The phone system is designed to transmit audio signals in the range of human hearing. To transmit digital data, a modem modulatesthe signal into analog audible tones that can be relayed through the phone system. The modem on the other end then demodulates them back again. This wastes bandwidth. Even though the phone wire is capable of carrying 25 MHz sound waves, an analog phone line never exceeds the 4-KHz range within the human hearing range. ADSL speeds data transmission by using a different part of the bandwidth to transmit data whilst still leaving the original 4-KHz band for voice.
BT runs two different types of network. Your local copper line which you use to connect to your local exchange, and a high-speed digital network over fiber-optic cables which it uses to link exchanges. At the local exchange, BT takes the digital signal and splits the signal into the 4-KHz analogue band and sends digital data down the remaining bandwidth. The expense of multiplexer used to split/combine the voice and data and the cost of the additional amplifiers needed to utilise the additional bandwidth are the reason that BT are reluctant to convert exchanges without a minimum customer base.
Because it is using higher frequency bands, the digital signal will not carry as far as the voice signal, hence the limitation of only being available out to around 5Km from the exchange - though a modified variant is being introduced reaching out to 8Km.
The additional signal can cause interference in equipment not designed to screen it out, hence the use of filters. The filters are used in-line on phones and other equipment designed to use the voice signal and filter out everything except the original 4-KHz signal.
Totally separate to the above ADSL technology, broadband digital signals can also be carried by cable operators who have their own high capacity fibre-optic networks to provide cable TV. Because they are not limited to using copper lines over the last few miles, they have better coverage in the areas they have cabled. They can also provide even greater bandwidth than BT. (BT is only just starting to offer a 1Mb service in some areas, whilst NTL is offering a 1Mb service as a standard option - at a price!). In most areas it is not economic for BT to convert exchanges in areas where a cable company such as NTL has already signed up most of the customer base.
AOL as an internet service provider, has contracts with both BT and NTL to provide the broadband connection to their customers. You obviously fall in a cable area and will receive a cable modem - not an ADSL modem. You will not require any filters if you have a BT phone as the two services are not connected. AOL will pay NTL, though NTL will undoubtedly try and persuade you to sign up for their own phone and TV services at the same time......
Last edited by ORAC; 17th October 2003 at 06:25.