topic Re: SNR at 15db in Broadband

SNR at 15db

weeally46 — Mon, 10 Jul 2023 19:40:26 GMT

Hello all,

The line has had an SNR of 15db for months now, ever since FTTP installs were getting carried out in the area, it is syncing at 15452kbps down which is manageable but the latency has dropped dramatically.
The SNR should be 6db.

When I run Plusnet's txt service it comes back as there are no problems with the line.

I've ran a quiet line test on the phone and there is a very slight hiss that I think has always been there.

Could anyone help?

Re: SNR at 15db

jab1 — Mon, 10 Jul 2023 19:45:23 GMT

@weeally46 What Hub/router are you using ?- the line stats would be useful.

Re: SNR at 15db

weeally46 — Mon, 10 Jul 2023 22:23:36 GMT

@jab1 I'm using a draytek vigor 130 in bridge mode to a draytek vigor 2860.

Re: SNR at 15db

jab1 — Tue, 11 Jul 2023 05:56:09 GMT

@weeally46 I don't know Draytek kit at all, does it have an error log of some kind? That screenshot shows an error-free connection - which maybe a red herring as you have a bridged setup - but I don't know.

Re: SNR at 15db

weeally46 — Thu, 13 Jul 2023 23:39:18 GMT

@jab1 the connection is solid and rarely drops, it's the latency that's took a noticeable hit. The Vigor 130 essentially acts a adsl modem then passes the connection through PPPoE to the 2860 that acts as the firewall/router for the network.

The SNR reading is usually spot on, I've tested the line with another router modem and the 2860's built in modem, they all give the same results for the SNR.

Re: SNR at 15db

jab1 — Fri, 14 Jul 2023 05:24:06 GMT

@weeally46 The only thing visible to me on that screenshot of any concern is the really high SNR - which is indicative of the DLM having kicked in to resolve/attempt to resolve an issue.

We really need to see an error log if we are going to get anywhere with this. I can't see a latency issue, but that is because there is nothing on there to show one.

Re: SNR at 15db

Baldrick1 — Fri, 14 Jul 2023 10:23:37 GMT

@weeally46

I would imagine that your line has been banded. Report a fault: https://www.plus.net/help/report-a-problem/broadband/#broadband.

Keeping resetting or changing hubs will slow any chance of automatic recovery.

Re: SNR at 15db

Anonymous — Fri, 14 Jul 2023 10:45:13 GMT

@weeally46 could you post a screenshot of your Vigor 130's console page showing the connection status ?

To get this info, login to your modem's web console, then in the top red banner on the right hand side, there are five buttons, click on the middle one that looks like a four channel mixing desk and shows as "Web Console" when you hover your mouse over it. This should bring up a new window with a > prompt. At the prompt, type V then space, then S and then hit 'Enter', which should display a page with most of the useful connection details.

Re: SNR at 15db

weeally46 — Fri, 14 Jul 2023 17:27:28 GMT

@Baldrick1 It's been like this for 3 months now, changed the routers over twice to check that was 2 months ago. I don't think automatic recovery is working.

Re: SNR at 15db

Anonymous — Fri, 14 Jul 2023 17:32:00 GMT

@weeally46 next time you are looking at your Vigor 130, check that it has the latest "3.8.5.1_BT" firmware installed,

available to download from here - https://www.draytek.co.uk/support/downloads/vigor-130

Re: SNR at 15db

weeally46 — Fri, 14 Jul 2023 17:49:35 GMT

@Anonymous It does have the latest firmware installed. It's nice to meet another Draytek user.

Re: SNR at 15db

Anonymous — Fri, 14 Jul 2023 18:06:26 GMT

Just to check your Vigor 130 configuration isn't fighting the DLM negotiation of SNR,

what does it say if you type -

v snr

at the command prompt ?

Re: SNR at 15db

weeally46 — Fri, 14 Jul 2023 19:00:52 GMT

@Anonymous

Re: SNR at 15db

Baldrick1 — Fri, 14 Jul 2023 19:58:45 GMT

@weeally46 wrote:

@Baldrick1 It's been like this for 3 months now, changed the routers over twice to check that was 2 months ago. I don't think automatic recovery is working.

That’s why I suggested reporting a fault.

