
also
a
promising
technology
for
elec-
tronic
books
(ebooks).
~~!:~~!~i~;
~~i~I~~:!;l~~~~~
J.
Digital Versatile
Disc
associations
Much
of
the
im-
petus
for

DVD
development
has
come
from
indus-
try
alliances
to
promote
and
develop
the
technology.
See
individual entries
for
organizations ofparticular
interest, which include the
DVD
Forum and the
DVD+RW
Alliance.
Digital Versatile
Disc
player,
DVD
player A
con-
sumer

electronics
component
for
decompressing
and
outputting
the
digital data
from
a
Digit8;1
Versatile
Disc
(DVD).
The
player
uses
a laser
light
beam
to
scan
and
read
the
infonnation coded
into
pits
and
grooves

in
the
DVD
medium.
DVD
players
may
play
a
single
disc
or
may
be
capable of
holding
multiple
discs
(similar
to
CD
disc
changers
and
jukebox
sys-
tems).
Most
DVD
players

will
also
play
audio
CDs
but
the
DVDs
themselves
are
not
directly
CD-compatible.
The
DVD
fonnat
uses
a higher density of
storage
on
the
disc
and
different modulation
and
error
correc-
tion
schemes
than

CD.
Consumer
DVDs
are
designed
in
much
the
same
way
as
laserdisc players
and
VCRs
to
play
movies
(multimedia)
and
audio
recordings
on
a television set or through a monitor
and
separate
audio
components.
DVD
players
are

also
available
for
personal computers
and
may
be
internal
(similar
to
a
CD-ROM
player)
or
external (similar
to
an
ex-
ternal
CD-ROM
or
cartridge player).
In
the
past,
DVD
players
that
worked
on

computers
couldn't
nec-
essarily play
the
same
DVDs
as
a
separate
consumer
DVD
player, but intercompatibility
is
now
more
common.
DVD
players
are
also
being designed
as
small
por-
table
entertainment
devices
(like
small

boom
boxes
or
portable television sets).
Movies
are
one
of
the
most
popular
types
ofDVD products
being
sold
and
rented.
Typically
movies
played
back
on
DVD
play-
ers
with
appropriate monitors support
high
quality
component

(Y
IC)
video
at
about
500
horizontal
lines
ofresolution
compared
with
only
210-225
horizon-
tal
lines
ofresolution
in
VHS.
DVDs
are
mastered
at
CCIR6014:2:2
ratios.
DVD-Video
supports
multiple
aspect
ratios.

When
movies
are
played
back,
the
con-
sumer
is
usually
given
a choice of
aspect
ratios,
se-
lected
from
letterbox (4:3),
pan
and
scan
(4:3),
and
anamorphic
(wide
screen
16:9)
fonnats.
DVD
play-

ers
sold
in
Europe
often
support
both
PAL
and
NTSC
while
those
sold
in North America
may
only
support
NTSC
(this
is
changing
as
higher definition
formats
are
finally beginning
to
catch
on).
Audio

frequency response
is
generally
in
the
range
of2
to
44,000 Hz
with
a signal-to-noise ratio
of
110
265
© 2003 by CRC Press LLC
Fiber
Optics
Illustrated
Dictionary
decibels
or
more.
The
higher-end
DVD
players
tend
to
have
added

features,
such
as
progressive
scan,
vari-
ous
filters,
digital
video
equalizers, surround
sound,
and
slow-motion
effects,
that
are
not
available
on
the
less
expensive
models.
DVD+RW,
one
of
the
newer
versions ofDigital

Ver-
satile
Disc
technology,
is
promoted
by
the
DVD+RW
Alliance
as
a
storage
and
digital
video
recording
for-
mat
that
will
be
compatible with
the
majority
of
ex-
isting
DVD
players. It

is
expected
that
DVD+RW
players/drives will be able to read CD-ROM,
CD-RW,
DVD-R,
DVD-RW,
CD-Audio
and
other
similar
discs.
See
Digital
Versatile
Disc,
DVD
Forum.
digital video
DV.
Technologies
that
enable
the
re-
cording
and
playback ofdigitally encoded
moving

image
infonnation
and
sound
fall
into
the
category
of
digital
video.
Some
of
the
big
barriers
to
inexpen-
sive
digital
video
have
been
the
large
amount
of
data
that
is

required
to
record even small segments of
video
and
the
wide
bandwidth
and
processing that
is
required
to
quickly
display
high
resolution color
im-
ages.
When
still
images
from
film
frames
using
cell
animation techniques
are
individually digitized

and
stored,
each
frame
may
require
up
to
24
Mbytes,
if
it
is
to
approximate closely
the
image
quality of35mm
film.
Since
each
second
of
animation
requires
between
24
and
30
individual still

frames,
as
much
as
720
M-
bytes
may
be
needed
to
store a second of
video.
A
full-length
movie
is
usually
ca.
7000
seconds
or
more,
requiring
more
than 5,000,000 Mbytes of storage.
That's a
lot,
and
that's

not
including
sound
or
data
that
might
be
added
to
provide
search
and
retrieval
markers
interspersed with
the
images.
Digi
tal
video
has
been
in
development longer
than
most
people
realize.
Much

of
the
research
in
this
area
originated
in
the
Research
Center
in
Yokohama,
Ja-
pan,
in
1977.
By
1979,
Ampex,
Bosch,
and
Sony
were
introducing technologies based
on
digital
video
con-
cepts,

and
there
was
a
move
to
introduce
new
stan-
dards
to
encompass
emerging
digital
technologies.
In
order
to
make
digital
video
technology possible, a
number
of
innovations and trade-offs have been
implemented.
Data
compression
and
decompression

techniques
are
used
to
store
images
in
less
space,
but
Digital Versatile Disc (DVD)
Formats
Format
Description
DVD-Audio
A
CD-like
DVD
with
improvements
in
audio
fidelity
and
higher
capacity
than
traditional
CDs.
DVD-R

