Hardware Reference
In-Depth Information
How DSL Works
DSL takes advantage of the broadband nature of the telephone system, using the system's
capability to carry signals at multiple frequencies to allow both high-speed Internet traffic
and phone calls at the same time. Two methods for sending and receiving signals are used
by the most common type of DSL, Asymmetric DSL (ADSL):
• Carrierless Amplitude/Phase (CAP)
• Discrete Multitone (DMT)
Most early DSL installations used CAP, which splits the telephone line into three fre-
quency 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 an-
swering 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.
Somesystemsusethe300Hz-700Hzrangefordownstreamdataandfrequenciesof1MHz
and above for upstream data.
Because voice, downstream data, and upstream data use different frequencies, the tele-
phone and Internet connections can be used at the same time.
DMT, the system used by most recent ADSL installations, divides the telephone line into
247channelsthatare4KHzwide.Ifaparticularchannelhasproblems,adifferentchannel
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 de-
pends 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 con-
nects to your computer).
