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Originally Posted by kwikbreaks

All that happens on BTw is that there is a default target noise margin of 6dB set and the router/DSLAM negotiate the highest sync rate with that margin at the time. Increasing noise may cause the margin to fall. If it falls far enough sync is lost and then re-established still with that noise margin so as the noise was higher than means a lower sync. This is an ongoing process. If the software determines there have been excessive resyncs the target margin gets increased in 3dB steps up to (iirc) 15dB in an attempt to achieve stability. A long period of stability (typically a couple of weeks) is rewarded with a reduced noise margin - only down to 6dB although some ISPs will manually set it at 3dB. Really bad lines can be set at a fixed rate rather than rate adaptive.

Not quite. The system doesn't tend to modify target SNR as often, but actually prefers to put your line on a capped profile, with a minimum and maximum speed it can sync at. With excessive resyncs, it'll both reduce your maximum line speed directly (not via adjusting SNRM) and increase interleaving/INP depending on the cause.

It can take weeks or months for a capped profile to be automatically removed from your line, and often it goes up gradually. As an example my line was capped down to about 47Mbps after I messed about with it for a while, and gradually went back up to the full 80Mbps by increasing in approximately 5Mbps steps every week or so.

A BRAS profile (maximum datarate) is set based on sync. The old system was an immediate reduction for lower sync and delayed increase - sometimes as long as 5 days. The newer system makes closer to instant changes but as it was developed after the days when I took much of an interest in this stuff I don't know the details. The equipment datarate gets passed to ISPs and they sometimes have delays updating theirs so you can get "stuck profiles" with lower data rates than the sync could support.

The newer system is instant. Or rather, every time you reconnect your PPPoE session, the server obtains the live sync rate from the DSLAM (well, not directly) and sets your IP/BRAS profile to be exactly 96.8% of that.

IP profiles are only adjusted upon PPPoE reconnect, so if your line resyncs without your PPPoE session resetting, your IP profile will not adjust until your next PPPoE reconnect.

It is worth noting IP profiles *only* apply to BTw connections. LLU providers (i.e. Sky) do not have a BRAS/IP profile.

I haven't been on FTTC long enough to see any problems at all so far.

BT, in my experience, have been generally good at keeping up with capacity increases and congestion. Longest period of congestion I ever saw during my 2.5 years on BT Infinity was around 3 days.

Openreach as well, have been very proactive in keeping the cabinet loops on FTTC up to scratch. This is made easy partly because of the massive overprovisioning of spare fibre when they built the network, as well as it being a relatively new, modern infrastructure that is easily upgradeable. Each cabinet has the ability to be fed with enough capacity to exceed a dozen VM nodes.

Personal experience aside, OFCOM's national figures have shown FTTC's peak time speed losses, a key marker of congestion, jitter, and packet loss to be consistently lower than on VM, and sometimes several orders of magnitude lower.