SNR at 15db

[#1]

@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)

[#2]

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@weeally46 wrote:

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

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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 !.

[#3]

@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.

[#4]

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.

[#5]

@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.

[#6]

Are you going to ask Plusnet for a DLM reset to unstick your exchange/modem SNR ?

[#7]

@Anonymous I have, 1 min after putting the phone down, 21Mbps, 6dB and back to normal. Happy days.

The info I have learned is invaluable to me, Thanks again.

[#8]

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@weeally46 wrote:

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?

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Uh, huh?

I am now on FTTC but the above sounds awfully like my experience when I was last on ADSL 2+ with Plusnet.

My ADSL line was "banded", at the very highest speed banding. More than that... it was ALWAYS (reset router, power down/up router, kick router, set fire to router etc.) the exact same speed: Slap bang against the endstop of the highest possible speed end of the "banded" speed range. i.e. Logically it could go faster - which I already knew - but it seemed it wasn't allowed to go faster (via the banding?).

So "banding" means the line had a fault? But the line didn't have any fault that I, or anyone else, could point to or discover. It  just was banded. The advertised maximum speed (which had been steadily decreased over the years) - from the BT/Openreach line information source - showed a speed lower than I knew my line could achieve. Banding seemed to 'solve' that dilemma. So... 

I eventually reached the conclusion that ADSL was just on the way out. So I am now on FTTC (40/10) which works very well.

This may not be particularly helpful to you, just my experience.

[#9]

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@Anonymous wrote:

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.

That thought had occurred to me (or at least, perhaps a change in DLM policy) when I was last on ADSL 2+. But nobody at Plusnet, or at least on these boards, would buy it at the time.

[#10]

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@weeally46 wrote:

21Mbps, 6dB and back to normal. Happy days.

Does your line normally stay at 6dB SNR or does it drift towards 3dB as DLM gets used to your low error rate ?

I used to run my ADSL2+ SNR between 0 and 2dB (using SNR offset tweak) and would typically get 23.5 to 24 Mbps

[#11]

@Anonymous Normally sits at 6dB, I've seen 23Mbps on a 6dB SNR. 

The line is sitting at 5dB and is trying to drift to 3dB, I'm expecting it to take a few days and then resync. Once the line settles I might use some SNR tweaks to target 3dB