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High downstream SNR

jab1
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Re: High downstream SNR


@ejs wrote:

@summers wrote:

Yes, but we know that the DLM is making changes to the SNR target margin.

No, we did not initially know if it was the DLM responsible for all those drops in connection with a re-connect at a slower speed. It looked like the DLM left the target SNRM at 3dB, and then presumably something else knocked out the connection (maybe SHINE), whatever it was had gone by the time the modem had finished re-connecting, leaving it at a lower speed and higher current SNRM, without the DLM changing anything. Then a modem reboot or some other re-connection would restore the speed and 3dB current SNRM. I think a lack of clarity on what's causing the connection drops, and a fixation on the DLM, is not helpful. Yes the line might survive whatever was causing the connection to drop if it wasn't connected at a 3dB SNRM. It's now been made even less clear, because it might have now been the DLM on the Stable policy increasing the target SNRM all the way up to 15dB.


I am too far away from the exchange for a 3dB margin, @ejs,  which I have known for as long as I've been with PN (and before, tbh), so why the DLM keeps trying to set it at that level, with a resulting unsustainable speed of around 13.5 is what we need to try and ascertain. Until that is sorted, we are all floundering around, getting nowhere!

John
ejs
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Re: High downstream SNR

@summers

Lucky you if you've never had anything besides the DLM cause a drop in connection.

It probably takes about 30 seconds for a modem to reconnect. It's possible for there to be some short burst of noise, big enough to drop the connection, and for it to linger around long enough to cause the modem to re-connect at a lower speed, but to have gone by the time the first stats are polled by typical stats monitoring programs reading the stats at more conventional intervals (not one second!).

ejs
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Re: High downstream SNR

@jab1

I am much further away from my exchange (58dB attenuation) than you are to yours, and a 3 dB SNRM is not a problem. It may well be that your line cannot handle a 3dB SNRM, but there is no general rule that you need X db of SNRM per Y km of line, despite what poorly trained Plusnet staff may say.

jab1
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Re: High downstream SNR

My FINAL comment on this before I return to the subject in the New Year. This information did not come from 'poorly trained Plusnet staff' but from two 'well trained' BT engineers and an independent telecoms engineer I spoke to some time ago, now sadly no longer available due to retirement.

John
summers
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Re: High downstream SNR

@ejs but we aren't talking about what causes the connection to drop, at least thats my reading of the thread.

The question is when the line syncs when coming up, why is the target noise margin changing?

Yes I've at times taken down my own line, like a few days ago when files saved on the router mean it got an oom error which took the connection down. Then it has synced back at the same speed.

Actually I'm quite surprised at how much it takes to take my connection down, since I've moved to 3dB margin. When it has synced during the day, at night at times it has dropped below 1dB  margin. Then I've had 11 seconds with 40,000 fec corrections, and the line stayed up for a bit. Let me dig out the CV , those 11seconds only had 44 cv.

Maybe the question to you should be what else, other than the DLM, can set the target margin?

ejs
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Re: High downstream SNR

You seem to be missing the point of what I've been saying. The target SNRM wasn't changing. At least not earlier on in the thread. Maybe it looked like it was, but then the first SNRM reading after the reconnection may have been about a minute old. Perhaps read message 9 again.

summers
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Re: High downstream SNR

Granted @MatthewWheeler said "your target SNR has been set and is still at 3db" and a few posts later @Gandalf also said "I've nudged the target SNR back up to 6dB and asked our suppliers again to pin it at that level."  I though got more information from @Gandalf post, as its showed that both the Red and green threshold for the MTBE and MTBR had been set so that both would *always* be amber. Now under usual reading on how the DLM works, if the link is amber then no changes are made to the target SNR. So reading between the lines (yes this is a step into the unknown) I guessed that when plusnet set the target SNR margin, they probably set the starting target SNR margin. Then if it never changes this is *like* fixing the margin.

On the other hand, we know that plusnet aren't comfortable with setting a specific SNR margin, this has come up in various topics, it isn't their natural way of setting the parameters they control.

Another example would be setting the interleaving on a line, plusnet have a way to do this kinda, but only talk about it abstractly - its not something these do usually as their day to day configuration of a line.

I think part of the difficulty here, is that the DLM is on Openreach domain, and we can only talk to the DLM via plusnet. Now this is eased, as us punters have  contract with plusnet, and plusnet has a contract with openreach. So each has is giving a service, and so tries to set things up in a way that works fluidly.

