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Edit: Sorry for the long post - it grew... This is very NLC but a few
folks seem interested on and off list. If you aren't interested in how
ADSL works, please stop reading now!
Right...
The contention at the exchange is (almost) never the issue. The issue
lies with the available bandwidth on the central pipes rented by the ISP
from BT (leaving LLU out of it for now) which all traffic from the BT
exchanges to your ISP travels across. If your ISP doesn't have enough
to support all its users at peak times, you will see slowdowns.
Each 155Mbps central costs a small fortune (millions). Each must carry
a certain number of users to make it pay for itself. As DSL speeds
increase, the usage patterns of users changes from bursty traffic like
mail, webpages etc to full-whack traffic like file-sharing, music
downloads, streaming media etc. This means that about 5% of your user
on a pipe can max out the entire pipe at peak times slowing everyone
down, including those who only want to check their email. There are a
few ways to deal with this problem.
1) Buy more centrals to deal with peak time demand. This is an
approach that the deeper-pocketed ISPs are taking short term while they
scrabble around for a better idea, but as there are still the same
number of users paying their £14.99 per month, nobody is paying for the
extra centrals. This is financial suicide. It could wipe out a small
ISP in a year, and even the biggest ISP eventually. ADSL is getting
faster. The faster it gets, the more people can, and do, download. At
the same time, the customer demands cheaper (or even free) broadband so
the amount of money available to pay for centrals drops. If you wanted
your fair share of a central based on contention, you'd have to pay
around £300 per month for 2Mb ADSL, so the exchange contention (50:1 or
20:1) doesn't even come into it. Nobody will pay £300 per month for
ADSL; that's crazy. Worse still with this approach, off peak (2am for
example) you have a load of empty centrals which are costing a *lot* of
money but nobody needs.
2) Limit your customers' usage (to 1GB, 2GB, 40GB etc per month) as
Robin suggested. This was the first thing PlusNet tried, but it didn't
quite work. It goes part way to solving the problem, and allows ISPs to
offer cheaper products, but gives no control over *when* the users
choose to use up their allowance. The chances are that at peak times,
or during an major online event (olympic games live etc), your ISPs
centrals will still be maxed out and users will be getting slow speeds.
Your customer who only wants to check their mail will be tearing their
hair out at the mercy of the handful of customers who are maxing out
their 8Mb connections filling up their iPods.
3) Start prioritising download traffic. This is where I can blow the
PlusNet trumpet a bit, as we are leading the world in developing this
technology. Simply, we have developed, and are currently trialling, the
ability to inspect every piece of traffic across our network (using deep
packet inspection hardware) and based on what that traffic is, make sure
it gets through in the right order.
We can now protect the guy checking his webmail from getting a sluggish
experience by stealing a bit of priority from somebody's Napster
download for example. Deep packet inspection is not that new, but we
can now prioritise based on what you are downloading, how time-sensitive
it is, which product you are on (thus how much you pay us) and how busy
our network is, all in real time. During the night when there are few
users 'logged-on', all traffic gets full priority as there is more than
enough central capacity so need to prioritise. At 4pm when the kids get
home from school and jump on the internet for a while, the centrals max
out. Our aim is make sure you never notice. This is generally termed
Quality of Service (QoS).
Nobody will notice if a large file takes 35 seconds instead of 30
seconds to arrive at peak times, but you can bet that if your Skype call
starts breaking up, you'll notice immediately, simply because voice and
streaming media are time-sensitive. So, by making sure that online
gaming (very time sensitive), streaming media (time-sensitive), then
email and web traffic (small, bursty), then all the other stuff gets
through ***in that order*** everyone gets a full speed experience even
at peak times. It also means that, within reason, we won't be that
concerned about how much you download.
Content providers are worried about this QoS problem too. They want to
launch Movies-Over-Broadband and other such services, but because the
ISPs own the delivery network which gets the movie to your home, the
content providers can't promise you will be able to enjoy the movie
without it stopping or glitching or pausing if your ISP network is busy.
This would cause users to demand their money back from the movie
company because they movie skipped a lot. What the content providers
need is an ISP partner who can guarantee that their movies will be
priortised over other traffic so that you can enjoy your movie without
interruptions, skipping etc.
You may ask "Why haven't I noticed?" or "Why doesn't my ISP have this
problem?"
The answer is that they do. They are dealing with it using solutions 1
or 2 above. The only solution that will work long term is 3.
So why isn't your ISP doing 3? They can't. Some have very primitive
blocking or limiting of peer-to-peer traffic for example, but the full
Monty is an NP-hard problem and we've been working on it for a long
time. More than a few ISPs know that we can (and soon will for real)
and they are already knocking on our door. We need to be careful about
giving our advantage away, but if the price is right...
<trumpet>
And that, my friends is why PlusNet can continue to offer the best value
broadband long-term. We are not just a reseller of BT wholesale
products; the occasional customer service issues pale into
insignificance when you realise that we are actually leading the
development of the entire industry. When you start getting streaming,
on-demand TV/movies over your broadband connection, regardless of who
your ISP is, it will be because a small group of people at PlusNet
helped make it possible.
</trumpet>
Questions/Comments/Flames on-or-off-list very welcome.
Graeme
Anyway the business pays for it 
