Performance Degradation

FTTC broadband arrived in our rural village at the end of 2017. Once live and after the training period completed, the line settled with a downstream rate of ~64Mbit/s and upstream rate of ~13Mbit/s, which I was very pleased with coming from regular ADSL! Over time, these did come down to ~52Mbit/s and ~10Mbit/s, most likely as a result of more people switching over to FTTC broadband (more crosstalk), but at least the line was stable. It would normally remain connected for more than 180 days...

However, over the past few months my line has become unreliable, and it is reconnecting randomly several times each week, which is very annoying when you spend a large part of the day on Teams calls (working from home)!

Initially when I reported this to my ISP (the phone line is also with them), they confirmed there was a 'battery contact' fault on the line, and this was resolved. Indeed, it seemed to be ok for a week or so, but then the problems started again. I checked back in with my ISP and following some remote diagnostics they confirmed there are now no faults on the line or broadband connection.

From that point onwards, I started troubleshooting myself by trying another modem and connecting into the test socket. This has made no difference. Luckily I use DSLstats to monitor my line statistics, and I think I have worked out why I'm getting the disconnects... Please see some plots here. The top bitloading graph is from April 2018, and the one below it from this morning. As you can see, the D3 band has become very noisy and I've lost most of the U2 band. Until yesterday, the downstream SNR margin target was sitting at 3db, as it has been since line training completed. With the D3 band being so noisy, the FEC errors / hour would be higher than 60,000,000, i.e. more than 1,000,000 per minute! It is my theory that because the FEC rate was so high, the line would eventually be dropped to try and bring it back under control. Since yesterday, it looks like the DLM system has upped the target SNR margin to 4db, and the FEC rate has remained normal and the line remains connected. I've also included the QLN and HLOG plots.

The question is whether this can be classified as a fault and thus escalated to Openreach by my ISP, or whether this would be interpreted as 'operating within acceptable parameters'? I reckon I'm about ~800m from the cabinet.

Thanks in advance for your help.

If DLM increasing the SNR resolves the instability and you still have speeds above your "guaranteed speed" that you were given at the start of the contract then most ISPs would not consider it a fault. DLM is doing what it does and hopefully will stabilise the line at a potential loss of some speed - the question is whether that loss takes you below the guaranteed speed.

After posting, I decided to perform another 'quiet line test' while monitoring DSLstats. To my surprise, I saw the downstream SNR margin drop, the FEC/min shot up and there was relatively significant change in the bitloading plot above ~7,000kHz, see here. Does this point to a particular issue?