Primarily
a
read
format.
It
is
a
professional
authoring
fonnat in
which
the
DVD
data,
when
ready
to
master,
is
recorded
once
to
the
DVD
and
then
read
as
many
times

as
desired.
DVD-RAM
A
type
of
hard
storage
with
random
read-write
access.
It
is
a
medium
that
can
be
written
to,
with
an
appropriate
DVD
drive,
more
than
100,000
times

and
functions
much
like
a
hard
drive,
except
that
the
discs
are
easily
swapped
and
stored
(like
cartridges).
DVD-RAM
discs
can
be
used
to
store
up
to
about
4.7
Gbytes

of
data.
DVD-RW
Similar
to
a
DVD-RAM,
in
that
it
is
a
type
of
hard
storage,
but
rather
than
random-access,
it
uses
sequential-access,
similar
to
streaming
tape
storage
for
up

to
about
4.7
Gbytes
per
side.
It
can
be
rewritten
up
to
about
1000
times
and
thus
is
an
appropriate
choice
for
data
that
doesn't
often
change,
such
as
backup

data,
or
database
information
that
is
read
more
often
than
it
is
written.
DVD+RW
This
rewritable
DVD
format
is
one
of
the
more
recent
versions,
promoted
by
the
DVD+RW
Alliance.

Hewlett-Packard
announced
the
release
of
its
first
DVD+RW
drive
in
August
2001.
DVD+RW
is
primarily
intended
for
data
storage
and
recording
digital
video
onto
4.7
-Gbyte
discs.
The
Alliance
plans

to
release
software
upgrades
to
support
DVD+R
as
well.
This
format
has
been
developed
without
the
endorsement
of
the
DVD
Forum
and
is
competitive
to
current
formats.
DVD-ROM
A
format

for
computer
data
storage
drives
that
is
generally
available
with
SCSI
and
ATAPI
interfaces
with
capacities
ranging
from
about
2.x
Gbytes
to
over
9
Gbytes.
Blank
rewritable
cartridges
are
available

in
both
single-
and
double-sided
optical
cartridges
and
recordable
discs.
DVD-ROMs
may
at
some
point
supersede
CD-ROMs.
DVD Video Text
Sometimes
called
DVD-
Text,
an
optional
means
to
store
and
access
textual

information
related
to
a
DVD.
It
can
consist
of
consumer
information
for
users
of
DVD
players
or
DVD-ROM
drives,
text
information
for
content
providers
or
DVD
authors,
or
textual
supplements

to
a
video
or
audio
data
stream
to
enhance
the
value
of
the
main
information
on
the
DVD.
A
guidebook
for
DVD-
Text
is
available
from
the
DVD
Forum.
266

© 2003 by CRC Press LLC
the
picture
quality
does
not
equal
film,
and
fast
pro-
cessors
and
frame
buffers
are
required
to
handle
play-
back
in
realtime.
In
spite
of
its
limitations
and
technical

requirements,
developers
are
forging
ahead
with
digital
video
prod-
ucts
partly
because
digital
video
can
be
edited
and
manipulated
in
remarkable
ways.
Special
effects
that
are
impossible
or
difficult
to

achieve
with
analog
film
are
possible
with
digital
video.
DV
also
has
greatly
increased possibilities
for
interactivity
and
access
through
the
Internet.
Of
further
importance
is
the
fact
that
it
doesn't

have
to
go
through
a
chemical
photo-
finishing
process
before
it
can
be
used.
See
animate,
celluloid, D-l, Digital Versatile Disc, interactive
video,
MPEG,
video-on-demand.
Digital Video
Broadcasting
DVB.
A family
of
compatible
television/media
delivery
standards
includ-

ing
cable
and
satellite
technologies.
Digital
Video
Broadcasting
standards
are
supported
by
the
Digital
Video
Broadcasting
Group
and
published
by
the
Eu-
ropean
Telecommunications
Standards
(ETSI).
In
conjunction
with
the

Centre
for
Electrotechnical
Stan-
dards
(CENELEC)
and
the
European
Broadcasting
Union
(EBU),
ETSI
has
formed
ajoint
technical
com-
mittee
to
handle
DVB
standards.
DVB
is
gaining
ac-
ceptance
in
Europe

as
a
digital
video
infrastructure
and
is
gradually
replacing
the
traditional
analog
en-
vironment.
There
are
a
number
of
subdivisions
of
DVB
technologies.
DVB Technology Subdivisions
Subdivision/Description
DVB-C
cable
transmission
standard
DVB-S

satellite
transmission
standard
DVB-
T Digital Terrestrial Broadcasting
(DTTB)
designed
to
be
adapted
to
the
needs
of
local frequency and geo-
graphical
environments.
It
enables
the
development
of
single-frequency
net-
works
(SFNs).
Configurable
to
support
legacy