Still I feel sure, how us punters view the DLM, is very different from the plusnet staff, is very different from openreach tech gurus. We can only talk to plusnet via interfaces like this forum. Plusnet can access the DLM parameters via whatever interface Openreach gives them, what those parameters are, we don't know - and actually its not surprising that plusnet only talk about them abstractly. Openreach tech gurus can probably log directly onto the DLM hardware, can rewrite code if necessary (e.g. we expect FFTC to differ from standard broadband).

So what do were learn from this, well going back to to the graph that @MatthewWheeler posted, its clear that the margin when bringing up the link on @jab1 line is varying, that is clear. Maybe us punters don't know exactly why, but we can guess. Plusnet have more information, that they may or may not share (e.g. they may have an authoritative answer to if only the DLM can change the target margin). Openreach (if they looked) could say exactly why the margin is changing - but we can't talk to them, and they probably have no reason to look into it.

So what do were learn here? Only us punters can get information where we can (e.g. via kitz). We chat with plusnet to try and get the line set up as we want. The plusnet staff, as their resourcing allows can try methods of configuring the openreach DLM interface to meet those aims. This isn't a perfect system, but what else can we do?

summers
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Re: High downstream SNR

Granted @MatthewWheeler said "your target SNR has been set and is still at 3db" and a few posts later @Gandalf also said "I've nudged the target SNR back up to 6dB and asked our suppliers again to pin it at that level."  I though got more information from @Gandalf post, as its showed that both the Red and green threshold for the MTBE and MTBR had been set so that both would *always* be amber. Now under usual reading on how the DLM works, if the link is amber then no changes are made to the target SNR. So reading between the lines (yes this is a step into the unknown) I guessed that when plusnet set the target SNR margin, they probably set the starting target SNR margin. Then if it never changes this is *like* fixing the margin.

On the other hand, we know that plusnet aren't comfortable with setting a specific SNR margin, this has come up in various topics, it isn't their natural way of setting the parameters they control.

Another example would be setting the interleaving on a line, plusnet have a way to do this kinda, but only talk about it abstractly - its not something these do usually as their day to day configuration of a line.

I think part of the difficulty here, is that the DLM is on Openreach domain, and we can only talk to the DLM via plusnet. Now this is eased, as us punters have  contract with plusnet, and plusnet has a contract with openreach. So each has is giving a service, and so tries to set things up in a way that works fluidly.

Still I feel sure, how us punters view the DLM, is very different from the plusnet staff, is very different from openreach tech gurus. We can only talk to plusnet via interfaces like this forum. Plusnet can access the DLM parameters via whatever interface Openreach gives them, what those parameters are, we don't know - and actually its not surprising that plusnet only talk about them abstractly. Openreach tech gurus can probably log directly onto the DLM hardware, can rewrite code if necessary (e.g. we expect FFTC to differ from standard broadband).

So what do were learn from this, well going back to to the graph that @MatthewWheeler posted, its clear that the margin when bringing up the link on @jab1 line is varying, that is clear. Maybe us punters don't know exactly why, but we can guess. Plusnet have more information, that they may or may not share (e.g. they may have an authoritative answer to if only the DLM can change the target margin). Openreach (if they looked) could say exactly why the margin is changing - but we can't talk to them, and they probably have no reason to look into it.

So what do were learn here? Only us punters can get information where we can (e.g. via kitz). We chat with plusnet to try and get the line set up as we want. The plusnet staff, as their resourcing allows can try methods of configuring the openreach DLM interface to meet those aims. This isn't a perfect system, but what else can we do?

summers
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Re: High downstream SNR

Granted @MatthewWheeler said "your target SNR has been set and is still at 3db" and a few posts later @Gandalf also said "I've nudged the target SNR back up to 6dB and asked our suppliers again to pin it at that level."  I though got more information from @Gandalf post, as its showed that both the Red and green threshold for the MTBE and MTBR had been set so that both would *always* be amber. Now under usual reading on how the DLM works, if the link is amber then no changes are made to the target SNR. So reading between the lines (yes this is a step into the unknown) I guessed that when plusnet set the target SNR margin, they probably set the initial target SNR margin. Then if it never changes this is *like* fixing the margin.

On the other hand, we know that plusnet aren't comfortable with setting a specific SNR margin, this has come up in various topics, it isn't their natural way of setting the parameters they control.

Another example would be setting the interleaving on a line, plusnet have a way to do this kinda, but only talk about it abstractly - its not something these do usually as their day to day configuration of a line.

I think part of the difficulty here, is that the DLM is on Openreach domain, and we can only talk to the DLM via plusnet. Now this is eased, as us punters have contract with plusnet, and plusnet has a contract with openreach. So each has is giving a service, and so tries to set things up in a way that works fluidly.

