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Author Topic: DLM management of target SNR  (Read 2652 times)

sevenlayermuddle

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DLM management of target SNR
« on: September 24, 2008, 11:08:29 AM »

I've recently been smitten by the 15dB noise margin after stupidly resynching my modem too many times, in an attempt to evaluate pros and cons of different microfiliters.

Target SNR is a topic that's been discussed many times on this and other forums.   The excellent BTW "myths and legends" pdf does explain how my margin got increased (ten resyncs in an hour),  but I've never found much said from BTW as to how DLM brings the margin down again.

However, there's a BT patent application which can be viewed at the following URL... http://www.freepatentsonline.com/EP1953959.html  Now if, as seems likely to me, that patent describes BTW's current practice then a few more facts arise that I'd not seen published on the forums.  Refer to the link in case I've misunderstood anything but, if I understand that patent correctly....

1) A line which is "unstable", i.e. has lots of CRC errors, may eventually pushed one step (3dB) up the noise margin ladder.

2) A line which is "very unstable", i.e. has had 10 retrains in an hour, may immediately be pushed two steps (6dB) up the ladder.

3) Your target margin won't ever be raised unless your sync speed is 800kbs better than your Fault Rate (which was established during the initial 10 day period).

4) Error rates are categorised as "very poor", "poor", "acceptable" and "very stable".  If your target margin's been increased then it won't ever be reduced again just because your line subsequently exhibits an "acceptable" error rate, it has to be "very stable" for a very long time (weeks/months) to get reduced.

5) Here's one that sounds interesting...  if a line completely fails to synch, it will have its target margin reduced in the hope of maintaining some service, and will be flagged for investigation.

I find (3) worrying because, sooner or later, everybody's going to see transient instability.   If you were 'lucky' during the ten day period and have a decent Fault Rate you'll be fine as DLM will leave you alone.  However, if you got allocated a poor Fault Rate, and your line's subsequently improved, you'll be punished and lose - maybe forever - all of the benefits that the improved line should have brought.

Tempting as it may be, I'd not recommend that anybody deliberately try emulating (5) to fool the DLM into dropping the target margin, as that could obviously cause grief for BTW, and might backfire on the cuplrit if it leads to manual intervention on a line that's wrongly believed to be dodgy, making things even worse.

Any thoughts, anybody?  Anybody from BTW and willing to pass comment?
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roseway

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Re: DLM management of target SNR
« Reply #1 on: September 24, 2008, 11:34:23 AM »

Hi and welcome

I'm going to need some time to read that document before commenting.
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  Eric

sevenlayermuddle

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Re: DLM management of target SNR
« Reply #2 on: September 25, 2008, 12:47:08 PM »

Further to my initial posting, as mentioned, I'd got an inappropriate 15dB target margin and wanted it fixed.  My ISP's tech support were worse than useless and clearly had no idea what I was talking about - their best suggestion was that I should take my modem back to where I bough it (Amazon) and ask them to test it for me, lol.   After I invoked their ISO 9001 complaints procedure I finally got to talk to somebody who not only understood the question, but was able to get it fixed.  My modem suddenly resynched and I was back at 9dB, immediately followed by a call from the ISP to confirm it had been actioned.  All this had taken about two weeks.

All well and good, but about 48 hours later (possibly to the minute), my modem spontaneously resynched back at 15dB.  I'd had only three resynchs in that 48 hours - one of my own doing, and two in the wee-small hours.   CRC errors were just a few hundred, so I really don't think it was line instability that triggered that second jump to 15dB.

I haven't got the energy to battle with my ISP all over again so I'll live with things for a while and see what happens.  In any case, I doubt whether BT would reset my target margin again so soon after having already done so, but has anybody any idea whether there's some kind of 48 hour threshold on  BT's manual overides?  Maybe my ISP were supposed to have got back in touch with them to confirm I was happy?

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roseway

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Re: DLM management of target SNR
« Reply #3 on: September 25, 2008, 12:56:49 PM »

I can't produce any evidence to prove this, but it's my impression that, once your line is flagged as being rather unstable, then it only takes a small number of disconnections to bump the target noise margin up. So I suspect that what's happened to you is simply the way it's set up to work.
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  Eric

sevenlayermuddle

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Re: DLM management of target SNR
« Reply #4 on: September 25, 2008, 02:40:11 PM »

Thanks, and sadly, that would explain things.  It seems to me the DLM process simply doesn't accomodate the possibility that a line might genuinely and permanently 'improve' after the 10 day period, such as often happens when one cuts the bell wire (who, me? ).

I'm beginning to feel a bit of sympathy for the ISPs who're regularly accused by press and TV campaigns of foul play, for advertising "up to 8Meg" and rarely delivering it.  In many cases, like mine, the blame for substandard service may lie squarely on the shoulders of flawed DLM algorithms controlled by the Telco, and yet it's the ISPs who take the flak. 

Meanwhile I've ordered a speedtouch modem with a broadcom chipset, just in case there's any merit in the recent hyseteria surrounding the AR9 which my 3Com uses.  Maybe that'll at least squeeze a bit more speed from my existing 15dB margin, or maybe it'll disconnest less often, leading eventually to DLM recognition of good behavior, if I don't die first.
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kitz

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Re: DLM management of target SNR
« Reply #5 on: September 27, 2008, 09:22:41 PM »

Interesting documentation youve come across there... thank you for that link.

I also will need to take time to read and digest before commenting further.
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