50-
or
60-Hz
systems.
See
Digital
Video
Broadcasting
Group.
Digital
Video
Broadcasting Group
DVBG.
A
Euro-
pean
trade
consortium that provides support
and
specifications
for
traditional
and
emerging
broadcast
technologies, such
as
cable and satellites, and
schemes
for

the
protection of
commercial
program-
ming.
See
Digital
Video
Broadcasting.
/>digital video format A
series
of
digital
video
stan-
dards
introduced
since
the
1980s,
including
0-1,0-2,
0-3,
etc.
See
Digital
Video
Formats
chart.
Digital Video Interactive

DVI.
A
digital
recording
and
playback
chipset technology
developed
at
the
David
SamoffResearch
Center.
The
technology
was
acquired
by
Intel
Corporation
which
subsequently
developed
it
into
Indeo
2
and
Indeo
3.

It
is
now
known
as
Intel
Video
Interactive
(IVI).
See
Intel
Video
In-
teractive.
digital videodisc
See
Digital
Versatile
Disc.
digital voice encoding A
process
of
sampling
and
quantizing
voice
signals
and
storing
them

as
digital
data.
Since
voice
encoding
can
require
much
memory,
the
information
is
usually
compressed
for
storage
and
decompressed
when
replayed.
Fractal
and
wavelet
compression techniques are becoming popular
choices.
Much
of
the
technology

for
digital
voice
encoding
has
come
from
the
music
industry.
Research
into
the
sampling
and
playback
of
synthesized
music
can
be
applied
effectively
in
voice
encoding
and
play-
back
applications.

Digital
voice
encoding
is
used
in
speech
and
voice
recognition
systems
and
is
used
to
send
conversations
over
digital
voice
telephone
channels,
such
as
Inter-
net
telephone
applications,
ISDN,
or

digital
PCS.
To
save
memory
and
cut
down
on
transmission
time,
in-
teresting
algorithms
for
removing
pauses
and
spaces
are
used
in
conjunction
with
digital
voice.
Encryp-
tion
to
ensure

privacy
is
also
possible
with
digital
voice
communications.
Digital
voice
encoding
can
provide
a
data
library
of
sounds,
phonemes,
words,
or
other
units,
for
use
in
digital
applications
such
as

automated
voice
menu
systems,
speaking
computer
applications,
digital
talk-
ing
books,
answering
machines,
voice
mail
systems,
and
more.
Digitally
encoded
voice
tends
to
be
more
pleasant
and
natural
sounding
than

mathematically
generated
voice
and,
hence,
is
favored
for
applica-
tions
where
instructions
or
responses
are
given
to
a
human
listener.
See
quantize, silence
suppression,
speech
recognition,
voice
recognition.
Digital8 Adigital
video
format

developed
by
Sony.
It
records
digital
video
data
and
stores
it
on
Hi8/8mm
tapes.
The
recording
time
is
about
45
minutes
due
to
the
faster
rolling
speed
of
the
tape

compared
to
Hi8/
8mm
(which
records
about
two
hours).
See
Hi8mm.
Digital-S
See
digital
video
format
(D-9).
digitize
lJ.
To
convert
from
analog
to
digital,
i.e.,
to
take
an
analog

signal
and
convert
it
into
data
that
con-
sists
of
discrete
units,
such
as
ones
and
zeros,
usu-
ally
by
sampling
the
analog
signal
at
discrete
points
and
assigning
a

value
to
the
data
at
that
point.
Since
most
broadcast
media
are
transmitted
as
waves,
they
are
analog
systems
of
communication.
However,
in
order
to
process
the
information
or
interface

it
with
networks,
it
must
be
converted
to
digital
format
at
some
point
in
the
transmission
process.
Thus,
digi-
tizing
a
phone
signal
makes
it
possible
to
add
fea-
tures

such
as
compression,
encryption,
and
voice
rec-
ognition.
Digitizing
a
video
signal
allows
processing
of
the
image:
palette
changes,
overlays,
image
com-
posites,
split
screen
viewing,
videoconferencing
and
more.
A

desktop
scanner
is
a
type
of
digitizer,
as
is
a
digital
camera.
See
analog,
digital,
digitizer,
pulse
code
modulation.
digitizerA
device
that
converts
a
signal
from
analog
to
digital,
usually

by
sampling
the
analog
signal
at
discrete
points
over
time
and
assigning
a
value
to
the
measurement.
Up
to
a
point,
the
more
frequent
the
267
© 2003 by CRC Press LLC
Fiber
Optics
Illustrated

