ADSL Architecture  

Delivery of ADSL services requires a single copper pair configuration of a standard voice circuit with an ADSL modem at each end of the line, splits the telephone line into three information channels: a high speed downstream channel, a medium speed upstream channel, and a Plain Old Telephone Service (POTS) channel for voice or an ISDN channel.

The ADSL network components are:

1.  The ADSL modem at the customer premises, that is called an ADSL transceiver unit-remote (ATU-R). It provides local loop termination on the customer side.
2. The modem of the central office that is called an ADSL Transmission Unit- central office (ATU–C). It terminates the ADSL local loop at the central office premises.
3. DSLAM – DSL access multiplexer. Many ATU-C units are inserted into the DSLAM. This unit can connect through an ATM or an ETHERNET access network to the internet.
4. Splitter – an electronic low pass filter that separates the analogue voice or ISDN signal from ADSL data frequencies when they get to the subscriber premises. For outgoing traffic, when they are transmitted from the subscriber premises, it combines the voice and the data frequencies onto one line. This allows a POTS phone connection to operate at the same time as ADSL digital data is transmitted or received on the same line.  One splitter is located at the central office and another at the subscriber premises. The splitter at the central office can be separate device or may be incorporated into the DSLAM.
   

Figure 1. loop architechture

How does it work?

An ADSL link is basically a computer, connected to a modem, connected over the old PSTN (public switched telephone network) copper wire to a DSLAM at the telephone company main office. The PSTN is separated from the data traffic using high/low pass filters (splitters). The voice data is routed to the regular PSTN switching network, while the data is routed to the ISP. The multiple virtual circuits (ADSL connections) created, are then aggregated and transferred to the correct ISP using a T3/E3 line.

The ADSL specification allows for voice (PSTN) and data communication to co-exist on the same pair of twisted copper lines. This is accomplished by a set of low-pass and high-pass splitters installed at both the customers' location and the central office. These splitters also protect the sensitive data communication from spikes originating in the PSTN network.
 

Figure 2. field configuration

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