The basic primer
below is intended for those new to DSL technologies. It is based on publicly
available US information published in late 1999, hence there have been subsequent
developments in some areas. DSL (Digital Subscriber Line) is
a technology for bringing high-bandwidth information - over 2 megabits (millions
of bits) per second downstream for video - to homes and small businesses over
ordinary copper twisted pair telephone lines.
xDSL refers to all the different
variations of DSL, such as ADSL, HDSL, VDSL and RADSL. If your home or office
is close enough to a telephone company exchange that has installed DSL equipment,
and offers the service, you may be able to receive data at rates up to 6.1 Mbps
(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. In the US Compaq,
Intel, and Microsoft among others are working with telephone companies to develop
a standard and easier-to-install form of ADSL called G.Lite which is accelerating
deployment. DSL is replacing ISDN in many areas and is set to compete with the
cable modem in bringing multimedia and 3-D to homes and small businesses.
How It Works
Traditional phone service (sometimes called POTS for 'plain old telephone service')
connect your home or office to a telephone company exchange over copper wires
that are wound around each other and called twisted pair. Traditional phone
services were 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
analogue signal. An input device such as a phone set takes an acoustic signal
(which is a natural analogue 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 analogue
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 analogue signal
and turn its values into the string of 0 and 1 values that is called digital
information.
Because analogue 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 analogue form for your telephone
line, and requires your modem to change it back into digital. In other words,
the analogue 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 analogue form and back. Digital data is transmitted to your computer
directly as digital data and this allows the telecoms provider 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 analogue
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 a signal splitter to be installed at a home or
office, requiring the expense of an operator 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 standardised 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 a repeater
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 fibre cable.
The Digital Subscriber Line Access Multiplexer
(DSLAM)
To interconnect multiple DSL users to a high-speed backbone network, the telephone
company uses a Digital Subscriber Line Access Multiplexer (DSLAM). Typically,
the DSLAM connects to an asynchronous transfer mode (ATM) network that can aggregate
data transmission at gigabit data rates. At the other end of each transmission,
a DSLAM demultiplexes the signals and forwards them to appropriate individual
DSL connections.
Types of DSL
ADSL
The most familiar and widespread variation of DSL is ADSL (Asymmetric Digital
Subscriber Line). 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 that a telephone line will be able to deliver
motion video, audio, and 3-D images to a computer or connected-in TV set. In
addition, a small portion of the downstream bandwidth can be devoted to voice
rather data, and users can hold phone conversations without requiring a separate
line. Unlike regular dialup phone service, ADSL provides continuously-available,
"always on" connection.
And unlike broadband services over cable TV fibre optics or coaxial cable, with
ADSL, users won't be competing for bandwidth with their neighbours as a dedicated
line is provided (broadband cable users share bandwidth from the street-side
cabinet). In many cases, the existing telephone lines will work with ADSL. In
some areas, especially where the copper is particularly old, they may need upgrading.
Several experiments with ADSL to real users began in 1996.
CDSL
CDSL (Consumer DSL) is a trademarked version of DSL that is somewhat slower
than ADSL (1 Mbps downstream, probably less upstream) but has the advantage
that a 'splitter' does not need to be installed at the user's end. Rockwell
in the US, which owns the technology and makes a chipset for it, believes that
US phone companies should be able to deliver it in the $40 to $45 a month price
range. CDSL uses its own carrier technology rather than DMT or CAP ADSL technology.
G.Lite or DSL Lite
G.Lite (also known as DSL Lite, splitterless ADSL, and Universal ADSL) is essentially
a slower ADSL that doesn't require splitting of the line at the user end but
manages to split it for the user remotely at the telephone company. This saves
the cost of what the phone companies call 'the truck roll.' G.Lite, officially
ITU-T standard G-992.2, provides a data rate from 1.544 Mbps to 6 Mpbs downstream
and from 128 Kbps to 384 Kbps upstream. G.Lite is expected to become the most
widely installed form of DSL.
HDSL
The earliest variation of DSL to be widely used has been HDSL (High bit-rate
DSL) which 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. For this reason, the maximum data rate is lower than for ADSL.
HDSL can carry as much on a single wire of twisted-pair as can be carried on
a T1 line in North America or an E1 line in Europe (2,320 Kbps).
IDSL
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.
RADSL
RADSL (Rate-Adaptive DSL) is an ADSL technology from Westell in which software
is able to determine the rate at which signals can be transmitted on a given
customer phone line and adjust the delivery rate accordingly. Westell's FlexCap2
system uses RADSL to deliver from 640 Kbps to 2.2 Mbps downstream and from 272
Kbps to 1.088 Mbps upstream over an existing line.
SDSL
SDSL (Symmetric DSL) is similar to HDSL with a single twisted-pair line, carrying
1.544 Mbps (US 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.
UDSL
UDSL (Unidirectional DSL) is a proposal from a European company. It's a unidirectional
version of HDSL.
VDSL
VDSL (Very high data rate DSL) is a developing technology that promises much
higher data rates over relatively short distances (between 51 and 55 Mbps over
lines up to 1,000 feet or 300 meters in length). It's envisioned that VDSL may
emerge somewhat after ADSL is widely deployed and co-exist with it. The transmission
technology (CAP, DMT, or other) and its effectiveness in some environments is
not yet determined. A number of standards organisations are working on it.
x2/DSL
x2/DSL is a modem from 3Com that supports 56 Kbps modem communication but is
upgradeable through new software installation to ADSL when it becomes available
in the user's area. 3Com calls it, "the last modem you will ever need."
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