Had 76 Mbps fibre once upon a time, can now only get 55 Mbps - WHY?
@Browni wrote:
Crosstalk can have severe effects on a line as this thread from the Kitz forum shows.
That kind of explains what has happened to me. 
I should have stuck with 76Mbps when I had it instead of downgrading. 
@Baldrick1 wrote:
This reply has just crossed with the previous post. Can crosstalk really get that bad that speeds can reduce from 76 to 55Mbps?
Based on link by @Browni, the answer is yes. 
@shah2000 wrote:
I should have stuck with 76Mbps when I had it instead of downgrading.
Downgrading won't have caused the problem.
Depending on how often you run a speedtest there would have been a gradual decrease in speed over time along with some sharp dips when a particularly noisy line becomes active.
Vectoring can reduce the impact of crosstalk but that is not widely available yet.
@shah2000 Regular Interference Observed Daily at Customer Premise could be practically anything like electrical interference, unfortunately we can't raise a fault to our suppliers solely based on this. Error Protection is a form of interleaving which helps keep the line stable, this is turned off. Retransmission is another form of error correction/interleaving which is switched on.
@Browni 55mbps isn't the maximum the line can handle, that's just the high range of the speed estimates.
@Browni wrote:
@shah2000 wrote:
I should have stuck with 76Mbps when I had it instead of downgrading.
Downgrading won't have caused the problem.
Depending on how often you run a speedtest there would have been a gradual decrease in speed over time along with some sharp dips when a particularly noisy line becomes active.
I wasn't saying it caused the problem, just that I would have had more chance of noticing any speed decrease if I had kept my speed at 76Mbps and possibly getting BT OpenReach to fix it.
Now I have to decide whether it is worth spending the extra few pounds for the extra 15 Mbps of speed or go back to 40 Mbps and save a few quid.
@Gandalf wrote:
@shah2000 Regular Interference Observed Daily at Customer Premise could be practically anything like electrical interference, unfortunately we can't raise a fault to our suppliers solely based on this. Error Protection is a form of interleaving which helps keep the line stable, this is turned off. Retransmission is another form of error correction/interleaving which is switched on.
@Browni 55mbps isn't the maximum the line can handle, that's just the high range of the speed estimates.
@Gandalf, Is there anything I can do find out what the cause of the Regular Interference is so that I can try and mitigate it? Would ring mains electricity or RF interference from a power brick for the router cause such interference?
What is the benefit of Retransmission over Error Protection in terms of latency and bandwidth?
@Browni, My results from Speedtest.net, and another method, both show a slightly improved download speed of 57 Mbps today, so it seems as though I am getting closer to the sync speed. But my upload speed hasn't improved. 
@shah2000 the interference could be something electrical nearby, it's difficult to advise.
Retransmission is a form of error correction/interleaving which is better because it can improve sync speeds but both retransmission and error correction/interleaving do have a negative impact on latency.
@shah2000 wrote:
if I move away anything that could cause interference by using shielded extension cables, would that be automatically detected and my speeds improved, or does something have to be done on your end to retrain the sync speed?
It may / may not. I had a particularly nasty case where the power supply unit causing the intererence was the one powering the router, replaced the power unit with a spare - problem gone. It can be a time consuming task so one approach is to switch off ALL possible electrical devices and test using a laptap / Tablet on battery power. Try a different router (see above). Hopefully the 'noise' will temporarily go away and then switching other devices back on in a controlled manner will highlight the probable culprit. 
