Although a fair usage limit on something labelled 'unlimited' is entirely antithetical, there are performance reasons behind providers' policies that may impact your internet usage.
Fair use and contention ratios
Customers share internet connections. The number of people sharing a connection is called the 'contention ratio', which can be up to 50 people per line. So, if one user is constantly downloading, it will slow the connection for the other users who are on at the same time.
Fair usage policies limit the usage of one user over another at peak times so that the performance is not affected for the rest of the group. Providers tend to group people according to usage, so people who download all the time are grouped together.
These restrictions are imposed to ensure that the best broadband quality is available to the maximum number of customers. So, even though you have a 24Mbps connection with unlimited downloads, your speed may be heavily limited at peak times.
This is frustrating for many people who have specifically bought high-speed packages because of their usage patterns. This kind of advertising is aimed at heavy users and encourages people to get super speeds and unlimited allowances.
Check with your ISP to see your contention ratio and what they classify as fair usage.
Internet usage of heavy users
Heavy or excessive users generally use peer-to-peer (P2P) or file-sharing software. They upload and download large files, such as movies and music, daily, taking up excessive bandwidth and slowing the connection down for everyone else.
Generally, after a couple of months of heavy use, ISPs will notify you if they feel your usage is excessive. This notification will usually point you toward their specific policy on 'fair usage' and suggest you refrain from uploading or downloading large files at peak times.
What is fair?
Surfing, checking e-mails, and occasional downloading will not get you into trouble with your ISP. However, it is almost certain that you can download a 3GB movie every day.
The key is to keep an eye on the amount you're downloading. Then, if you're classed as a heavy or excessive user, restructure your internet usage pattern so that you are not uploading or downloading at peak times.
If you don't take heed of their warnings, your ISP will slow down the speed of your connection at peak times. In serious cases, your ISP may terminate your internet connection.
Be warned - unlimited is not quite unlimited.
