ISP's using Datastream connections (Tiscali, Bulldog etc) are the ones that are being complained about most on many ADSL forums. This is because BT, under OFCOM rulings, had to open their exchanges up for the sake of competition. Since they had to do this, it now allows ISP's to install there own equipment in the exchange. This is the key to Datastream services.
A Datastream provider can install a 2Mbit circuit into a telephone exchange, but this immediatly causes contention issues, as the capacity to run thousands of connections is not specifically there from the offset.
The main issue currently facing Datastream broadband providers is the speed that users can access the Internet and email services. Because the ISP routes traffic through its own network, the more users on board directly affects the speed in which users can use the Internet. The busier a telephone exchange, the worse the connection speed becomes.
IPStream Broadband Providers (BT, Proweb etc) channel all customers down a 155Mbit network circuit. This is fed directly onto a BT controlled network, which gives us a balanced speed. The number of users connected to the exchange does not affect the connection speeds, as it is a completely balanced network, regulated by OFCOM.