How does contention work with BT IPStream etc

OK so 50 (or 20 if you are lucky) users share bandwidth.. Couple fo questions if anyone knows, rather than guesses, how it works..

Does that mean its the same 50 you are grouped with or is it dynamic?

Are you grouped with others from same provider (eg Tiscali or Entanet) or with all BT IPstream customers on that exchange?

I'm considering ADSL with O2 Home Access or Entanet or Tiscali as I am a heavy user but I want to know how it works. Leaning towards O2 as they seem a little less FUP happy than some and Tiscali sounds like speeds are dire.

In most modern exchanges, the users are grouped into all IPStream users (Enta et al) and then separate VP's for other data services (Datastream, leased lines etc) so yes, you are in effect contending with a multiple of the user rates you suggested. However the 50:1 and 20:1 has long since ben dropped in favour of "Home" and "Premium" (Office or 20:1) as was. Office gets a higher priority.

Don't touch Tiscali. O2 seems to be a reasonable choice if you don't mind rubbish DNS servers and will use Opendns or similar.

the 20 or 50 is a historical figure no longer used. It never meant groups of 20 or 50 in any case, it was a ratio, so x amount of actual bandwidth existed to supply 20x or 50x of sold bandwidth.
The 50 was always "up to 50" and in practice was nearer 20.

Contention can occur in the connection to the exchange on one or more virtual paths, this will affect users on those VPs which may be from several ISPs.

Tiscali tend to use Datastream with their own dedicated VP which they pile high and sell cheap.