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Author Topic: Resynchs improve speed ?  (Read 617 times)

Weaver

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Resynchs improve speed ?
« on: September 16, 2018, 12:59:14 AM »

Prevailed on Mrs Weaver to power cycle all modems. Got a combined downstream sync rate improvement of about 300kbps, after adding up all four modems.

It is interesting how fairly regular resynchs do improve speeds. But I suspect that you really must have a long interval of continuous up time or forcing a resynch can I think be very counterproductive. I don’t know what the minimum on-time is. I wonder if perhaps resynching makes sure that the modems are now fully attuned to changed crosstalk or interference conditions. In the past, various people have suggested resynching at certain favorable times of day. I suspect that the rules are not simple though and I would not be surprised to find out that things differ according to how good or bad your line is, how variable it is in various ways and maybe even according to what your target SNRM is.
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re0

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Re: Resynchs improve speed ?
« Reply #1 on: September 16, 2018, 05:12:51 AM »

I apologise in advance if this is just repeating your knowledge.

I have heard nowhere about there being speed improvements by regularly resync'ing. There are speed improvements to be had from a resync under circumstances where the conditions have changed leading to reduced interference (e.g. if a crosstalker has gone offline, background noise has reduced). Though there is a chance the speed at resync could be less if the conditions have worsened (essentially in instances contrary to the previously mentioned example improvements).

There may be cases where a resync will improve speed in reference to throughput if you are experiencing a lot of noise that reduces the SNR margin and creates errors between both ends (since a barrage of CRCs are going to be essentially be lost data and the data will need to be re-transmitted causing a loss of throughput). Under this circumstance, the sync speed would actually be lower by a reasonable degree if you actually decided to resync.

During training, modems at both ends have to communicate and work out what can utilised based on what they can "hear" and then they will synchronise (if it is possible, of course). Bitswapping has the job of keeping the connection stable since conditions do change after the line synchronises; it simply allows for bits to be swapped to other subchannels in the event where the noise margin is too low (potentially due to a burst of interference) within a specific tone to transmit the allocated number of bits. I have read it even can increase the gain if necessary.

The thing is, you do not have Seamless Rate Adaption (SRA) so you do not actually see the sync speed drop and rise (when conditions change) without a resync. I understand that 300 kbps would account for about 10% if it was just in reference to a single line of yours but this is across all 4 lines so per line it is rather insignificant; 75 Kbps is nada. Next time you do a "regular" resync, you could lose that 75 Kbps.
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Weaver

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Re: Resynchs improve speed ?
« Reply #2 on: September 16, 2018, 10:25:09 AM »

I could easily lose all that I have gained. I am really certain that this is highly likely if the modems have only been up for a short while.

However, the long term observation, formed over a number of years, is that in my case if I leave modems on for a really long period that is without problems and so no spontaneous resynchs, then they will tend to resynch higher when a resynch is forced. Obviously there is a ceiling, otherwise by that argument sync rates would be pushed up and up for ever. I think in part this is the intelligent god overriding statistics, because if sync rate is low then you choose to wait a good length of time and then resync and if the rate is high then you do not resynch, so perhaps it is that choice intervention process that biases things and it is as much about the properties of the god of modems (me) than about DSL.

I wonder if this is totally not true in the case of other lines, even if done with the same intelligent intervention strategy.

The other candidate explanation is the ‘change in conditions’ one mentioned earlier, and for that it would have to be the case that there is long-term drift away from the initial state, so the original plan becomes less appropriate (and if fact shifts to something beyond what bitswap can adapt to), rather than cyclic variations.
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re0

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Re: Resynchs improve speed ?
« Reply #3 on: September 16, 2018, 05:40:37 PM »

Obviously there is a ceiling, otherwise by that argument sync rates would be pushed up and up for ever.
I did actually think about this from your original post. :D But I know that you know it's against physics. :)

The other candidate explanation is the ‘change in conditions’ one mentioned earlier, and for that it would have to be the case that there is long-term drift away from the initial state, so the original plan becomes less appropriate (and if fact shifts to something beyond what bitswap can adapt to), rather than cyclic variations.
I see what you are saying and believe I understand it. Though I can't say that I have the answer. :(

I was under the impression that modems mark channels as unusable when the SNR drops below the minimum amount to allocate bits (so bitswaps are not possible there). Though I have no idea if the modem can make channels as usable again once the SNR is high enough again (sadly this is the extent of my knowledge, though it would make sense of it was possible to do so). If they cannot mark channels as usable again (even if there is spare SNR), then the idea of forcing a resync would make complete sense.
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kitzuser87430

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Re: Resynchs improve speed ?
« Reply #4 on: September 16, 2018, 06:11:29 PM »

@weaver

You really need to use DslStats to monitor your line(s), specifically the delta between daytime SNR margin and night time SNR margin.