Dictionary
sampling,
the
better
the
encoding
represents
the
origi-
nal.
Digitizers
are
commonly
used
in
video
and
au-
dio
applications.
The
sound
patches
used
in
electronic
music
are
digitized
from

analog
sound
samples.
Digitizers
are
not
constrained
to
only
two
dimensions.
Apen
or
robot
arm
can
trace
contours
on
a
physical
object
and
convert
the
spatial
information
into
a
3D

coordinate
system
for
rendering,
ray
tracing,
or
CAD
programs.
One
of
the
first
widely
distributed
2D
digi-
tizers
for
microcomputers
(1986)
was
NewTek's
DigiView
for
the
Amiga.
It
is
historically

significant
as
the
forerunner
to
the
Video
Toaster,
which
initi-
ated
the
desktop
video
industry.
See
sampling.
Dll
See
defense
information
infrastructure.
Dijkstra'sAlgorithm
InATM
networking,
an
algo-
rithm
sometimes
used

in
conjunction
with
link
and
nodal
state
topology
information
to
calculate
routes.
DIM
See
document
image
management.
dim fiber
Fiber
optic
cable
in
which
the
carrier
pro-
vides
the
means
to

carry
the
signal
through
the
fiber
but
does
not
originate
the
signals
at
either
end
of
the
circuit.
See
dark
fiber.
dimmed
Lowered
in
illumination
or
visibility.
Re-
duced
in

intensity.
See
ghosted.
DIMS
1.
Document
Image
Management
System.
A
hypermedia-based
document
management
system
jointly
developed
by
HyperMedia
and
Aramco
for
the
purpose
of
capturing
the
geological,
geophysical,
and
reservoir

engineering
data
at
Aramco.
2.
document
and
image
management
system.
A
system
for
auto-
mating
and
integrating
the
management
of
images
of
text and graphical documents that are typically
scanned
and
stored
as
digital
data.
DIN

1.
Deutsches
Institute
fUr
Normung.
German
in-
stitute
for
standardization,
one
of
the
major
standards
bodies
in
Europe.
DIN
is
located
in
Berlin.
The
DIN
specification
is
a
widely
used

standard
for
computer
connectors.
2.
dual
inline.
DINA
1.
Danish
Informatics
Network
in
the
Agricul-
tural
Sciences.
2.
Distributed
Intelligence
Network
Architecture.
3.
Dynamic
Intelligent
Network
Archi-
tecture.
A
research

project
topic
at
Carnegie-Mellon
University.
dinosaur
slang
A
large,
obsolete,
or
aging
system
that
requires
excessive resources
to
keep
in
operation.
Given
the
speed
of technological obsolescence
in
computer
technology,
systems
quickly
become

dino-
saurs,
but
firms
often
hang
on
to
them
because
the
cost
ofinstalling
and
learning
new
technology
may
be
higher
than
continuing
to
use
old,
slow,
but
tried-
and-true
systems.

The
term
dinosaur
applies
espe-
cially
to
old
room-sized
supercomputers
that
are
now
less
powerful
than
many
desktop
systems.
diode, Fleming tube
Historically,
an
electron
tube
with only
two
electrodes,
the
cathode (electron-
emitting)

and
the
anode
(electron-attracting)
in
which
Digital Video
Formats
Type
Description
D-l
A
component
digital
video
format
approved
by
SMPTE
in
1985
and
introduced
by
Sony
as
the
DVR-l
000
in

1987.
It
is
favored
by
digital
effects
videographers.
D-l
records
uncompressed
8-bit
video.
The
cassette
tapes
are
large,
weighing
up
to
six
pounds.
D-2
A
composite
digital
video
format
introduced

by
Ampex
(1986)
and
Sony
(1988)
that
enjoyed
broad
popularity
until
the
late
1990s,
superseding
composite
video
tape
recorders
(VTRs)
using
1"
tapes.
D-2
(CCIR
601)
records
uncompressed
video.
The

cassette
tapes
are
large,
weighing
up
to
six
pounds.
D-3
A
composite
digital
video
format
developed
by
Matsushita
and
introduced
by
Pana-
sonic
as
a
competitor
to
Sony's
D-2
format.

D-3
records
uncompressed
video.
D-5
A
component
digital
video
format
developed
by
Matsushita
and
introduced
by
Pana-
sonic
as
a
competitor
to
Sony's
D-l
format.
D-5
records
uncompressed
10-bit
video.

D-5
has
caught
on
in
High
Definition
Television
(HDTV).
Some
D-5
systems
can
play
D-3
tapes.
Panasonic
has
introduced
a
high
definition
4:
1
compression
ratio
version
ofD-5.
D-6
A

high-definition
digital
video
format
developed
by
Toshiba/BTS
and
introduced
by
Philips.
D-6
records
up
to
64
minutes
per
19
mm
cassette
through
a
system
of
helical
scan
tracks
with
a

track
pitch
of
22
micrometers.
There
is
a
longitudinal
analog
cue
track,
a
control
track,
and
a
time
code
track.
D-7,DVCPRO
(DVCPRO
is
the
more
common
name,
D-7
is
the

SMPTE
designation,
but
it
is
placed
here
for
easier
comparison
with
other
D-
formats.)
A
component
digital
video
fonnat
introduced
by
Panasonic.
D-7
records
compressed
8-bit
video
at
34
mmJ

second.
D-7
supports
16-bit,
48
kHz
digital
audio.
Streaming
video
is
supported
through
SDTI
and
IEEE-1394
(FireWire)
standards.
D-9, Digital-S
A
component
digital
video
format
introduced
by
Jve.
D-9
records
3.3:

