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the receiving channel can climb to 6 Mbit/s, but actual performance depends on the condition of the line
being used.
There was a DSL Forum formed towards the end of 1994, and this was set up to help telephone companies
and their suppliers realize the great market potential of ADSL. This forum has two forms of assistance which
are technical and marketing. The Forum's marketing programs attempt to uncomplicate the technical
complexity of it and to spread the news concerning ADSL. The output of the forum to the public mixes the
tutorial with the promotion of this device.[2]
The ADSL Technology:
ADSL specifications states that it can support transmission rates of 8Mbps downstream from a central office
to residence and upto 640kbps for upstream transmission. The key feature of ADSL is its ability to works with
Plain Old Telephone Services(POTS) on the same copper line using a filter called splitter. ADSL uses Frequency
division Multiplexing (FDM) and a Splitter is responsible for separating the frequency rates for data and
telephone signals. The ADSL functions at the central office are handled by the ATU-C (ADSL Terminal Unit).
The ATU-C transmits high-speed simplex channels downstream, and sends and receives lower-speed duplex
channels. The ATU-C units may either be standalone, or mounted with others in an equipment shelf. In the
future, the ATU-C will be integrated into access nodes and remote access nodes. The ATU-C is paired with its
mirror image, an ADSL remote terminal unit (ATU-R). The ATU-R is generally a standalone device, and
receives downstream data and transceives corresponding duplex data. Refer the below figure for an overview
of ADSL.
Figure 70: Architecture
Frequency Division Multiplexing (FDM) is used for ADSL over a 1 MHz spectrum. Diagram 3 illustrates the
general allocation of the frequency spectrum above the voice band. The downstream (high capacity) data
rate is largely dependent on the length of the subscriber line from the central office and the gauge of the
twisted pair cable.
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