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DADDY BOB'S COMPUTER Q & A
What Is DSL?
DSL (Digital Subscriber Line)
is a technology that brings high-bandwidth
information to homes and small businesses
over ordinary copper telephone lines. There
are different variations of DSL, such as
ADSL, HDSL, G.Lite and SDSL explained
herein. Assuming your home or small business
is close enough to a telephone company
central office that offers DSL service,
(usually
3.5 miles or
18,000 feet)
you may be able to receive
data at rates up to 6.1 Mbps with a
theoretical max rate of 8.448 Mbps, enabling
continuous transmission of motion video,
audio, and even 3-D effects.
Most locally typically,
individual connections will provide from 512
Kbps to 6.0
Mbps and about 128 to 512
Kbps upstream. (The current 56K modem in
wide use has a downstream speed of about 50
Kbps, and an upstream speed of 28 Kbps.) One
Mbps (Megabit per second) is equal to 1024
Kbps (Kilobits per second) or 1,048,576 bits
per second. Eight (8) bits equal a Byte,
which can roughly be compared to a character
or number. Therefore, the downstream speed
of 512 Kbps to 6.0 Mbps mentioned above is
roughly equivalent to 66.000 to 786,000
characters per second.
To put this in better
perspective, consider that I am located
about 3 miles from the local exchange, and
my current ADSL connection is averaging 6.0
Mbps downstream speed. This is some 122
times faster than my 56K dial-up modem. This
means my granddaughter can download an
average size MP3 file (2.5 –3 MB) in about 5
seconds.
A DSL line can carry both
data and voice signals and the data part of
the line is continuously connected. This is
sometimes referred to as being “Always On”.
A Brief Description of How It
Works
Traditional
phone service (called
POTS for "plain old telephone
service") connects your home or small
business to a telephone company office over
copper wires that are wound around each
other and called
twisted pair. Traditional phone
service was created to let you exchange
voice information with other phone users and
the type of signal used for this kind of
transmission is called an
analog signal.
To better
understand the difference between Digital
and Analog, consider this scenario. A man is
standing on a high cliff and then jumps off.
When he is on the cliff before the jump, and
when he is on the ground after the jump, he
is digital. While he is falling, he is
analog. Another comparison could be this.
Answering a question with “Yes or No” is
digital, while answering with “I don’t know
or I don’t care” is analog.
An input device
such as a phone set takes an acoustic
signal, which is a natural analog signal,
and converts it into an electrical
equivalent in terms of volume (signal
amplitude) and pitch (frequency of wave
change). Since the telephone company's
equipment is already set up for this analog
wave transmission, it's easier for it to use
it as the way to get information back and
forth between your telephone and the
telephone company.
However, your
computer needs digital data, and that's why
your computer has to have a
modem (MOdulate DEModulate)-
so that it can demodulate the analog signal
and turn it into the string of 0 and 1
values that is called
digital information. So, in
the simplest of terms, the modem is a D-A
(Digital to Analog) converter on upstream
data, and an A-D (Analog to Digital)
converter on downstream data.
Analog transmission only uses
a small portion of the available amount of
information that could be transmitted over
copper wires, so this, and other reasons we
won’t go into here, the maximum amount of
data that you can receive using ordinary
modems is about 52 Kbps (thousands of bits
per second).
The ability of your computer
to receive information is constrained by the
fact that the telephone company converts
information that arrives as digital data,
into analog form for your telephone line,
and then your modem is required to change it
back into digital. In other words, the
analog transmission between your home or
business and the phone company is a
bandwidth bottleneck. You must remember that
the telephone system was designed for analog
voice transmissions, and we are forcing it
to do things it wasn’t designed to do,
namely transmit digital data.
Digital Subscriber Line is a
technology that recognizes that digital data
for the computer does not require changing
into analog form and back. Digital data is
transmitted to your computer directly as
digital data and this allows the phone
company to use a much wider bandwidth for
transmitting it to you. In most
applications, the signal is separated so
that some of the bandwidth is used to
transmit an analog signal along with the
digital signal, allowing you to use your
telephone and computer on the same line and
at the same time.
Factors Affecting the
Transmission Data Rate
DSL modems follow
the data rate multiples established by North
American and European standards. In general,
the maximum range for DSL without a
repeater is 18,000 feet, or
about 3.5 miles. As distance decreases
between you and the telephone company
exchange, the data rate increases. Another
factor is the gauge of the copper wire. The
heavier 24-gauge wire carries the same data
rate farther than 26-gauge wire. It is hard
to believe that all this data and voice
communications comes into you home on just 2
wires, not much bigger than a sewing thread.
Types of DSL
The variation
called ADSL (Asymmetric Digital Subscriber
Line) is the form of DSL that will become
most familiar to home and small business
users. ADSL is called "asymmetric" because
most of its two-way or
duplex bandwidth is devoted
to the downstream direction, sending data to
the user. Only a small portion of bandwidth
is available for upstream or
user-interaction messages. However, most
Internet and especially graphics- or
multi-media intensive Web data need lots of
downstream bandwidth, but user requests and
responses are small and require little
upstream bandwidth.
Using ADSL, up to
6.1 megabits per second of data can be sent
downstream and up to 640 Kbps upstream. The
high downstream bandwidth means that your
telephone line will be able to bring motion
video, audio, and 3-D images to your
computer or hooked-in TV set. In addition, a
small portion of the downstream bandwidth
can be devoted to voice, so you can hold
phone conversations without requiring a
separate line. A special filter usually
called a “Z” filter is used in the home to
separate the voice from the data. Without
this filter, the telephone would have a
severe buzz on it making it unusable.
Unlike a similar service over
your cable TV line, using ADSL, you won't be
competing for bandwidth with neighbors in
your area. In many cases, your existing
telephone lines will work with ADSL. In some
areas, they may need upgrading.
HDSL
HDSL (High
bit-rate Digital Subscriber Line), one of
the earliest forms of DSL, is used for
wideband digital transmission within a
corporate site and between the telephone
company and a customer. The main
characteristic of HDSL is that it is
symmetrical: an equal amount of bandwidth is
available in both directions.
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