The one area in which there may be sharing of bandwidth on DSL is at
the CO/ISP - because ISPs tend to underallocate their outbound
bandwidth to the Internet backbone. They have to do this to make
any money at all - if they had outbound bandwidth to support the
full offered bandwidth for all their users, it would be unused
most of the time, just eating money.
That is, there is some number X that is the factor by which they
multiply the size of their pipe to the backbone to determine how
many customers they can serve. Different ISPs will use different
values of X; the bigger X is, the less likely it is that any single
customer will get to use their pipe fully.
For example, if a theoretical ISP has a 1 megabit/sec pipe, they can serve
8 128-kbit/sec customers without any sharing. They probably would run
with X=5 or so, depending on their other costs, so they would provision
40 128-kbit/sec customers on that 1 mbps pipe. If all 40 customers
try to download at once, they're all going to see less than their
rated speed.
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