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Author Topic: SNR reduction, giving DLM a real chance...  (Read 9765 times)

4candles

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Re: SNR reduction, giving DLM a real chance...
« Reply #30 on: October 09, 2009, 02:29:29 PM »

Not having been around these forums for a while,  :-[  I was fascinated to find this thread today. It's very frustrating that the workings of DLM are well documented in respect of sync, SNRM, IP Profile etc, and seem to more or less work, most of the time; yet the way that DLM handles the Target Margin seems shrouded in mystery, myth and magic.

I wonder if any of the techies here can make anything of my recent experience -

I'd been coasting along with my 525dB line syncing at around 5200, 6dB Target, until the cable just down the road was JCB'd by a careless contractor on 20 May. After the repair, I was down to 250kbps and a 12dB Target, and as expected the sync increased after 3 days, to 3776, with a low error count. As long as YouTube and BBC iPlayer worked, I was quite happy, and interested, to see how long it took for the Target to reduce.

Until then I'd always left the router on 24/7, so when nothing had happened by mid-August, I tried re-booting every day around mid-morning for a few weeks - still no change. At the beginning of October, I thought I'd better throw in the towel and ask Plusnet to request a reduction by BTw, but before I got round to it, I lost ISP connection around lunchtime on 6 Oct, due to a MUX failure in the local exchange. Sync was OK, but was at 576/288 with a margin of 31dB - just as though I was suddenly on a 05Mb fixed rate line!  ???

The MUX was fixed by late afternoon, and I was back to 3776 at 12dB, but within a matter of minutes, the router re-synced of its own accord at around 4350 at 9dB. The following day around lunchtime, again without prompting, the router re-synced at 5120 at 6dB!  :) :)

It would seem, then, that the DSLAM was stuck for nearly 6 months, and was somehow unstuck by a MUX failure. 
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jeffbb

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Re: SNR reduction, giving DLM a real chance...
« Reply #31 on: October 09, 2009, 02:41:20 PM »

Hi

Some people are just lucky I guess  ;D  keep fingers crossed

Regards Jeff
edit  btw you don't happen to be on Anglesey?
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4candles

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Re: SNR reduction, giving DLM a real chance...
« Reply #32 on: October 09, 2009, 02:52:39 PM »

Yeah - everything's crossed.   ;D

Anglesey? No, Moray, NE Scotland.
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jeffbb

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Re: SNR reduction, giving DLM a real chance...
« Reply #33 on: October 09, 2009, 03:08:20 PM »

Hi

quote  until the cable just down the road was JCB'd by a careless contractor on 20 May they must have their mates down here  :lol:

Regards Jeff
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sevenlayermuddle

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Re: SNR reduction, giving DLM a real chance...
« Reply #34 on: October 09, 2009, 03:08:29 PM »

That's very interesting 4candles.  One thing we have in common was that we had a period with a very high SNR (with low sync speed), that  seemed to have preempted the target reduction.  In your case however the reduction was very quick, so I'd say you're probably right about it being a result of the MUX failure.  

One of the features I believe DLM implements  is that if, as a result of a raised target, your line fails to sync, then the target will be reduced and your line will be 'flagged for investigation'.  Maybe that's what happened to you, and maybe even resulted in human (common sense) intervention?

My own target has stayed contently at 9dB, despite the less than perfect stability that I expect with such a margin.  It often drops to 6dB or so late evening, by which time error rates are usually 10-20 ES per hour.  But in one 24 hour period earlier in the week I noticed around 100 ES per hour, and over 1400 ES in that 24 hour interval.    That's the worst error rates I've ever seen but probably not unreasonable for an occasional occurrence, considering it's a 56dB line and synced at c3800, but I'm relieved to see DLM is no longer hyper-sensitive.

I conclude that, in my case at least, the SNR reduction that I got by request of my ISP was ineffective as it shot back up again after 48 hours.  by allowing DLM to make up it's own mind to reduce, the reduction looks like it has been permanent.

edited:  changed 'syncs below FTR' to 'fails to sync', which I believe to be more accurate.
« Last Edit: October 09, 2009, 03:48:26 PM by sevenlayermuddle »
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4candles

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Re: SNR reduction, giving DLM a real chance...
« Reply #35 on: October 09, 2009, 06:17:40 PM »

One of the features I believe DLM implements  is that if, as a result of a raised target, your line fails to sync, then the target will be reduced and your line will be 'flagged for investigation'.  Maybe that's what happened to you, and maybe even resulted in human (common sense) intervention?

I conclude that, in my case at least, the SNR reduction that I got by request of my ISP was ineffective as it shot back up again after 48 hours.  by allowing DLM to make up it's own mind to reduce, the reduction looks like it has been permanent.

edited:  changed 'syncs below FTR' to 'fails to sync', which I believe to be more accurate.

I see what you mean, but sync was never lost during this episode, and as my initial 10 day training period completed before I understood such things, I'm saddled with an FTR of 2640, :( but don't get me started on that!

I think you're probably right re your own reduction - hope it stays down.   :)
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