Originally Posted by Richard B
Can someone correct me if I'm wrong...
There are only really two Broadband suppliers - Virgin Media (formerly NTL) and BT.
Virgin Media (formerly NTL) use the same cable infrastructure used to distribute cable TV and bandwidth is shared with your neighbours, so some "throttling" can occur when all the people in your segment (usually your road or street) are using the service.
BT supply direct (BT and BT Yahoo) as well re-packaging the service through suppliers such as AOL, Orange, Plus Net, Zen, Global, Waitrose and Pipex. It's a single (not shared) connection back to the local exchange, but bandwidth is dependant on distance from the exchange.
There's also LLU (Local loop unbundling) which allows operators to make the last connection from the exchange to you without BT. IIRC the service provider puts their own kit in BT exchange. Generally, this is only broadband and not the regular voice lines. It's one of the reason's TalkTalk had such a nightmare because they were unbundling exchanges, not just setting up connections.
The other issue on telephone lines is that of 'contention ratios' - ie how many people are sharing the bandwidth at various points on the route. Pay more for less contention.
There'is another option if you really want speed and certainty - leased lines, but you can add a couple of noughts to your usual phone bill for that.