Asymmetric Digital Subscriber Line (ADSL) is a broadband digital connection in which the downstream
and upstream bit rate are unequal. For example, a 256k/64k ADSL connection consists of a 256Kb downstream
and 64Kb upstream bit rate. ADSL is also a distance-sensitive technology. This by definition means that as
a connection's length increases, the signal quality decreases, thus reducing the connection speed. The limit
for an ADSL service is approximately 3500 metres from your exchange.
Voice calls are not limited by distance and the reason for this lies in small amplifiers
called loading coils. These coils are used to boost voice signals if the distance from your phone line
to the exchange is over the ADSL distance limit. Unfortunately voice coil technology is not compatible
with ADSL signals so if a voice coil is required between your telephone line and the exchange this will
prevent you from connecting an ADSL service.
ADSL operates by dividing the signals on the telephone line into two distinct bands. Voice conversations
are carried in the 0 to 4 KHz (kilohertz) band. The data
band operates between 20KHz and 2.2MHz (megahertz). The use of these bands minimizes the possibility of
interference between the channels.
To use both the voice and data bands simultaneously (ie. be on the Internet and the phone at the same time)
you require a small low-pass filter (LPF) attached to the line the phone is connected to. To connect an ADSL
modem and phone to the same physical phone line a splitter (which incorporates a LPF) is required.
Low-pass filters are used to block all signals above 4 KHz, preventing data signals from interfering with standard telephone calls.
To create an ADSL connection a DSL modem or router connected to your phone line initiates a connection.
A DSL Access Multiplexer (DSLAM) located at the telco accepts the connection and then passes it through
to ISPDr where it is finally terminated at an access server for login.
An ADSL modem or router can connect to your computer equipment in several ways, though most residential
installations use either USB or Ethernet connections.
A DSLAM accepts many ADSL connections 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.
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