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ADSL
EXPLAINED
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ADSL is a new technology, developed by Bellcore Labs of Morristown, New Jersey, and was originally intended for use with interactive TV. It’s the latest big breakthrough in high-bandwidth connectivity, with the potential to bring as much as 7MB of data across the copper telephone wires that are already installed in most homes.
ADSL stands for Asymmetric Digital Subscriber Line. It’s asymmetric in that it uses most of the channel to transmit downstream to the user, and only a small part to receive information from the user. However, the outgoing (the slower direction) is still considerably faster than an analogue modem or ISDN connection.
It works by dividing the bandwidth of copper telephone lines into different frequency ranges, known as carriers. The separate carriers each carry different parts of the same data transmission simultaneously, in a process known as Frequency Division Multiplexing (FDM). This gives more available bandwidth and higher transmission speeds for the end user, and allows several different signals to be sent on the same wire. In this way, ADSL can accommodate simultaneous high-speed transmission of data, video, and fax, all without interrupting the regular telephone service on the same line. There is a frequency range reserved for Plain Old Telephone Services (POTS) so that you can still make a phone call during a high-speed data transmission; in fact, you can still make a phone call in the event of an electrical power outage.
It is, in effect, an upgrade to your existing analogue phone line. Your existing BT wallbox is modified to split-off the 'ADSL' signal, and your existing telephone line and number will still work as normal.
Advantages
ADSL offers several advantages over a conventional dial-up or ISDN connection:
Improved speeds of data transmissions. Presently, ADSL is capable of delivering downstream speeds of up to 2MB burst, and upstream speeds of 256K burst; and as the technology develops in years to come, you can expect downstream speeds of up to 32MB, and upstream speeds as high as 2MB.
No dial-up connection. ADSL builds high bandwidth into your existing phone line, and once you install your ADSL modem it's always on. This eliminates the need to dial up in order to establish a connection to the Internet or to your corporate LAN; so the Internet is always available when you need it.
Maximum utilisation of resources. Regular telephone services use about 1% of the actual capacity of the telephone lines; ADSL picks up the slack, using the remaining 99% for high-speed data transfer. This is accomplished by setting different frequency channels for different functions, so that voice communication still happens at the lower end of the frequency spectrum, while the rest of the wire is used to carry higher frequency data transmissions.
Multitasking. Because of the different frequency channels used on the same wire, ADSL allows the simultaneous use of voice and data. Using a single phone line, you can make and receive phone calls, send and receive faxes, browse the Web and download information, send and receive email, upload web pages to your site, and access data on your corporate LAN.
Fixed monthly charges, so you know in advance how much you are going to spend each month on your Internet calls.
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