How does contention work on FTTC

[#1]

Contention on FTTC is something I've often wondered about, maybe someone can answer.
Here is a very simple example.
Say we have 2 people connecting on a FTTC cabinet, lets say one is syncing at 40mbit, the other at 80mbit.  Then say there is contention and neither can receive their full speed.
Is each connection slowed down related to their respective sync speed, or is the connection syncing the fastest slowed down first?   In other words is the fastest connection reduced first, or are all connections reduced regardless of the sync speed?  
Can anyone shed any light on the contention algorithm used on a FTTC cabinet.  Just curious that's all.
Thanks
Jason

[#2]

It's more likely that the contention is at the exchange or even further away.

[#3]

Any contention would be at the exchange rather then the cabinet.
However crosstalk can occur in a cabinet which whilst not the same as contention can drop your speeds over time as more and more users swap over to fibre.

[#4]

Matthew,  I have no issues with my own internet,  just curious how contention works on FTTC.
Going back to original question.  When contention happens is the fastest connection slowed down first, or are all connections slowed down regardless of connection speed.