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Author Topic: Noisy Line Tactics  (Read 4769 times)

aesmith

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Noisy Line Tactics
« on: December 13, 2015, 05:09:25 PM »

Hi,

Our line is slightly noisy, and also suffers from reduced SNR during the evening and overnight, typically dropping from the target NM of 6dB down to a highly variable 4.5-5.5dB.   The line doesn't drop, but my ISP reports that we're getting errors.  How many errors I don't know, my router doesn't report them, and the ISP's view appears to just be the number of errors within the current 15 min period when they look (so could be from the last 900 seconds, or only a few, depending on when in that slot they grab the data).

Anyway, thinking about this, if there's noise on the line when the modem trains up, it should avoid the noisy frequencies - is that correct?

If so, then maybe rather than making sure I always reconnect during daylight to get the best speed, should I reconnect in the evening to give it a chance to work around the dodgy frequencies.   I can force target NM and basically get it to connect at whatever speed I want, but realistically it's not going to run fast enough to get a 4.0meg IP profile, so connecting at just over 4000K synch speed could be the best compromise.

Does this make sense?

Thanks,  Tony S
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Weaver

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Re: Noisy Line Tactics
« Reply #1 on: December 13, 2015, 05:28:00 PM »

Your modem will employ bitswap to move away from bad tones (frequencies) aka "bins". You are probably familiar with this mechanism. If you have a large target SNRM, then you will plenty of freedom to move away from future bad frequencies, if it turns out that conditions change too much for bitswap to be able to find sufficient high quality alternative frequencies to switch to, then it will have to re-sync. In this case DLM on a BTW line will increase the target SNRM.

You probably know all this.

I'm not sure that turning on during the evening will make a significant difference. It could be that the interference is transient. In that case it may be that you need a higher level of interleave and DLM will arrange this I believe.

Basically what I am saying is that you can either let DLM do its thing, or get the ISP to increase the level of interleave or failing that, to increase the target SNRM. For example, many "business" accounts with ISPs are routinely set at 9dBs instead of 6.

Best.
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kitzuser87430

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Re: Noisy Line Tactics
« Reply #2 on: December 13, 2015, 05:32:03 PM »

Quote
6dB down to a highly variable 4.5-5.5dB
That is not too bad actually; mine used to drop to about 1.5-2.0 in the dark hours (with a target of 6 dB).

You really need a modem/router that displays error seconds, crc errors and fec errors, what router are you using?

When my line was at its poorest, i used to force the target snr to about 7 dB (daytime synch) so the night time errors were reduced (snr dropping to about 3dB).

Ian
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aesmith

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Re: Noisy Line Tactics
« Reply #3 on: December 13, 2015, 05:41:12 PM »

That is not too bad actually;
I was quite happy with that variation until suddenly DLM starting getting upset and moved me onto first 9dB then 12dB target.   I'm in the dark until I get a router with decent stats, but it really goes against the grain to have to buy kit instead of my ISP fixing things.  What I really want at this stage is to get the line stable and at a reasonable speed so I can change providers.  I want to make the change while the line's running at 4meg, not at 2.5 as it did when DLM turned against me.

What do you think of my theory - that resynching during the noisier time show help the router avoid the noisy frequencies?   My router gives a plot of SNR and bit loading across the spectrum, which as far as I can see is a snapshot taken at the time it trains up.  See attached for what it shows at the moment.

(Edit - sorry for some reason I didn't see your post, Weaver.  I need to consider what you've written)
« Last Edit: December 13, 2015, 05:43:25 PM by aesmith »
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aesmith

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Re: Noisy Line Tactics
« Reply #4 on: December 13, 2015, 05:46:15 PM »

One specific question - when DLM changes the target NM, does it force a reconnection?  Or does it only make the change if the router loses synch?   Story from the ISP is that the error rate was such that DLM threw me off, and raised the target on reconnection.   I can't actually contradict that because the NM plot when that happened was no different to previous nights so no reason to think the router dropped the connection.  (See attachment - regrading near the end takes the NM off the scale)
« Last Edit: December 13, 2015, 05:52:05 PM by aesmith »
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Weaver

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Re: Noisy Line Tactics
« Reply #5 on: December 13, 2015, 06:04:24 PM »

As far as I understand it, the target SNRM is set at connection time and can't be changed, nit unless you reconnect / re-sync and then at that time employ a new target SNRM.

There is a thing called SRA which gives much more freedom and avoids the necessity to re-sync, but BT don't have it available, and I'm not sure anyone else does either, LLU operators included.
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kitzuser87430

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Re: Noisy Line Tactics
« Reply #6 on: December 13, 2015, 06:08:23 PM »

Quote
does it force a reconnection?
Yes normally between 2 and 6 AM (as per your graph)

Quote
resynching during the noisier time show help the router avoid the noisy frequencies?

It will higher the target snrm and yes it should help the router and create less errors, but, if DLM has upped your target to 12 dB recently  the increase will not be large enough to make any real difference.

Have you optimized your wiring ie. using a filtered faceplate, and as short a twisted pair rj11 from the master socket to the router?

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Weaver

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Re: Noisy Line Tactics
« Reply #7 on: December 13, 2015, 06:29:53 PM »

If your SNRM is going up as high as 12 dB, then your line needs sorting or the interference source needs to be located and zapped. That's not good enough at all. Your interleave settings may need to be higher.

You know what I'm going to say, a good ISP can sort this out for you. Apologies for sounding like a cracked record. Predictable.

