Just annoyed with TT support really they are basically saying your getting above the "BT Wholesale" estimate [censored] off were not going to help you :/
There is a very fine line to be drawn here.
In FTTC, most people do not suffer from physical line problems with the service, so there is little work needed to rectify such faults. It does happen to some people, but support staff don't get too much practice at dealing with problems like that.
In truth, we're all largely "early adopters" still, so we're getting put onto cabinets that don't yet have much take-up. The combination of early-adoption, and largely good quality lines means that most people are seeing speeds higher than the estimates. For now.
However, the biggest factor that will affect our speeds isn't a physical problem with the line, but is crosstalk - the signal for other subscribers that appears as noise to our modem. As take-up increases, the noise increases, and the effect on our modem increases too.
Unfortunately, the effect is random, and can occur at any time - totally out of control of the ISPs or Openreach. It depends on your neighbours, and what they do.
The BT estimate seems to take into account a large amount of crosstalk, though not quite the theoretical worst-case amount.
So... we all need to expect that crosstalk will arrive, and increase, as take-up increases (and there's room for a lot more yet). We need to expect that the high speeds we initially see, the above-estimate speeds, will not stay. We need to expect that FEC and interleaving will become the norm, and could be added at any time.
Hence the fine line. Your slowdown could be due to a fully-expected increase in crosstalk, for which there is no remedy as yet. Or it could be caused by a physical fault.
In one case, it is pointless sending an engineer, and a waste of resources. In the other case, it might be worthwhile.
How else can we judge which case it is? The estimate is one reasonable way to do so.
I also heard that BT will investigate if there has been a 25% drop in speed within a 7 day period. I'm not sure if this is a current threhold, or one that only applied sometime in the past.
I thought FTTC could in theory handle speeds way above 80/20.
Yes, but it is still distance-limited.
The top downstream speed for 17MHz VDSL2 is theoretically around 170Mbps very close to the cabinet, without crosstalk. 100Mbps is possible at 400-500 metres, again without crosstalk. Unfortunately, crosstalk can take away half of those theoretical speeds.
Vectoring is a big solution to these, which is being trialled later this summer. It holds good promise, but we should expect it to take 2-4 more years to upgrade the BT network, if it gets the go-ahead.