ADSL - What is it and do I need it?

By: Shane Dempsey

Let's face it, in Ireland home Internet access is just too S-L-O-W. If you have a 56 Kbps (Kilo bits per second) modem you're unlikely to get download speeds above 4 Kilobytes per second (8 bits to a byte and congested networks). Start adding images to web pages and they quickly grow to well over 50 Kilobytes making access quite slow even if common images are stored on your computer or cached. The good news is that the situation will improve and there are faster Internet access methods. ISDN is one of possibility but this is a relatively old technology, fast by comparison with 56K modems.

Asymmetric Digital Subscriber Loop (ADSL) is a much newer modem technology, which has been developed over the last 10 years to provide a broadband (transmission over a wide range of frequencies), always-on connection over an ordinary telephone line, on top of the existing telephone service. As the name suggests, it's asymmetric and provides a greater downstream capacity (towards the customer) than upstream capacity. The cable linking the Local Exchange or Central Office of the telephone network with the customer's premises is designed to deliver the 4 kHz of bandwidth needed for the standard analogue voice telephone service but can carry signals at much higher frequencies. Above 20 kHz, the signals are severely distorted. ADSL modems carry the broadband signals at frequencies between 20kHz and 1.104MHz (M stands for Mega or million) and use sophisticated encoding/decoding techniques to overcome the distortion.

The modulation scheme (how data is transmitted over the wire) is constantly adapted to compensate for the distortion at these high frequencies. ADSL modems can theoretically deliver up to about 8Mbit/s downstream and almost 1Mbit/s upstream but the actual transmission rate that can be achieved is strongly dependent on the length and quality of the copper cable. Customers must be within a few kilometers of their local exchange. In general, ADSL modems contain a Public Switched Telephony Network (PSTN) splitter, allowing ADSL and telephone signals to be carried on the same copper pair. The splitter acts as a filter, routing low frequency PSTN signals to the telephone and high frequency ADSL signals into the ADSL encoder/decoder.

Most ADSL operators offer a range of service options. Each option provides varying levels of downstream and upstream transmission rates. Eircom is still carrying out trials on their ADSL service, called I-Stream. Home customers of the service can expect to pay around EUR250 for equipment and setup and around EUR100 per month. Business customers wishing to connect a network of 4 or more users to the web via ADSL will pay about 390 Euro for equipment and setup and a further EUR225 a month. These products offer 512/128 Kbps and 1Mbps/256 Kbps transmission rates, respectively.

This is not the complete story however as ADSL is 'always-on' technology. This means that your machine or network is always connected to the Internet. This means that you don't have to wait 30 seconds or so while your computer dials up and you receive e-mails instantly. The downside is that you absolutely require security software that can act as a firewall to stop from stealing information or tampering with your computer.