Hi,
I wonder if anyone can offer any advice on a very strange issue with my FTTC.
Firstly, we're in suburbia in the UK, and we believe the line length to the cabinet is 0.8 miles, but there is aluminium cable in the line, resulting in increased losses. Hence, we see sync speeds around 10 to 12Mbps down but only 600kbps up. So, this is not a great FTTC line for speed. Historically, the line has been stable, but has always had interleaving applied on the downstream side.
The cabinet is Infineon, and our modem is a Draytek Vigor 130, connected to an Openreach face plate on the master socket. This has been fine for many years. We've had fTTC since 2012, originally a Huawei HG613, but changed at least four years back to the Draytek which seemed to sync faster.
On occasion, due to the Interleave, it appears that the DLM would decide to try removing all interleave every so often to see if the line would "improve" - and then we'd go through a patch where the line wasn't as stable, but would eventually settle down as it decided to reinstate the interleaving.
Back in March 2020, we had several days of no service - the FTTC was in sync, but there was no response to the PPPoE frames - and required a line card to be swapped. The replacement line card reduced our speed slightly, both upstream and downstream - downstream became around 10Mbps, upstream became around 591kbps.
Then, a couple of weeks ago, DLM decided to remove the interleave - and that is when the current problems started. The line won't stabilise. We get lots of CRC, ES, SES, FECS being reported by the Vigor 130. We get many resyncs a day. Sometimes the speed goes down, sometimes it goes up. The noise margin doesn't improve. The "attainable" speeds that the Vigor 130 gives tend to be initially just above the current speed, but will fall below, sometimes for considerable periods of time.
Interleave has returned, but at random depths.
Unfortunately, I wasn't graphing the line's performance, but I have recently started in detail, not only the stats from "vdsl status" but also "vdsl status more", and I've also started to log DMT-style SNR and bit loadings. The Draytek doesn't give attenuation per tone, but gives "gain" - and reading the specs, this should be between k-14.5dB and 2.5dB for a tone with bits > 0 - but I regularly see figures outside of that. (is that one of the BT modifications?)
I have wandered around with a radio tuned to 612kHz... found there to be noise emanating from the Vigor 130's wall wart, so replaced that - no difference in modem behaviour. The area around the modem is quiet when everything is unplugged, and only the barest minimum is now connected, nothing that sounds like REIN though The modem has been re-sited to be as far away from any interference as possible. I've checked the connections - all seems fine and secure, no effects from moving the DSL cable around. Tried 17070 quiet line test - I can hear the VDSL signal faintly through the phone, I can hear the single tones it uses while training (if it's training while I'm on quiet line), and then when it goes into showtime - reminds me of an old 56k modem.
In any case, I attach a few files. First two are from my hacked-up dmt-ux tool to log the line profile. The "attenuation" is not, it's actually the gain figure scaled and offset (= 85 + gi * 5).
The first is the line profile just before the last resync and then after the resync. The third is my grafana dashboard logging of almost every statistic that the modem gives me (so is rather big in terms of pixel size, sorry!) I've included the last 7 days of stats, so you can see when the interleave was removed.
One interesting detail is that occasionally when I run the KBD tests, it'll report back that the consistency check has failed - it claims there's a mismatch between the DSL and BRAS. Re-run the test, and the problem has gone, and it says no fault found. BT's voice line checks come back clear.
I suspect there are some SHINE events (I've seen three today), one where the SNR dropped (somehow) to -2dB, but didn't cause the link to fail... which makes me question whether the figures that Draytek's firmware is reporting have any real basis in reality?
I'm rather concerned that reporting this to my ISP will end up with an Openreach charge due to "no fault found" (because they can't identify the problem).
Anyone seen anything similar to this - does it look like the DLM is operating as it should - note that we are now at a faster download rate than a week ago before the interleaving was removed... despite all the Error seconds and resyncs - kitz DLM information suggests that MTBR and MTBE should be red/orange) ?
My next idea is to try another VDSL modem to see whether it's a modem issue, but that'll be a while.
Thanks for any comments.
