It really depends what the problem is.

Crosstalk is the dominant disturber for FTTC services, but it only appears, and then increases further, as take-up increases.

If that is the cause of your problem, then it isn't going away. It is a natural result of getting more subscribers onto your cabinet - and is the reason why BT specify a speed range, rather than just a single speed.

This behaviour means we cannot expect to get a single speed, and for that speed to be achievable forever.

Unfortunately, neither BT nor the ISPs educate people that this can, and will, happen. To everyone.

Or, of course, your problem can be caused by something else, such as a line fault. Depending on the symptoms, an engineer may be sent to investigate your line in detail, as happened in your case. He can fix faults (if he can find one), but he can't fix crosstalk.

I have been made aware of crosstalk since my speed dropped, but the line tests (to my uneducated eyes) seem to suggest none was detected

Test Outcome Pass
Test Outcome Code GTC_FTTC_SERVICE_0003
Description No problem found, OAM test is not currently supported on this line.
Main Fault Location OK
Sync Status In Sync
Downstream Speed 55.2 Mbps
Upstream Speed 19.8 Mbps
Appointment Required N
Fault Report Advised N
NTE Power Status PowerOn
Voice Line Test Result Pass
Bridge Tap Not Detected
Radio Frequency Ingress Not Detected
Repetitive Electrical Impulse Noise Not Detected
Cross Talk Not Detected
Profile Name 0.128M-80M Downstream, Interleaving Low - 0.128M-20M Upstream, Interleaving On
Time Stamp

In the past my linespeed was always 76.4 now the plusnet member page says

Estimated line speed:
There's no speed estimate currently held on your account.
Current line speed:
53.2 Mb

I would like some sort of response from PN, but both on the ticket & on the forums, they are not being very responsive.

PS. The engineer said there was nothing wrong with the line and that my limited speed was a result of plusnet 'capping' it.

Someone on the plusnet forums said that was not possible, however plusnet staff have not responded either way. I'm not a techie, I have no idea what is possible or who to believe.

Edited by deleted (Wed 01-Apr-15 17:58:10)

apparently it is a Huawei

Try common knowledge as a defense in court. I know the safety procedure on an airplane lke the back of my hand, but they always give it, isn't it common knowledge?

In reply to a post by janimal:

I have been made aware of crosstalk since my speed dropped, but the line tests (to my uneducated eyes) seem to suggest none was detected

Test Outcome Pass
Test Outcome Code GTC_FTTC_SERVICE_0003
Description No problem found, OAM test is not currently supported on this line.
Main Fault Location OK
Sync Status In Sync
Downstream Speed 55.2 Mbps
Upstream Speed 19.8 Mbps
Appointment Required N
Fault Report Advised N
NTE Power Status PowerOn
Voice Line Test Result Pass
Bridge Tap Not Detected
Radio Frequency Ingress Not Detected
Repetitive Electrical Impulse Noise Not Detected
Cross Talk Not Detected
Profile Name 0.128M-80M Downstream, Interleaving Low - 0.128M-20M Upstream, Interleaving On
Time Stamp

The result of "Crosstalk not detected" is laughable on that result, IMHO. Crosstalk is an infinitely variable affair, creating noise of different levels at different frequencies.

It is caused by having two or more VDSL2 subscribers in the same cable for some distance; if the twisted pairs are closer within the cable, then more crosstalk results.

But the design employed by BT - having tie pairs that run from FTTC cabinet to the PCP - guarantees that once there are two customers, then there is some shared cable, so there is crosstalk. Beyond the PCP, the shared cables spread out, reducing the likelihood of further "contamination", but it doesn't eliminate it.

The amount of crosstalk added by each subscriber might vary, but its existence doesn't. Add each subscriber, then you add crosstalk - sometimes tiny, sometimes small, sometimes large.

When you see the graphs, you realise that, in the absolute worst case, more than half your speed can disappear from the extra noise and interference. Seeing 10Mbps over 2 years isn't that bad a result.

Worse, there is a double-whammy. As well as causing reduced SNR levels (so lower speeds), crosstalk causes more errors to appear on the line. Eventually, the higher error level trips DLM into action; the old-style DLM solution was to turn on FEC and interleaving ... which would reduce speeds by 10-15% in one hit (on top of any reductions from raw crosstalk) and add 8-16ms onto latency.

From the result quoted above, DLM has intervened on your line, and set this kind of old-style solution. You will have lost a chunk of your sync speed to this.

Right now, BT are deploying G.INP retransmission, which is an alternative approach for DLM to choose to correct errors; this should not have such a large impact on sync speed or latency - so that chunk of performance can be recovered. However, it doesn't overcome crosstalk.

To properly overcome crosstalk requires BT to deploy vectoring. That acts as noise cancellation for subscribers, so it ought to undo the effect of crosstalk, restoring speed back to your starting speed.

Thanks for taking the time to explain.

What I am really annoyed about is PN support (and I guess point of sale as well)

1) They make no mention at the point of sale ff the fact that speed will reduce over time.
2) They make no effort to explain the possible cause of the loss of speed in response to the fault.
3) They see no irony in declaring my current throughput of 50 mbps as a satisfactory resolution to a reported drop of speed from 73 mbps to 55mbps. In fact the quote was "Your speeds are great" just 23 mbps less great than they were 10 days ago and the previous 17 months before that.

4) Surely someone must know if extra customers have been added to the cab since the speed dropped so that they could say with confidence if it was likely to be crosstalk.

One question...
I am about 90 -100m from the cab, my friend is 20 - 25m closer. If crosstalk was the issue would we expect to see his sync speed reduce as well? (It hasn't) Or is it simply a case of where exactly your connection is placed in the cab?

In reply to a post by janimal:

apparently it is a Huawei

Is it unlocked?

I don't know.

I certainly haven't done anything to it myself.

Is there a telnet or web interface to the thing or do I have to do anything to unlock it?

Yes, you have to load the unlocked firmware which basically lowers the firewall allowing access from the local LAN.

Unfortunately, neither BT nor the ISPs educate people that this can, and will, happen. To everyone

I guess they don't want to loose potential customers and revenue