ADSL (Assymetric Digital Subscriber Line) was devised as a means for transporting broadband over conventional telephone lines. Obviously this makes great sense as almost all of us have a telephone line. But that line, consisting of a pair of twisted copper wires, was only ever originally intended to carry voice calls.
ADSL exploits the fact that voice calls only use a limited range of frequencies (under 4kHz) whereas the wires themselves can carry much higher frequencies. How high depends on how far you are from the exchange (higher frequencies degrade more rapidly over distance). This is where the confusion arises over speeds being quoted as ‘up to.’
Most ISPs currently offering ADSL use ADSL2+ which has a maximum speed of 24 Mbit/s downstream and 1 Mbit/s upstream. But that depends on you being close to the exchange and having a good quality copper line. When you visit the ISPs website, they will give you a prediction based on what they know, but no ISP can guarantee a speed.
This may become clearer with a brief explanation of how it work in practise:
If you have ADSL you will have a filter in your house, either integrated into the master socket or a separate ‘microfilter.’ This splits off the lower frequencies for you telephone(s) and send the higher ones to your ADSL modem. This is usually supplied by the ISP and most often ‘built-in’ to a wireless router.
Your ADSL modem uses the next block of frequencies (26-138kHz) for your 1 Mbit/s uplink. All of the frequencies above that are available for download. The number of frequencies actually usable for download will depend on the length of the line, its quality and any outside sources of interference. Your modem will go though a process called ‘training’ where it tests each frequency and decides how much data each one can reliable carry. It can take some time to settle down to the best speed it can reliably achieve. This is why when you first get ADSL installed you should try not to disturb or restart the modem until it has had time to complete this process.
ADSL can be adversely affected by any noise on the line, so if you have a crackly phone line you might need to get that fixed before you subscribe to ADSL.