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The ISP, formerly ntl:Telewest, said that it occasionally moderates downstream speeds for subscribers 'downloading an unusually large amount' of data, sometimes as much as 3GB, during the peak 4pm-to-midnight period.

'Our boffins recently ran a trial to see how much our service was being affected,' it explained. 'They discovered that, in certain areas, just five per cent of customers were downloading such a large amount of content that it was affecting the service for other users in these areas.'

Traffic shaping - the industry's term for temporarily reducing individual user's speeds - will be applied at varying thresholds. Subscribers to the

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M package will have their download speed cut to 1Mbps (from 2Mbps) for four hours if they download 350MB during peak hours; L subscribers will be limited to 2Mbps (from 4Mbps) for exceeding 750MB and XL subscribers to 5Mbps (from 10 and in some cases 20Mbps) for exceeding 3GB. Upstream speeds will also be cut.

The ISP said it would monitor the effects of these restrictions and would raise or lower the thresholds if necessary. It said that it has already noted a 'positive response' from customers, with fewer calls to its technical support teams regarding problems with broadband performance. Indeed the broadband speed was faster for the vast majority of users, it said.

More information can be found at allyours.virginmedia.com/html/internet/traffic.html.

Virgin is not alone in applying traffic shaping to manage bandwidth during peak hours, and its restrictions are much less severe than other ISPs apply. However the problem may soon disappear, with BT promising that its 21st Century Network, which it began implementing late last year, will provide enough capacity to make restrictions unnecessary.