Still I feel sure, how us punters view the DLM, is very different from the plusnet staff, is very different from openreach tech gurus. We can only talk to plusnet via interfaces like this forum. Plusnet can access the DLM parameters via whatever interface Openreach gives them, what those parameters are, we don't know - and actually its not surprising that plusnet only talk about them abstractly. Openreach tech gurus can probably log directly onto the DLM hardware, can rewrite code if necessary (e.g. we expect FFTC to differ from standard broadband).

So what do were learn from this, well going back to to the graph that @MatthewWheeler posted, its clear that the margin when bringing up the link on @jab1 line is varying, that is clear. Maybe us punters don't know exactly why, but we can guess. Plusnet have more information, that they may or may not share (e.g. they may have an authoritative answer to if only the DLM can change the target margin). Openreach (if they looked) could say exactly why the margin is changing - but we can't talk to them, and they probably have no reason to look into it.

So what do were learn here? Only us punters can get information where we can (e.g. via kitz). We chat with plusnet to try and get the line set up as we want. The plusnet staff, as their resourcing allows can try methods of configuring the openreach DLM interface to meet those aims. This isn't a perfect system, but what else can we do?

ejs
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Re: High downstream SNR

Have you read and understood anything at all that I've said?

The graph that MatthewWheeler posted was of the current SNRM recorded, it was not a graph of varying target SNRM. There is no 13 or 14 target SNRM setting.

Actually the ADSL DLM is operated by BTWholesale, not Openreach. Openreach maintain the telephone cables, BTWholesale operate the ADSL equipment in the exchanges. I think an Openreach person attending someone's line fault will be able to do exactly the same things to the ADSL DLM as someone at a ISP can.

Baldrick1
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Re: High downstream SNR

Let me put my two penneth in from a position of ignorance of how DLM works in the hope that my knowledge can be expanded.

As I see it there are two things that determine the SNR. First the signal level, the second factor is background noise, which can be introduced at any point between the fibre cabinet and the modem.

From my experience in fighting EMC issues in a previous life I assume that depending on where noise is injected, it's quite feasible at the frequencies in use and the LC effects of the long length of telephone cable for the level of background noise at the modem to be different to that at the fibre cabinet. So how does this work? I assume that the signal level for both downstream and upstream data is determined by the DLM. Consequently it is quite feasible to my mind that the SNR at the modem can, depending on where noise in introduced, be different to that at the DLM. If correct then this begs the question: what is the data reported at the modem/router  Is it the local router SNR, in which case the two ends of the line could report different levels, or that at the DLM and therefore not representative of the SNR with which the modem must tolerate?

I am also confused as to how a fixed SNR works. If the SNR is fixed then the DLM must, I assume, be required to jack the signal level up and down to compensate for a varying noise level. The DLM must have a limited dynamic range, for example there has to be a maximum level set such that one single bad line doesn't introduce unacceptable levels of crosstalk into other lines. So if background level is high and the signal level hits its end stop then the SNR simply has to drop if it's to try to keep holding sync.

Clarification would help my addled brain!

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jab1
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Re: High downstream SNR

@Baldrick1 I kind of understand what you are saying, and if I read it right, it introduces even more complications Cry, but I don't know if it's going to help.Undecided

FYI, I'm on ADSL, not fibre, I can't justify the added cost for my use.

John
ejs
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Re: High downstream SNR

OK, well done, you all win, I'll give up and find something else to do.

summers
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Re: High downstream SNR

@ejs So if I understand what you are saying, you think @MatthewWheeler set the target SNR margin to 3db, which is what the link would sync to when it brings up the line, then the conditions on the line improved to give 13-14 of margin. @jab1 said his line was changing speed, so it must have resynced, but if the target margin was set to 3dB would it not have resynced at 3dB, and then decreasing noise on the line  taken it to 13-14dB.

This makes no sense at all. I see something like 1-2dB of variance day to night with the ionosphere change.

The only thing I can understand would change the margin by the ~10dB shown is that the sync margin was different. So this means what you are saying that you don't think the sync margin when the connection comes up is tied to the target margin. If this is the case, what is the target margin used for?

I'm not sure we are making any progress here at all. @jab1 still has his speed issue. We here have no understanding of why. plusnet doesn't understand and suggest and engineer. So is there any point in continuing this thread, or shall we just call it an end due to mass confusion?

jab1
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Re: High downstream SNR

@summers I did say previously I was leaving this to 'sink' until I returned in the New Year, and it is becoming along and complicated story, maybe it should be allowed a natural death.

I will start another thread, referencing this, when I decide to revisit the subject then.

John