Re: SNR at 15db

weeally46 — Fri, 14 Jul 2023 20:24:25 GMT

@Baldrick1 The diagnostics bot just tells me the line from the network to my home is working.
Do I need to phone to report a fault? There used to be an online trouble shooter to report faults.

Re: SNR at 15db

Anonymous — Fri, 14 Jul 2023 20:25:52 GMT

@weeally46 what upload and download speeds do you get when this is working ?

The thing that is confusing me is why you only have 1Mbps upload sync, when the "Far SNR Margin" is 6dB.

Yes the 15Mbps download combined with "Cur SNR Margin" 15dB, looks like the BT DLM has got stuck.

The other thing that doesn't look right is "NE Current Attenuation" at 2dB - unless you are living IN the fibre cabinet !

I've also never seen all four error counts being at zero with the current firmware (as I got DrayTek to fix those counters)

Re: SNR at 15db

Anonymous — Fri, 14 Jul 2023 20:34:34 GMT

@weeally46 wrote:

Do I need to phone to report a fault? There used to be an online trouble shooter to report faults.

Try this - Quickest way - troubleshoot by text

See if that process clears the fault, or just says there is no problem detected.

I used this facility to fix a stuck line with my Vigor 130 this week !.

Re: SNR at 15db

weeally46 — Fri, 14 Jul 2023 20:37:40 GMT

@Anonymous It's an ADSL 2+ line, years ago it would sing and dance at 23Mbps and 1.4Mbps. Over the last few years 21Mbps max then 18Mbps max down, the uploads have always sat around 1Mbps.

The line itself is usually stable at 6dB

My house is very close to the exchange less than 200m.

I don't know what's going on with the error counts, running the modem in the vigor 2860 throws up at least some.

Re: SNR at 15db

Anonymous — Fri, 14 Jul 2023 21:35:25 GMT

D'oH! I missed seeing that it was ADSL2+ in your console stats - I've been so used to looking at FTTC for the last few years.

Wow, what you are seeing takes me back a few years, and I've never seen anyone else have this problem (other than me !).

I think what you are seeing is a design fault in the ADSL2+ exchange DLM algorithm.

I bet that even when it was working at 23Mbps that you had near zero errors on the line ?

What this problem is, is that the ADSL2+ DLM process LIKES to have a little bit of noise, so that the exchange can balance the SNR but at the same time reduce the power level of the ADSL signal on your line - in an attempt to reduce the crosstalk to adjacent phone wires from other customers, theoretically making it better for everyone.

Normally when there is some level noise detected the line signal strength is increased to compensate and the SNR number gets smaller, whereas if the noise is below some threshold then the signal strength is reduced and the SNR number gets bigger, and for MOST connections this balance works to give the best bandwidth at the minimum power levels.

What I had (and you probably have) is a line that was so quiet, that the DLM kept reducing the signal level on the line, expecting at some point that some level of noise would eventually appear - BUT NEVER DID, so the SNR number gets bigger and bigger, until eventually that SNR runs out of range in either the exchange end, or in the modem, and you end up with a big SNR value, the sync rate collapses, you still see no detectable errors on the line, and the algorithm is stuck.

I suspect that the 15dB SNR you are seeing is the limit (probably a 4 bit integer ) in the Vigor 130 code.

I've had various broadcom based modems, including Netgear and Billion, where the SNR got to 19dB

The bad news, is that the ONLY fix is to ask Plusnet to schedule a line DLM reset.

I can't remember off the top of my head, does the SNR tweak on a Vigor 130 work on ADSL ? - I know it does on FTTC but I've got an idea that it might not on ADSL. The reason I bring this up, is that on broadcom based modem chips (such as the Billion and Netgear routers) you can tweak the ADSL2+ SNR by setting an SNR offset that prevents the faulty ADSL2+ DLM algorithm wandering off the end of its range. See https://www.increasebroadbandspeed.co.uk/SNR-tweak for the method. If the Vigor 130 SNR tweak works on ADSL2, then you might be able to prevent this problem repeating itself.

Re: SNR at 15db

weeally46 — Sat, 15 Jul 2023 15:58:47 GMT

@Anonymous Interesting.

Very little errors at any speed, maybe 500 for the month and it was as stable as a rock.

I messed around with the vigor 130 SNR tweak last year for abit, most of the time it wouldn't affect the SNR margins and just confused the DLM.

Thank you so much for the information, it's great to speak to another draytek user especially one as knowledgeable as yourself.