|
What Is...DSL and xDSL (Digital Subscriber Line) (a definition)
DSL and xDSL (Digital
Subscriber Line and its variations)
How
It Works
ADSL | IDSL | SDSL | Summary Table
|
DSL (Digital Subscriber
Line) is a technology for bringing high-bandwidth information to homes and small
businesses over ordinary copper telephone lines. xDSL refers to different variations
of DSL, such as ADSL, HDSL, and RADSL. Assuming your home or small business
is close enough to a telephone company central office that offers DSL service,
you may be able to receive data at rates up to 6.1 megabits (millions of bits) per second
(of a theoretical 8.448 megabits per second), enabling continuous transmission
of motion video, audio, and even 3-D effects. More typically, individual connections
will provide from 1.544 Mbps to 512 Kbps downstream and about 128
Kbps upstream. A DSL line can carry both data and voice signals and the data
part of the line is continuously connected. DSL installations began in 1998
and will continue at a greatly increased pace through the next decade in a number
of communities in the U.S. and elsewhere. Compaq, Intel, and Microsoft working
with telephone companies have developed a standard and easier-to-install form
of ADSL called G.Lite that is accelerating
deployment. DSL is expected to replace ISDN in many areas and to compete with the
cable modem in bringing multimedia
and 3-D to homes and small businesses. Dataquest, a market research firm, forecasts
5.8 million lines installed by the end of 1999.
Traditional phone service (sometimes
called "Plain Old Telephone Service" or POTS) 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. 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 signalling is already set up for this analog wave transmission, it's
easier for it to use that as the way to get information back and forth between
your telephone and the telephone company. That's why your computer has to have
a modem - so that it can demodulate the analog
signal and turn its values into the string of 0 and 1 values that is called digital
information.
Because analog transmission
only uses a small portion of the available amount of information that could
be transmitted over copper wires, the maximum amount of data that you can receive
using ordinary modems is about 56 Kbps (thousands of bits per second). (With
ISDN, which one might think of as a
limited precursor to DSL, you can receive up to 128 Kbps.) The ability of your
computer to receive information is constrained by the fact that the telephone
company filters information that arrives as digital data, puts it into analog
form for your telephone line, and requires your modem 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.
Digital Subscriber Line
is a technology that assumes digital data does not require change 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. Meanwhile, if you choose, the signal can be separated so that some
of the bandwidth is used to transmit an analog signal so that you can use your
telephone and computer on the same line and at the same time.
Splitter-based vs. Splitterless
DSL
Most DSL technologies require
that a signal splitter be installed at a home or business, requiring the expense
of a phone company visit and installation. However, it is possible to manage the
splitting remotely from the central office. This is known as splitterless DSL,
"DSL Lite," G.Lite, or Universal ADSL and has recently been made a standard.
Modulation Technologies
Several modulation technologies are used by
various kinds of DSL, although these are being standardized by the International
Telecommunication Union (ITU). Different
DSL modem makers are using either Discrete Multitone Technology (DMT) or Carrierless
Amplitude Modulation (CAP). A third technology, known as Multiple Virtual Line
(MVL), is another possibility.
Factors Affecting the Experienced
Data Rate
DSL modems follow the data
rate multiples established by North American and European standards. In general,
the maximum range for DSL without repeaters is 5.5 km (18,000 feet). As
distance decreases toward the telephone company office, 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. If you live beyond the 5.5 kilometer
range, you may still be able to have DSL if your phone company has extended the
local loop with optical fiber cable.
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 rather data, and you can hold
phone conversations without requiring a separate line.
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.
IDSL (ISDN DSL) is somewhat
of a misnomer since it's really closer to ISDN
data rates and service at 128 Kbps than to the much higher rates of ADSL.
SDSL (Symmetric DSL) is similar
to HDSL with a single twisted-pair line, carrying 1.544 Mbps (U.S. and Canada)
or 2.048 Mbps (Europe) each direction on a duplex line. It's symmetric because
the data rate is the same in both directions.
| DSL Type |
Description |
Data Rate
Downstream;
Upstream |
Distance Limit |
Application |
| IDSL |
ISDN Digital Subscriber Line |
128 Kbps |
18,000 feet on 24 gauge wire |
Similar to the ISDN BRI service but data only (no voice on the same
line) |
| SDSL |
Symmetric DSL |
1.544 Mbps duplex (U.S. and Canada); 2.048 Mbps (Europe) on a single
duplex line downstream and upstream |
12,000 feet on 24 gauge wire |
Same as for HDSL but requiring only one line of twisted-pair |
| ADSL |
Asymmetric Digital Subscriber Line |
1.544 to 6.1 Mbps downstream;
16 to 640 Kbps upstream |
1.544 Mbps at 18,000 feet;
2.048 Mbps at 16,000 feet;
6.312 Mpbs at 12,000 feet;
8.448 Mbps at 9,000 feet |
Used for Internet and Web access, motion video, video on demand, remote
LAN access |
|
