When you connect to the Internet,
you might connect through a regular
modem, through a
network connection in your office, through a
or through a digital subscriber line (DSL) connection. DSL is a very
high-speed connection that uses the same wires as a regular
Here are some advantages of DSL:
- You can leave your Internet connection open and still use the phone
line for voice calls.
- The speed is much higher than a regular modem (1.5 Mbps vs. 56 Kbps)
- DSL doesn't necessarily require new wiring; it can use the phone line
you already have.
- The company that offers DSL will usually provide the modem as part of
But there are disadvantages:
- A DSL connection works better when you are closer to the provider's
- The connection is faster for receiving data than it is for sending
data over the Internet.
- The service is not available everywhere.
In this edition of
HowStuffWorks, we explain how a DSL connection manages to squeeze
more information through a standard phone line -- and lets you make regular
telephone calls even when you're online!
Skinny Voice, Broad Band
If you have read
How Telephones Work, then you know that a standard telephone
installation in the United States consists of a pair of copper wires that
the phone company installs in your home. The copper wires have lots of room
for carrying more than your phone conversations -- they are capable of
handling a much greater bandwidth, or range of frequencies, than that
demanded for voice. DSL exploits this "extra capacity" to carry information
on the wire without disturbing the line's ability to carry conversations.
The entire plan is based on matching particular frequencies to specific
To understand DSL, you first need to know a couple of things about a
normal telephone line -- the kind that telephone professionals call POTS,
for Plain Old Telephone Service. One of the ways that POTS makes the most of
the telephone company's wires and equipment is by limiting the frequencies
that the switches, telephones and other equipment will carry. Human voices,
speaking in normal conversational tones, can be carried in a frequency range
of 0 to 3,400 Hertz (cycles per second -- see
Work for a great demonstration of this). This range of frequencies is
tiny. For example, compare this to the range of most stereo
cover from roughly 20 Hertz to 20,000 Hertz. And the wires themselves have
the potential to handle frequencies up to several million Hertz in most
cases. The use of such a small portion of the wire's total bandwidth is
historical -- remember that the telephone system has been in place, using a
pair of copper wires to each home, for about a century. By limiting the
frequencies carried over the lines, the telephone system can pack lots of
wires into a very small space without worrying about interference between
lines. Modern equipment that sends digital rather than analog data can
safely use much more of the telephone line's capacity. DSL does just that.
Most homes and small business users are connected to an asymmetric DSL
(ADSL) line. ADSL divides up the available frequencies in a line on the
assumption that most Internet users look at, or download, much more
information than they send, or upload. Under this assumption, if the
connection speed from the Internet to the user is three to four times faster
than the connection from the user back to the Internet, then the user will
see the most benefit (most of the time).
Voice and Data
Other Types of DSL
Very high bit-rate DSL (VDSL)
- This is a fast connection, but works only over a short distance.
Symmetric DSL (SDSL) - This connection, used mainly by small
businesses, doesn't allow you to use the phone at the same time, but the
speed of receiving and sending data is the same.
Rate-adaptive DSL (RADSL) - This is a variation of ADSL, but
the modem can adjust the speed of the connection depending on the length
and quality of the line.
Precisely how much benefit you see will greatly depend on how far you are
from the central office of the company providing the ADSL service. ADSL is a
distance-sensitive technology: As the connection's length increases,
the signal quality decreases and the connection speed goes down. The limit
for ADSL service is 18,000 feet (5,460 meters), though for speed and
quality of service reasons many ADSL providers place a lower limit on the
distances for the service. At the extremes of the distance limits, ADSL
customers may see speeds far below the promised maximums, while customers
nearer the central office have faster connections and may see extremely high
speeds in the future. ADSL technology can provide maximum downstream
(Internet to customer) speeds of up to 8 megabits per second (Mbps) at a
distance of about 6,000 feet (1,820 meters), and upstream speeds of up to
640 kilobits per second (Kbps). In practice, the best speeds widely offered
today are 1.5 Mbps downstream, with upstream speeds varying between 64 and
You might wonder, if distance is a limitation for DSL, why it's not also
a limitation for voice telephone calls. The answer lies in small amplifiers
called loading coils that the telephone company uses to boost voice
signals. Unfortunately, these loading coils are incompatible with ADSL
signals, so a voice coil in the loop between your telephone and the
telephone company's central office will disqualify you from receiving ADSL.
Other factors that might disqualify you from receiving ADSL include:
- Bridge taps - These are extensions, between you and the central
office, that extend service to other customers. While you wouldn't notice
these bridge taps in normal phone service, they may take the total length
of the circuit beyond the distance limits of the service provider.
Fiber-optic cables - ADSL signals can't pass through the
conversion from analog to digital and back to analog that occurs if a
portion of your telephone circuit comes through fiber-optic cables.