1
ratio
com-
pressed
1
O-bit
video.
Promoted
as
similar
in
quality
to
Digital
Betacam.
268
© 2003 by CRC Press LLC
the
electrons
flowed
freely
and
uncontrolled,
which
wasn't
very
useful.
L.
de
Forest

developed
a
Fleming
tube
into
a
triode,
a
three-electron
tube,
which
enabled
control
of
the
electron
flow.
In
transistor electronics, a
diode
is
a piece of
semi-
conductor
material,
positive
on
one
side
and

nega-
tive
on
the
other,
with
a terminal
at
each
end.
Where
the
electrical positive
and
negative regions
come
to-
gether
in
the
semiconductor,
it
is
called a
p-n
(posi-
tive-negative)
junction.
Like
a two-element electron

tube,
the
electrons
normally
flow
in
one
direction,
serving
as
a
rectifier,
enabling conversion of
energy
from
one
fonn
to
another
-
an
essential
aspect
of
com-
munications
technology.
An
old crystal
radio

recti-
fier
can
change
radio
waves
into
audio
waves
that
can
be
heard
through
earphones.
Asemiconductor recti-
fier
can
change
alternating current
(AC)
to
direct
cur-
rent
(DC)
for
powering electronic
equipment.
Forcing

the
flow
of electrons
to
go
opposite
to
the
natural
direction
can
be
accomplished
in
some
cir-
cumstances
with
sufficient
voltage,
resulting
in
re-
verse
bias.
This
practice
is
sometimes
useful

in
semi-
conductor technology
for
altering
the
information
stored
in
a
chip.
See
avalanche
breakdown,
erasable
programmable
read-only
memory,
Zener
diode.
Fleming
Valve
- Two-Electron
Tube
evacuated
globe
metal
sleeve
(anode)
for

attracting
electrons
filament
(cathode)
for
emitting
electrons
connecting
leads

The
Fleming valve
was
an
important historical
an-
tecedent
to
three-element
vacuum
tubes
but lacked
the
controlling grid that made itpossible
to
control elec-
tronjlow.
diode laser module
DLM.
See

laser
diode.
diode transistor logic
DTL.
Acircuit
board
configu-
ration
wherein
the
logic
is
accomplished
with
diodes
as
opposed
to
resistors, reducing interaction
between
inputs
and
permitting
many
diodes
to
be
used.
How-
ever,

depending
on
the
design
and
application,
there
may
be
a trade-off
in
speed.
DTL
calculators
during
the
1960s
can
be
found
that
include
both
transistors
and
integrated
circuits
and
represent
some

of
the
early
DTL
devices.
Sometimes
referred
to
as
diode
logic
(DL),
the
early
implementations
tended
to
be
discrete-
component
bipolar
logic
circuits (eventually
super-
seded
by
integrated circuits).
As
integrated circuits
(ICs)

became
more
commonly
used,
DTL
became
an
IC
implementation
of
the
basic
diode
logic.
See
tran-
sistor-transistor
logic.
DIP
1.
document
image
processing.
2.
dual
inline
package.
See
DIP
switch.

DIPswitch
dual
inline
package
switch.
A
very
small
switch
that
is
meant
to
be
toggled
to
one
side
or
the
other
(hence
"dual").
It
can
be
changed
with
a
pencil

or
other pointed
object.
A
few
DIP
switches
are
large
enough
to
be
toggled
with
the
fingers.
The
early
desk-
top
microcomputers,
such
as
the
Kenbak
-1
and
the
Altair,
were

programmed
with
DIP
switches.
The
smaller
DIP
switches
are
often
found
on
SCSI
devices,
for
setting
SCSI
ill
numbers
or
for
adjust-
ing
other settings.
Graphics
controller
cards
some-
times
have

DIP
switches
to
adjust resolution
or
scan
rate
settings.
dipole antenna
In
its
basic
sense,
an
antenna
with
two
poles
mounted horizontally
to
produce
one
long
rod
that
is
a subdivision of
the
length of
the

wave-
length
it
is
designed
to
receive.
Ideally
the
impedance
(
expressed
in
ohms)
of
the
lead
connecting
the
dipole
antenna
to
the
receiving
equipment
(radio,
television,
etc.) should
match
the

impedance
of
the antenna
at
its
strongest point (usually
the
center in a
symmetri-
cal antenna). The frequency response ofa simple
dipole
antenna
can
be
increased by
placing
the
two
rods
proximate
and
parallel,
and
connecting
them
at
both
ends
to
form

afolded dipole
antenna.
DirecPC A commercial service
from
Hughes
Net-
work
Systems
allowing
data
communications
through
Very
Small
Aperture
Terminal
(VSAT)
satellite
sys-
tems
with
a personal
computer.
The
satellite
signal
is
received
by
a two-foot parabolic

dish
antenna
that
feeds
the
transmission
to
a peripheral card
in
a
com-
puter
where
the
signal
is
demodulated,
demultiplexed,
decoded,
and
then
sent
to
the
software
interface.
The
software
can
also

