What is ADSL?

At the simplest level, ADSL is a high-speed replacement for your modem or ISDN adapter that allows you to access the Internet faster.

ADSL stands for Asymmetrical Digital Subscriber Line - it is the transmission technique used on the line from your modem to your service provider.

Asymmetrical:
The speed of transmission is not the same in both directions. The downstream (from network to consumer) speed can be more than ten times as fast as upstream (from consumer to network). This coincides nicely with the requirements for Internet access where a single mouse click (therefore a small amount of data sent by the consumer) can result in the receipt of a large amount of data from the Internet.

Digital:

ADSL modems operate on a bit stream, and are intended for carrying digital information between digital equipment such as PCs. In this respect they are no different from conventional modems.

Subscriber Line:
ADSL itself only operates over the subscriber's normal telephone line to the local exchange. The telephone line can continue to be used for voice calls through the use of devices called 'splitters' that separate the data and voice on the line.

Beyond the point at which the subscriber's line is terminated in the exchange, other technologies are responsible for the data transmission.

What does ADSL do?

ADSL defines how data can be transmitted between a user's premises (home or office) and the local telephone exchange over the normal telephone wiring. The telephone companies call this telephone wiring 'the local loop'.

Getting data to and from the local telephone exchange is not in itself of much use. The purpose of ADSL services is to enable high-speed access to the Internet, so discussions of ADSL (including this one) generally include how the data connection is extended to an Internet Service Provider, and therefore, to the Internet.

So although we assume that ADSL is used to carry data using Internet protocols, how this is done is not in fact part of the ADSL specification. This gives rise to some of the variations that occur in practical implementations of ADSL.

ADSL was originally devised as a way of delivering digital television over telephone wires and this may be a significant application in the future. For now, the main use of ADSL is Internet access.

How does ADSL work

ADSL exploits the unused analogue bandwidth that is potentially available in the wires that run from the user premises to the local exchange. This wiring was designed to carry that portion of the frequency spectrum that is occupied by normal speech. The wires can, however, carry frequencies above this rather limited spectrum. This is the portion that ADSL uses.

voice typically uses the range 300Hz to 3,400Hz.

We can now see how voice and ADSL data can share the same telephone line - in fact, splitters are used to ensure that the data and voice do not interfere with each other.

The frequencies that the local loop can carry - and hence, the amount of data transmission capacity that is available - depend on a number of factors such as:

• The distance from the local exchange
• The type and thickness of wires used
• The number and type of joins in the wire
• The proximity of the wire to other wires carrying ADSL, ISDN and
  other non-voice signals
• The proximity of the wires to radio transmitters.