The full cost of provisioning adsl is quite expensive and therefore to make it available and more affordable for more people, the costs and the available bandwidth is shared.
IPStream adsl relies on traffic being bursty and not too many packets being requested at the same time. You cant get more data through the pipes than the physical size of the pipe and if too many users request data at the same time, then each user gets a smaller portion of the available bandwidth, and speeds start to suffer.
Some people may be a little confused as to what contention is and how it affects us. I've written a more technical explanation of contention from the BT side of things but below is an analogy of contention reproduced from a post I made in late 2005 from the ISPs perspective.
I'm trying to think of a way to explain it in simple terms.
The following may not be the best analogy - its full of flaws, but until some one thinks of something better it will have to do.
There's a village with 1000 people in the middle of no-where which has no water.
To get water piped to the village a company [BT] says I can provide you with a water pipe which will give you up to 155 pints of water at anyone time. (The pipe isn't big enough to hold more than 155 pints), but the cost for this pipe will be £40,000 per month to every house that I lay the pipe to.
The villagers think there is no way we can each afford to pay £40,000 for a water pipe each, so the village chief [ISP] says, I tell you what I will pay for the main pipe to be installed and the water... but you all have to give me £40 per month.
So all the villagers pay their £40 and hook up a smaller hose [modem] which will run 2 pints of water at any one time so that they can each have water, For a long time this works ok, When John runs a bath, his neighbour isn't using the water, Sue does her washing at a different time, and they all draw what water when they like... but its sporadic and bursty.
Then one day a group of 50 travelers pass through the village and they like it so much that they decide they are going to stay, and they each give their £40 to the Chief and get their own 2 pint pipe each.
A few months later these new people decide they are going grow a crop which requires watering 24/7.
So they each use their hoses to water the crop.. The only problem now is that these 50 hoses are drawing 2 pints of water which is 100 pints at any one time. This only leaves 55 pints for the rest of the 1000 in the village. Which some of the time is ok.
The rest of the village don't mind during the night when they are asleep. But problems start to occur at "peak times" such as Monday mornings when all the housewives wish to do their washing, or at 6pm on a sunday evening when most of the mums bath the kids.
The remaining 55 pints isn't enough for the rest of them.
So what happens then is that everyones water flow reduces when they run a bath, and it takes longer to fill it.
You could still fit more hoses on the main pipe, but what happens is it takes longer for everyone to draw their water, and the bath now takes an hour to run rather than 5 mins.
In other words you can always put more people on the big pipe at the back, but the more that use it at the same time, then that portion becomes less for every one.