orient
the
satellite
dish.
An
innovative
version
of
this
system
for
Internet
use
combines
the
satellite receiving
system
and
a
normal
analog
connection
to
an
Internet
Services
Provider
(ISP)
as
an

upload/download hybrid
system.
This
service
allows
the
user
to
connect
to
an
ISP
through
the
normal
phone
line
with a conventional
modem,
to
interact
through
a
Web
browser.
When
files
are
re-
quested, rather

than
downloading through
the
ISP
phone
connection,
the
files
are
transferred
from
the
DirecPC
network operations
to
the
VSAT
satellite,
and
then
to
the
user's
DirecPC
dish,
thus
providing
downloads
at
400

kbps
compared
to
about
39.6
kbps.
direct access storage device DASD.
Quick
access
computer storage devices such
as
hard
drives
and
memory
chips.
direct
broadcast
satellite DBS. Originally this
phrase
was
intended
to
describe a particular service
transmitting
in
the
12.2
to
12.7

GHz
range,
a
fre-
quency
approved
by
the
World
Administrative
Radio
Conference (WARe).
However,
since
the
availabil-
ity
of
smaller,
more
convenient receiving
dishes,
the
term
is
also
used
in
abroader
context,

to
describe
any
269
© 2003 by CRC Press LLC
Fiber
Optics
Illustrated
Dictionary
satellite
that
transmits
a
signal
that
can
be
picked
up
by
individual
home
and
business
subscribers,
with-
out
going
through
an

intermediary
station.
Since
direct
broadcasts
are
transmitted
to
small
con-
sumer
dishes,
and
the
size
of
the
dish
is
related
to
the
length
of
the
radiant
waves
being
received,
the

higher
wavelengths
are
used,
such
as
Ku-band
frequencies.
This
permits
a
dish
as
small
as
two
or
four
feet
in
di-
ameter
to
be
effective.
Further,
as
DBS
systems
are

transmitted
to
the
small
consumer
dishes
rather
than
to
a
station
with
a
powerful
receiving
antenna,
it
was
necessary
for
them
to
use
high
power
transmissions,
usually
around
40
to

160
watts,
far
more
than
was
being
used
for
C-band
communications
in
the
1980s
when
DBS
began
to
develop.
(Today
satellites
can
deliver
almost
ten
times
that
power.)
DBS
to

the
home
presents
a
number
of
the
same
moral
challenges
as
widespread
access
to
the
Internet.
DBS
providers
are
concerned
about
illegal
consumer
copy-
ing
and
redistribution
of
programming,
and

consum-
ers
are
concerned
about
program
content.
These
is-
sues
are
still
being
studied
and
resolved.
DBS
system
guidelines
for
Europe
were
originally
established
by
segmenting
the
11.7
to
12.5

GHz
fre-
quency spectrum into
40
channels,
to
be
shared
among
the
various
European
member
nations.
U.S.
DBS
systems
fall
under
the
jurisdiction of
the
Federal Communications Commission, through
guidelines
established
in
the
various
Telecommuni-
cations

Acts,
and
must
conform
to
internationally
established technical standards.
See
C-band,
Ku-
band,
microwave.
direct connect modemA
type
of
modem
which
be-
came
popular
on
desktop
systems
in
the
early
1980s,
gradually
superseding
acoustic

modems.
The
acous-
tic
modem
was
used
to
connect
data
and
phone
lines
by
setting
the
handset
of a
telephone
into
two
suc-
tion-cup
style
holders.
Room
noise
could
be
a

prob-
lem
and
the
holders
didn't
always
provide
a
good
con-
nection.
In
contrast,
the
direct
connect
modem
was
cabled
electrically
between
the
computer
serial
port
and
the
modem,
if

it
was
an
external
modem,
or
be-
tween
the
computer
and
the
internal
modem
card
to
the
telephone
line
through
the
telephone
jack
that
normally
connected
to
a
phone.
This

system
offered
faster
data
rates
and
was
more
convenient
and
effec-
tive
in
reducing
noise
and
ensuring
a
good
connec-
tion.
See
acoustic
modem.
direct current
DC,
dc.
Amore-or-Iess
constant
elec-

trical
current
flowing
in
one
direction.
DC
current
is
supplied
to
many
small
appliances
by
batteries.
Much
of
early
communications
history
was
based
on
direct
current
(DC)
as
a
power

source.
Telephones
had
talking
batteries
and
common
batteries.
The
batter-
ies
tended
to
be
large,
leaky
wet
cells,
that
caused
inconveniences
if
moved
or
subjected
to
temperature
fluctuations.
More
than

fifty
years
after
the
inven-
tion
of
the
telegraph,
AC
power
for
telegraph
systems
was
still
considered
a
novel
idea,
but
the
shortage
of
batteries,
and
their
cost,
provoked
French

and
Swiss
engineers
to
experiment
with
AC
generators,
as
de-
scribed
in
the
Annales
des
Postes,
Telegraphes
et
270
Telephones
in
September
1919.
Eventually
the
ad-
vantages
of
AC
power

were
better
understood,
its
use
became
common,
and
DC
took
its
place
as
a
current
source
for
small,
portable
electric
conveniences
such
as
calculators,
radios,
wristwatches,
laptops,
cameras,
etc.
See