I force my single line to sync at about 5 dB (7323 kbps) and this drops to about 2 dB in the hours of darkness currently the line has been up for 14 days and 10 hours with a running total of only 660 error seconds.

I think i could risk a sync at 4 dB and gain approximately 500 kbps,but i prefer stability over speed.

Monitoring your lines will enable you to set a target SNRM offset to gain every kbps of speed that you seem to be chasing.

Ian

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ejs

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Re: Resynchs improve speed ?
« Reply #5 on: September 16, 2018, 06:45:14 PM »

I was under the impression that modems mark channels as unusable when the SNR drops below the minimum amount to allocate bits (so bitswaps are not possible there). Though I have no idea if the modem can make channels as usable again once the SNR is high enough again (sadly this is the extent of my knowledge, though it would make sense of it was possible to do so). If they cannot mark channels as usable again (even if there is spare SNR), then the idea of forcing a resync would make complete sense.

All except very old and obsolete modems can do this. It's that "monitorTone" capability you can see and switch on/off with the xdslctl program.

At this time of year, as it's getting colder, it's possible that the increases in speed are due to the attenuation decreasing with the colder temperature.
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re0

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Re: Resynchs improve speed ?
« Reply #6 on: September 16, 2018, 06:51:28 PM »

All except very old and obsolete modems can do this. It's that "monitorTone" capability you can see and switch on/off with the xdslctl program.
Hmm, I see:
Code: [Select]
Capability:
        bitswap         On
        sra             On
        trellis         On
        sesdrop         Off
        CoMinMgn        Off
        24k             On
        phyReXmt(Us/Ds) Off/On
        TpsTc           AvPvAa
        monitorTone:    On
        dynamicD:       On
        dynamicF:       Off
        SOS:            On
        Training Margin(Q4 in dB):      -1(DEFAULT)

So you are saying that this can allow for the re-using of tones that were previously marked as unusable?
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ejs

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Re: Resynchs improve speed ?
« Reply #7 on: September 16, 2018, 09:23:18 PM »

Yes that's it. The name makes sense in that the tones swapped down to zero bits need to be monitored to determine if they can carry bits again.
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Weaver

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Re: Resynchs improve speed ?
« Reply #8 on: September 17, 2018, 02:12:55 AM »

My previous modems, the DLink DSL-320B-Z1, I believe, did not have monitored tones capability and so just got worse and worse until they resynched on their own. So this would be another argument in favour of my assertion and another mechanism, which I had forgotten about.

So by that argument, I should expect to see this benefit much less in future.
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re0

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Re: Resynchs improve speed ?
« Reply #9 on: September 17, 2018, 06:22:29 PM »

From my perspective, the short of this topic is that resync'ing at regular intervals isn't going to improve sync speeds on its own. I just can't see a reason for it, unless it happened to occur after "positive" changes outside of the typical variance otherwise you will get around the same speed.

Stability is a different question, but bitswapping is likely to take care of issues relating to that to some degree to prevent the line from disconnecting as often in a situation compared to a situation where it is absent.
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Weaver

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Re: Resynchs improve speed ?
« Reply #10 on: September 20, 2018, 12:49:19 AM »

Last night there was some thunder and lightning. My hardware lightning alert unit woke Janet up and she turned the modems off and unplugged them for the whole of the night.

This morning they were turned back on. Sync rate downstream worse on line #1, better on lines 2, 4. All lines improved or no change on upstream.

Line #1 downstream sync 3092k → 3077k; IP rate 2663957 → 2650992 :(
Line #2 downstream sync 2803k → 2963k; IP rate 2415021 → 2553319  ;D  ;D
Line #3 downstream sync 2938k - no change
Line #4 downstream sync 2968k → 2982k; IP rate 2560234 → 2569742  ;D

Line #1 upstream sync no change
Line #2 upstream sync 535k → 551k :(
Line #3 upstream sync no change
Line #4 upstream sync no change
« Last Edit: September 20, 2018, 01:26:13 AM by Weaver »
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re0

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Re: Resynchs improve speed ?
« Reply #11 on: September 20, 2018, 10:00:06 PM »

I'd probably do the same in your situation (and have done so in the past). I've never had a situation (and touch wood) where lightening has caused damage to any of my hardware. Though rather not risk it if the prevention is as easy as pulling the plug.

Sync speeds are all within a nice margin of the previous sync speed. :)
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