Openreach VDSL - SNRm reduction & G.INP

I'm on OpenReach FTTC with Zen. My upstream DSLAM is reporting Broadcom 10.9.33

Until 13 March, and stable for 117 days (before that was just because of moving stuff around!) I had a connection reporting

Current Throughput:
71389 kbit/s (receive direction)
14243 kbit/s (send direction)
Line attenuation:
21 dB (receive direction)
27 dB (send direction)
Signal-to-noise ratio:
4 dB (receive direction)
6 dB (send direction)

G,INP was on at 46/46 rx/tx, and snr around 3/6

Around 15-18 march I had a lot of resyncs. Why? Well I tried the fritz 7530 'beta' (which I had before) but it had a whacky time with Zen's TR069 - with the latter constantly trying to update the router, and the router rebooting. Pretty much every 3 minutes from 0300-0600 !

Zen tells me there is no profile on the line

However I am currently still on a slower speed -
Down: 61.23
Up: 11.99
SNRm 6 (rtx) / 7 (tx)
G.INP 44 (Rx) / 55 (Tx)

I do remember it taking a while for G.INP to cut in previously, that the G.INP values changed over time, and similar with the SNRm targets

I did contact zen but unfortunately got quite a generic response. My understanding is that there's little they can do except probably a line reset

Any suggestions on whether I should just leave it and hope it recovers, or do anything (like a reset) to get things moving again? We're now just under 4 weeks from the problem. The line remains totally stable.

No faster option available here sadly.. (possibly 5G with ext antenna. maybe... Virgin stops 50m away - private road, and cityfibre/BT not yet deployed, but it's brighton so hopefully in next few years!)

Edited by planetf1 (Tue 12-Apr-22 09:23:45)

Looks like you're on a 6dB SNR target. If you leave it alone long enough, you should be moved to 3dB in due course.

I've now been stable since 18 Mar with very low error rates, no sync drops (except once - I rebooted to see if sync changed on 9/4) - so 6-7 weeks.

Still on ~61/12 with SNRm at 6(up)/7(down) currently - target probably being 6/6

Prior to mid March I was on ~71/14 - due to different G.INP settings & a lower SNRm target

Are the 'timers' this long typically for snrm/g.inp - still wondering whether I should try and persuade zen to do a reset - no idea if they will though, and of course who knows what will happen automatically over the coming weeks. I wonder now if I am way outside any purely time-based timers and there's some other factor at work. Perhaps I was close to a threshold (but seems odd as it's a clean line/newbuild)

I also presume the other end will resync as needed, rather than changes only taking effect when I resync from my end?

How do your "actual" and "attainable" line rates compare?

If you have a modem that had the ability to call sync rates then applying a cap to your sync rate can give the DLM a "nudge" in to trying the lower SNRM targets again.
However it's also possible the line won't ever go below 6dB again.
The DLM uses a lines history as well as its current error rates when determining if it should lower the target.

Thanks @j0hn83

I had 2-3 days of horrendous activity as seen by the far end -- the router got itself into an update/retry cycle (beta fw *vs* reinstall via TR069) leading to reboots every 3 minutes between 0300-0600 .... !

If longer term history is used, would a DLM reset 'reset' this aspect? I am thinking it's worth a try, even though it may mean lower speeds/some line drops for the next week or two.

Edited by planetf1 (Fri 06-May-22 11:25:00)

Thanks @candlerb -- Currently seeing

Attainable throughput kbit/s 61980 14459
Current throughput kbit/s 61230 11999

Latency fast fast
Impulse Noise Protection (INP) 44 55
G.INP on on
Signal-to-noise ratio dB 6 7

Previously I managed to get to 71/14 with a 3db SNRm d/s. G.INP stayed on at 46/46. Last stats I see had attainable as 73/14

I think the G.INP difference may have affected the U/S, whilst the new 3dB snrm would change the downstream (including attainable speed)

Edited by planetf1 (Fri 06-May-22 11:25:24)

The amounts of reboots would've caused DLM to intervene. I'm surprised that it only increased the SNRM and not band the line! It may have treated this as a "traumatic" event and may never relent, as it will be concerned that the same thing may happen again if you get me? One day, we'll hopefully be all away from this DLM mess, I'm very thankful that I have!

Edited by Grimers (Fri 06-May-22 16:07:19)

I decided to contact zen.

Fortunately being a good ISP - and since BT now offer the capability - they just did a DLM reset for me. They weren't sure it would help, but happy to give it a go.

Actioned immediately & I'm already on an improved upload - gone back to 14.2 Mbps.
(A resync a few weeks ago didn't help)
Of course can be con-incidence, and DLM will take an age to settle now, but worth a shot!
At least until fttp from BT or CityFibre is here....

Thanks for the tips. Will update over coming weeks

Yes a DLM reset would reset the history and make the DLM more like to lower the target sooner or at all.

The resyncs have also caused the upstream to be capped/banded at 12Mb/s (11,999).
It will sync at this every time until DLM removes the banding.
It won't lower the SNRM target while banded.

Edit: just noticed your other post about the DLM reset. Good news.

Edited by j0hn83 (Fri 06-May-22 17:29:21)