AC
to
DC
converter,
alternating
current;
re-
sistance;
Tesla,
Nicola.
Direct Distance Dialing
DDD.
A
commercial
name
to
indicate
the
capability
ofa
network
to
connect
a
long-distance
call
without
operator
intervention.
Direct

Inward
Dialing
DID.
In
the
past,
calls
going
through
a
central
office
(CO)
to
aprivate
branch
ex-
tension
had
to
go
through
an
attendant.
With
increas-
ing
automation,
this
routing

is
now
rarely
necessary.
With
DID,
the
called
digits
are
passed
through
cen-
tral
office
DID
lines
directly
into
the
private
branch
exchange
(PBX).
DID
lines
do
not
offer
a

dial
tone
and
hence
cannot
be
used
for
direct
outgoing
calls.
Direct
Inward
System Access
DISA.
A
telephone
setup
in
which
outside
callers
can
dial
into
a
telephone
system,
usually
a

private
branch
exchange
(PBX),
and
have
the
use
of
the
system's
services
as
though
they
were
on
the
premises
using
the
system
from
the
inside.
DirectMarketingAssociation, Inc.
DMA.
The
larg-
est

trade
association
for
users/vendors
in
the
direct,
database,
and
interactive
marketing
fields,
founded
in
1917.
Due
to
the
significant
impact
of
the
Internet
on
marketing
technology
and
venues,
the
Association

has
also
acquired
two
electronic
commerce
trade
as-
sociations,
the
Association
for
Interactive Media
(AIM)
and
the
Internet
Alliance
(IA).
Telecommuni-
cations
are
an
integral
aspect
of
marketing,
and
thus
progress

in
electronics
has
shaped
and
been
shaped
by
the
activities
of
the
DMA.
The
DMA
encourages
and
supports
the
growth
and
profitability of
its
membership
and
advocates
adher-
ence
to
high

ethical
standards.
It
provides
leadership
on
behalfof
the
membership
in
government
and
pub-
lic
affairs.
http://www.
the-dma.org/
direct memory access
DMA.
A
means
to
bypass
the
central
processing
unit
(CPU)
in
a

computer
and
in-
teract
directly
with
memory.
This
access
is
used
to
reduce
processing
time
and
increase
speed.
direct outward dialing
DOD.
The
capability
of a
private
branch
exchange
(PBX)
to
dial
calls

outside
the
exchange
without first
dialing
an
access
code
(typically
"9"),
or
going
through
an
operator.
Direct
Print
Protocol
DPP.
A''thin''
networking
pro-
tocol
for
data
transfer
with
a
pair
of

DPP
command
sets,
Direct
Print
Command,
and
File
Transfer
Com-
mand,
for
printing
images
over
an
IEEE
1394
serial
bus,
developed
by
HCL
technologies.
As
the
result
ofDPP
interoperability
events,

a
number
of
commer-
cial
fIrmS
have
implemented
DPP.
There
are
other
protocols
that
transmit
data
over
IEEE
1394,
includ-
ing
IP
over
1394,
SBP-2,
and
AV/C.
Each
of
these

has
its
own
strengths
and
limitations
in
terms
of
the
types
of
data
that
are
efficiently
handled.
DPP
has
mainly
been
of
interest
to
digital
imaging
and
print-
ing
developers.

direct sequencing A spread
spectrum
frequency-
changing
broadcast
technology.
Spread spectrum
© 2003 by CRC Press LLC
broadcasts
spread
a
transmission
over
a
broader
range
of
frequencies
than
is
typically
needed
to
contain
the
broadcast.
A
pseudorandom
digital
sequence

directs
a
phase
modulator
to
distribute
the
original
RF
trans-
mission
over
a
bandwidth
that
is
proportional
to
the
clock
frequency
of
that
sequence.
The
receiver
must
then
be
synchronized

to
the
same
pattern
as
the
broad-
cast
generator
in
order
to
remove
the
phase
modula-
tion
and
recreate
the
original
signal.
Although
it's
not
commonly
done,
it
is
possible

to
combine
direct
sequencing
with
frequency
hopping.
This
combination
would
likely
be
used
only
in
very
high
security
transmissions,
as
the
synchronization
and
receiving techniques
are
not
trivial.
See
fre-
quency

hopping,
spread
spectrum.
direct
set
In
ATM
networking,
a
set
of
host
interfaces
that
can
establish
direct
communications
at
layer
two
for
unicast.
direct
view
storage tube
DVST.
A
cathode-ray
dis-

play
device
introduced
in
the
late
1960s
to
overcome
the
slow
refresh
and
large
storage
buffer
needs
of
early
vector
display
monitors.
By
employing
a
slow-
moving
beam
and
a

storage
mesh,
the
DVST
design
substantially
decreased
the
cost
of
display
devices.
See
vector
display.
directional antennaA
radio
antenna
that
is
designed
to
concentrate
its
signal transmission
or
receiving
strength,
resulting
in

a
stronger
signal,
but
one
which
is
not
of
equal
magnitude
in
all
directions.
Commonly
found
in
AM
radio
transmissions.
directory
1.
A
list,
usually
of
names
and
associated
information,

