Sorry only just seen this - still have catching up to do :/
Contention ratios aren't in use any more. They went out when BTw brought in CBC (Capacity Based Charging) for the ISP centrals and it was basically down to the ISP how many users they placed on their pipes. Some ISPs managed it much more efficiently than others eg: Zen - v - Virgin dsl.
Slightly later when maxdsl came in then you cant put an official contention ratio on ISP/BT bandwidth, because different users connect at so many different sync speed making it impossible to do the old type maths of say 50 @ 512kbps.
Instead now its majorly up to the ISPs to monitor how much bandwidth all of their users are using on their pipes. BT provide graphing abilities and once a pipe bandwidth approaches 98% full then the pipe can start to run hot and you may see some packet loss. Time for the ISP to order more bandwidth. When it reaches 100% capacity EU speeds drop for all users.
Bandwidth is expensive for the ISPs so they try deter overuse by customers and its why you now get restrictions such as traffic shaping or caps. Prior to CBC these were unheard of.
Business users pay more, and the ISPs can deal with this in a few ways
1) Higher download caps
2) Less traffic shaping
3) Have pipes reserved purely for their business accounts which have more spare capacity than the home users. This spare capacity means that business users would be less likely to see congestion than their home users.
Congestion can still be an issue at an exchange level, sometimes BT may be caught out and certain VPs may run 'hot' until BTw get around to lighting more capacity. (This is what dark fibre was all about). Hot VPs are/were more likely to occur on 20CN exchanges as their VPs were smaller and based around the line cards that people shared.
21CN is more efficient in that the MSAN has large 'super' VPs rather than split into chunks like with the DSLAMs.
BS is right there are figures for the ISP - its called session limits and BTw police these. It used to be a maximum of 25,600 EUs for a 622Mb central. The ISP estimates that at anyone time say only 70% of their users will actually have their routers switched on, and take this into account when planning how much bandwidth to buy for their centrals. Problems occur though when an ISP has a large customer base who keep routers on 24/7 and their may be times when some users cant connect. When CBC first came in, it wasnt unusual to have to try a couple of times to connect, but these days its much rarer as the ISPs now have a better idea of their customer base habits.
Theres more info here about the
BT Centrals. Be aware though that Centrals are gradually being replaced and within the next year or so
WBC/WBMC will mean that all BTw ISPs will be expected to purchase MSILs or dedicated host links.
Theres more info ~
What is a MSIL? together with the 3 options an ISP has of purchasing bandwidth & host links. There is also currently a 4th option (IPSc) but that will be also gradually be wound down after BTw stop supplying new centrals in 2014. Lifetime of a central is about 2 yrs, so centrals/IPStream/Datastream/IPSc will likely be totally gone by 2016 - if not sooner. I think all but the smallest ISPs are already moving over... the larger ones already have.
And of course things are different again for LLU.