I am able to get stats out of my modems via Andrews and Arnold's servers, which suck out the data remotely, so I can get the information I need even without a connection directly from the modems. And example would be the number of errors and the number of HEC errors, the sync rate, the current IP profile. (This is via BTW's systems to A & A.)
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aesmith

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Re: Noisy Line Tactics
« Reply #8 on: December 13, 2015, 07:27:58 PM »

Thanks.  To be honest this is all pretty frustrating, I thought we were in a reasonably stable condition, but now we've suddenly got audible noise on the phone line, a harsh buzzing.   Just had an OR visit to investigate, but since his tests showed a pass for all parameters he said he wasn't allowed to carry out any further checks.   Our line has a zillion joints and what's happened in the past with noise issues is the guy goes to a midway point, pops the joint open and test from there.  We have had four or five instances like that since we moved here, and they've never previously even needed access to the house let alone being required to test from there as the first stage.  So I have to assume that this audible noise issue take a while to get resolved.

ISP is very vague about what caused the DLM intervention.   I asked them to reset to 6dB which they did on 3rd December, and it's not been regraded since, even though nothing's been done to fix the line.   So my current aim is to keep the line running as best it can, without upsetting DLM again.
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Weaver

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Re: Noisy Line Tactics
« Reply #9 on: December 13, 2015, 08:16:06 PM »

<RevWeaver>Sincere apologies for further proselytising, this is exactly the kind of situation that (reportedly) Andrews & Arnold specialise in. They used to have a "fix your line for you or your money back" offer, possibly still have it. Reports say they are very determined to get the job done properly.
</RevWeaver>

Of course there are other things one can spend one's money on. And you know best what's right for you.

A target SNRM of 12 dB is an indication that things are very wrong, as I said earlier. It would be good to know what the interleave is at the moment, that is the interleave depth, as if it can be increased this might be a solution to transient interference problems. This might not be enough and it would still mean that you are left with a slow line if interleave can't take care of it. We haven't considered crosstalk either, and we should.

The good news is that when the line is sorted out then you'll have a much higher speed. It's also good that there is an audible problem, as BT Openreach can't simply deny the severity of a major issue such as yours.

I don't understand the logic behind wanting to wait before changing ISP if you're having a problem. You don't need to worry about this it won't make a difference. Just remind me, are you on ADSL two or ADSL one?
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ejs

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Re: Noisy Line Tactics
« Reply #10 on: December 13, 2015, 08:38:45 PM »

It's 20CN, so ADSL1, where I think interleaving can only be switched on or off, an ISP doesn't have control of the level of interleaving on 20CN.

It wouldn't surprise me if interleaving has been switched off due to previous problems with latency.
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aesmith

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Re: Noisy Line Tactics
« Reply #11 on: December 13, 2015, 08:51:22 PM »

Interleaving is currently on, it "fell off" for some reason a month or so ago (clearly visible in the data as a reduction in RTT).

The concern about change is that currently both phone and broadband are with the same provider.  I don't know how (for example) A&A would deal with a BB issue that's caused by a phone line fault.  Equally I don't know whether Pulse 8 would be effective pursuing a phone fault if the line tests clear in spite of the audible noise, given that they won't care whether it affects broadband.

I think Openreach position is that the line is within spec in terms of AC and DC balance, capacitance etc etc (the guy showed me the parameters).   He did say that it could still be an exchange fault for example, since the line's disconnected during his tests.  On the other hand if it's interference sufficient for a within-spec line to pick it up, then OR don't seem to think it's their responsibility.

We've now had four OR visits since September.  (1) fixed "battery contact", (2) broadband people, found and fixed blow fuse of some sort at the exchange, (3) SFI who said there's nothing wrong that enabling interleaving wouldn't fix (but that was before we got this noise on the line) and finally last week supposedly to address the noise but in fact not prepared to take any action.
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burakkucat

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Re: Noisy Line Tactics
« Reply #12 on: December 13, 2015, 09:46:55 PM »

As it is audible noise that is affecting your telephony service, you should report it as such to your communications provider and do not mention your broadband connection to the Internet.

The telephony service you are currently receiving is out of specification and it must be fixed by the infrastructure maintaining entity -- Openreach.

To be absolutely clear, assuming that you have tested with two different wired telephones connected directly, one at a time, to the "test" socket of the NTE5/A (with no other equipment connected) and the audible noise is still present then the telephony service is defective. The defect must be removed (corrected) by the telephony service provider.
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Weaver

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Re: Noisy Line Tactic
« Reply #13 on: December 13, 2015, 10:20:15 PM »

Burrakucat is 100% in that post.

<RevWeaver>One way of getting out of the situation is to do as I did, and transfer the PSTN line to A & A. You simply ask A & A to take over the line completely. Now I have access to one-stop-shop and there are none of these "who is responsible" games, as Andrews and Arnold can just get on and deal with things immediately with no buck-passing. Of course in your case you would then have to try and sort out a solution for voice. I can recommend Andrews and Arnold’s VoIP service.

There comes a point when you decide that you've had enough of the hassles and want to leave it to someone who is getting paid to sort everything out and take it away from you. That was me, and now I live a quiet life, any problems I just hand them over. ispreview.
</RevWeaver>

[ There he goes, at it again! I ask you? A disgrace.  >:( ;D ]
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aesmith

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Re: Noisy Line Tactics
« Reply #14 on: December 14, 2015, 08:54:59 AM »

I am pursuing the noisy line, but the first attempt didn't achieve anything.  Hence my interest in tactically getting the best DSL until it's fixed.

It does strike me that by this stage it should be accepted that broadband is a valid application for a PSTN line, not just a "nice to have" which rides on top of the voice which is it's real purpose.   Thinking at random, supposing the telephone engineer could remove the current noise by applying some sort of filter which in turn crippled DSL, like the loading coils that used to be installed.
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