- Distance - Even if you know where your central office is (don't
be surprised if you don't -- the telephone companies don't advertise their
locations), looking at a map is no indication of the distance a signal
must travel between your house and the office.
Splitting the Signal
There are two competing and incompatible standards for ADSL. The official
ANSI standard for ADSL is a system called discrete multitone, or
DMT. According to equipment manufacturers, most of the ADSL equipment
installed today uses DMT. An earlier and more easily implemented standard
was the carrierless amplitude/phase (CAP) system, which was used on
many of the early installations of ADSL.
CAP operates by dividing the signals on the telephone line into three
distinct bands: Voice conversations are carried in the 0 to 4 KHz
(kilohertz) band, as they are in all POTS circuits. The upstream channel
(from the user back to the server) is carried in a band between 25 and 160
KHz. The downstream channel (from the server to the user) begins at 240 KHz
and goes up to a point that varies depending on a number of conditions (line
length, line noise, number of users in a particular telephone company
switch) but has a maximum of about 1.5 MHz (megahertz). This system, with
the three channels widely separated, minimizes the possibility of
interference between the channels on one line, or between the signals on
DMT also divides signals into separate channels, but doesn't use two
fairly broad channels for upstream and downstream data. Instead, DMT divides
the data into 247 separate channels, each 4 KHz wide. One way to think about
it is to imagine that the phone company divides your copper line into 247
different 4-KHz lines and then attaches a modem to each one. You get the
equivalent of 247 modems connected to your computer at once! Each channel is
monitored and, if the quality is too impaired, the signal is shifted to
another channel. This system constantly shifts signals between different
channels, searching for the best channels for transmission and reception. In
addition, some of the lower channels (those starting at about 8 KHz), are
used as bidirectional channels, for upstream and downstream information.
Monitoring and sorting out the information on the bidirectional channels,
and keeping up with the quality of all 247 channels, makes DMT more complex
to implement than CAP, but gives it more flexibility on lines of differing
CAP and DMT are similar in one way that you can see as a DSL user. If you
have ADSL installed, you were almost certainly given small filters to attach
to the outlets that don't provide the signal to your ADSL modem. These
filters are low-pass filters -- simple filters that block all signals
above a certain frequency. Since all voice conversations take place below 4
KHz, the low-pass (LP) filters are built to block everything above 4 KHz,
preventing the data signals from interfering with standard telephone calls.
ADSL uses two pieces of equipment, one on the customer end and one at the
Internet service provider, telephone company or other provider of DSL
services. At the customer's location there is a DSL transceiver,
which may also provide other services. The DSL service provider has a DSL
Access Multiplexer (DSLAM) to receive customer connections.
Most residential customers call their DSL transceiver a "DSL modem." The
engineers at the telephone company or ISP call it an ATU-R.
Regardless of what it's called, it's the point where data from the user's
computer or network is connected to the DSL line. The transceiver can
connect to a customer's equipment in several ways, though most residential
installation uses USB
or 10 base-T
Ethernet connections. While most of the ADSL transceivers sold by ISPs
and telephone companies are simply transceivers, the devices used by
businesses may combine network
other networking equipment in the same platform.
The DSLAM at the access provider is the equipment that really allows DSL to
happen. A DSLAM takes connections from many customers and aggregates them
onto a single, high-capacity connection to the Internet. DSLAMs are
generally flexible and able to support multiple types of DSL in a single
central office, and different varieties of protocol and modulation -- both
CAP and DMT, for example -- in the same type of DSL. In addition, the DSLAM
may provide additional functions including routing or dynamic
assignment for the customers.
The DSLAM provides one of the main differences between user service
through ADSL and through
Because cable-modem users generally share a network loop that runs through a
neighborhood, adding users means lowering performance in many instances.
ADSL provides a dedicated connection from each user back to the DSLAM,
meaning that users won't see a performance decrease as new users are added
-- until the total number of users begins to saturate the single, high-speed
connection to the Internet. At that point, an upgrade by the service
provider can provide additional performance for all the users connected to
ADSL is competing with technologies such as cable-modem access and
Internet access for high-speed connections from consumers to the
Internet. According to IDC, a market-analysis firm based in Framingham, MA,
approximately 330,000 households in the United States were connected to the
Internet via DSL in 1999, compared to 1,350,000 households with cable
modems. By 2003, IDC estimates that the number of households with cable
modems will have risen to 8,980,000, while DSL will have raced into the
broadband lead with 9,300,000 households.
Currently, ADSL is limited (by U.S. Federal Communications Commission
regulations) to a maximum of 1.5 megabits per second. Current technology can
provide a theoretical maximum of up to 7 megabits per second, and research
promises even greater performance in the future with protocols like G.Lite
For information on ADSL rates and availability in the United States, go
Broadband Reports. This site can provide information on ADSL service
companies in your area, the rates they charge, and customer satisfaction, as
well as estimating how far you are from the nearest central office.