Hardware Reference
In-Depth Information
• Discrete Multitone (DMT)
Most early DSL installations used CAP, which splits the telephone line into three frequency bands.
Exact frequency usage varies by system, but most typically, the divisions resemble the following:
• Voice calls use frequencies from 30Hz to 4KHz. This frequency is also used by answering
machines, fax machines, and alarm systems.
• Upstream data such as web page requests and sent email uses frequencies between 25Hz and
160Hz.
• Downstream data such as received web pages and email uses frequencies between 240KHz and
1.5MHz.
Some systems use the 300Hz-700Hz range for downstream data and frequencies of 1MHz and above
for upstream data.
Because voice, downstream data, and upstream data use different frequencies, the telephone and
Internet connections can be used at the same time.
DMT, the system used by most recent ADSL installations, divides the telephone line into 247
channels that are 4KHz wide. If a particular channel has problems, a different channel with better
signal quality is used automatically. Unlike CAP, DMT uses some channels starting at around 8KHz
to send and receive information.
Both types of signaling can have problems with interference from telephones and similar devices, so
devices called
low-pass filters
prevent telephone signals from interfering with signals above the
4KHz range, where DSL signals begin. The location of these filters depends on the type of DSL you
use and whether you are installing DSL service yourself.
At the central switch, DSL data is transferred to a device called a
DSL access multiplexer
(DSLAM), which transfers outgoing signals to the Internet and sends incoming signals to the correct
DSL
transceiver
(the correct name for the so-called “DSL modem” that connects to your computer).
DSL Availability
Just as distance to a telephone company's central switch (CS) is an important consideration for
people purchasing an ISDN connection, distance also affects who can use DSL in the markets offering
it. For example, most DSL service types require that you be within about 18,000 feet (about 3 miles)
wire distance to a telephone company (telco) offering DSL; some won't offer it if you're beyond
15,000 feet wire distance because the speed drops significantly at longer distances. Repeaters or a
local loop that has been extended by the telco with fiber-optic line might provide longer distances.
The speed of your DSL connection varies with distance: The closer you are to the telco, the faster
your DSL access is. Many telcos that offer some type of DSL service provide websites that help you
determine whether, and what type of, DSL is available to you.
If you want to locate DSL service providers in your area, compare rates, and see reviews from users
of the hundreds of ISPs now providing DSL service, set your browser to
www.dslreports.com
. The
site provides a verdict on many of the ISPs reviewed, summarizing users' experiences and ranking
each ISP in five categories.
Major Types of DSL
Although the term
DSL
is used in advertising and popular discussions to refer to any form of DSL,
many, many variations of DSL are used in different markets and for different situations. This section
