First: Actual FTTC Speeds
Yes, unfortunately these can drop for no apparent reason.
There is of course a reason, in fact there are two, but neither are obvious to the average user.
Reason 1: Crosstalk
As take-up increases, the number of modems transmitting VDSL signals increases. Modems on neighbouring lines all interfere with your signal, degrading things to a greater or lesser extent ... This increase in noise generally reduces the speeds you can achieve, and the effect is almost entirely random - the timing can be random, and the amount of impact can be random. Worst case can suck 30, 40 or even 50% of your speed, over time.
Reason 2: DLM
BT's DLM monitors the error statistics reported by your modem 24x7, with the aim of keeping your connection stable. If the number of errors gets beyond a threshold, DLM intervenes ... which can eat anywhere between 10% and 20% of your speed.
As takeup increases, that extra noise can cause errors, which means you are more likely to trigger DLM. A double whammy.
Early adopters (within a particular cabinet) are likely to see speeds higher than the estimate at first, but can then drop considerably over time.
Solution: Employ vectoring. This technology attempts to negate crosstalk, giving people their speeds back (and likely more). It is being trialled, and isn't ready for mass deployment yet.
BT provides the estimates to ISP's like Zen, who "interpret" them for you. They originally start in the form of an A range and a B range, while Zen have obviously only passed on a single figure.
BT make estimates based on the performance of "similar lines"; the top of the range is the speed that the 80th percentile achieves, while the bottom of the range is the 20th percentile (which means that 20% of similar lines achieve lower speeds!)
Remember - an estimate is not a promise.
Third: Getting BT to investigate
BT know the effects of crosstalk & DLM, and know that subscribers will be hit by them, so expect speeds to drop. There is nothing they can do about these effects (until vectoring gets deployed), so they really want to avoid sending engineers for speed drops that are unavoidable.
Until recently, BT had set thresholds: If your speed was above the minimum of the range (which is the 20th percentile), they wouldn't investigate. If it dropped more than 25% in one go, they would investigate.
There was a story yesterday reporting that BT had changed the threshold that they would act at - from the "minimum of the range" (ie the 20th percentile) to the 10th percentile, which is rather lower. It appears (from this report of the story
) that the 10th percentile also happens to be the Ofcom-recommended threshold for determining remedial action.
That probably explains why Zen refer to an Ofcom range as being 43 to 61Mbps: the 43Mbps would be the speed of the 10th percentile of similar lines (ie 10% of similar lines get less than 43, and 90% get more).
It seems that take-up of FTTC is now hitting the critical threshold - and people are being affected by this issue more and more.
It really needs ISPs to be honest; they need to tell people that whatever speed they get initially may drop, and can drop to anything within the range and still be considered acceptable.
Unfortunately, people have been conditioned by ADSL behaviour, where they achieve a certain speed, and believe it is their inalienable right to achieve that speed forever more. This is no longer true.
Edited by WWWombat (Fri 05-Dec-14 00:08:52)