often
sorted
or
organized
and
displayed
to
enhance
visual
clarity.
These
may
be
the
names
of
people, companies, institutions, files, or sub-
directories,
etc.
2.
A
table
of
organizational
identifi-
ers
that
provides
addresses
to

individual
items
within
the
organizational
path.
This
path
is
frequently
hier-
archical
in
structure.
directory,
file
On
a
computer
file
system,
an
organi-
zational
structure,
comprising
a
file
storage
area,

un-
der
which
there
can
be
further
files
or
directories.
A
directory
listing
typically
includes
other
information
about the directory and its associated files or
subdirectories,
such
as
creation
date,
permissions,
file
type,
and
byte
size.
The

display
may
be
in
text
or
graphical
mode,
and
an
icon
resembling
a
file
folder
is
often
used
to
symbolize
the
directory.
Directory
Agent
DA.
1.
A
Novell
Directory
Services

(NDS)
database
service
that
accepts
advertisements
for
Service
Location
Protocol
(SLP)
devices
from
Service
Agents,
negotiates
registration/deregistration,
and
answers
queries.
See
Service
Location
Protocol,
Service
Agent,
User
Agent.
2.
A

database
service
for
negotiating
links
between
buyer
and
seller
agents
on
a
distributed
network.
Directory Assistance
DA.
A telephone service
in
which
the
caller
dials
directory
assistance
to
request
the
number
ofa
person

outside
the
local
calling
area,
or
within
the
local
calling
area
if
the
number
is
not
listed.
The
number
may
not
be
listed
because
the
in-
dividual
just
moved
in,

just
changed
a
phone
num-
ber,
or
installed
the
phone
just
after
the
directory
was
published.
Subscriber
unlisted
numbers
may
not
be
given
out
through
Directory
Assistance.
In
most
ar-

eas,
there
is
now
about
a
$.75
charge
associated
with
a
Directory
Assistance
request,
and
there
is
usually
a
two-number
limit
on
each
request.
Directory
Assistance
used
to
be
handled

entirely
by
human
operators,
but
there
are
now
automated
systems
which
will
request
the
city
and
name
of
the
person
whose
number
is
being
sought
and
dispense
the
num-
ber,

or
hand
over
the
call
to
the
operator
to
complete
the
transaction,
or
to
clarify
the
information
provided
by
the
caller. These automated systems
combine
speech
recognition
and
speech
synthesis
to
carry
out

their
tasks.
directory caching A
timesaving
function
of
many
operating
systems
that
stores
the
local
directory
list-
ing
in
memory
so
that
each
time
the
user
accesses
the
directory
information
(for listing
parts

or
all
of
the
files),
it
will
be
displayed
very
quickly.
For
small
di-
rectories
there
isn't
much
difference,
but
for
very
long
directory
listings
it
can
be
faster,
especially if

the
in-
formation
is
being
output
to
a
shell
or
MS-DOS
text
window.
Further,
the
information
can
be
used
to
ac-
cess
more
quickly
the
files
in
that
directory.
Since

the
directory information
is
read
from
memory
rather
than
from
disk,
transfer
time
can
be
faster.
Directory Number Call Forwarding
DNCF.
An
interim
Service
Provider
Number
Portability
(SPNP)
which
is
provided
through
existing
available

tele-
phone
services,
such
as
call
routing
and
call
forward-
ing.
The
DN
CF
is
set
up
so
that
calls
are
forwarded
to
a
new
number.
While
it
is
called

Directory
Num-
ber
Call
Forwarding,
unlisted
numbers
can
also
be
set
up
with
the
service.
Directory
System
Agent
DSA.
A
directory
applica-
tion
process
in
the
Open
Systems
Interconnection
(OSI)

system.
OSI
utilizes
a
system
of
agents
(helper
applications)
that
can
query
a
local
database,
com-
municate
with
other
agents,
or
hand
off
requests
to
other
agents
when
appropriate.
The

DSA
specifically
provides
associated
Directory
User
Agents
with
ac-
cess
to
the
directory
information
base
(Om).
directory tree
The
name
given
to
a
hierarchical
di-
rectory
file
structure.
The
"tree"
includes

the
current
directory
and
subdirectories
and
files
associated
with
the
current
directory
both
above
it
and
below
it
in
the
hierarchy.
Directory
tree
commands
are
used
for
file
creation,
deletion,

renaming,
protection,
or
for
search-
ing
a
series
of
files
or
all
the
files
located
in
the
tree.
Directory UserAgent
DUA.
A
directory
application
process
in
the
Open
Systems
Interconnection
(OSI)

system.
OSI
utilizes
a
system
of
agents
(helper
ap-
plications)
that
can
query
a
local
database,
commu-
nicate
with
other
agents,
or
hand
off
requests
to
other
agents
when
appropriate.

The
DSU
specifically
as-
sists
and
represents
the
user
in
accessing
a
database
through
a
Directory
System
Agent
(DSA).
See
Di-
rectory
System
Agent.
dirty power Electrical
power
that
is
spiky,
bursty,

noisy,
or
otherwise
unreliable.
Dirty
power
can
be
dangerous
to
delicate electronic components.
Lap-
tops
plugged
in
on
ferries
and
trains
should
be
used
271
© 2003 by CRC Press LLC