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Author Topic: Larger than usual SNR swing - weather related?  (Read 871 times)

tubaman

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Larger than usual SNR swing - weather related?
« on: January 11, 2021, 08:27:49 AM »

After being stable at 3dB SNR margin and low ES for over 50 days my line resynced about 12 days ago following a largish bout of ES. DLM moved me from 3dB to 4dB, losing me 2Mbps of speed in the process, and since then I have been keeping a close eye on things.
What I am seeing is a larger than usual swing in the SNR margin most evenings - gradually dropping from around 4.3dB to 2.6dB as it gets dark. In the morning its back to about 4.3dB again and appears to remain that way during the day.
My question is is this likely to be just weather related, as I know ADSL can be prone to evening interference from AM radio but I didn't think the VDSL frequencies suffered from this.
 :)
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benji09

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Re: Larger than usual SNR swing - weather related?
« Reply #1 on: January 11, 2021, 09:20:33 PM »


  In the good old days, until the late '60's - before scam calls from abroad were the norm -  intercontinental calls, both telephone and telex! were carried via HF radio. The cost of these calls was 1 per minute. These HF radio links used at least two radio frequencies to to cater for ionosphere changes during the day night cycle. These changes also occurred during summer and winter. So yes HF  frequencies are affected by the time of day, as are the old medium waves!   
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tubaman

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Re: Larger than usual SNR swing - weather related?
« Reply #2 on: January 12, 2021, 08:57:12 AM »

Last night at 03:13hrs I had a resync that took the service down for over 6 minutes, so I'm not sure what that was about? Now connected at 3dB again and speed loss restored, but I suspect this won't last for long if I'm still getting ~1.5dB drops in the evening as the ES will be too high.
Stupid DLM  :wall:
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burakkucat

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Re: Larger than usual SNR swing - weather related?
« Reply #3 on: January 12, 2021, 04:56:43 PM »

Last night at 03:13hrs I had a resync that took the service down for over 6 minutes, so I'm not sure what that was about?

It could have been a remotely initiaited upgrade of the DSLAM software, followed by a warm restart. There's no way for us to know.
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tubaman

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Re: Larger than usual SNR swing - weather related?
« Reply #4 on: January 12, 2021, 05:34:09 PM »

It could have been a remotely initiaited upgrade of the DSLAM software, followed by a warm restart. There's no way for us to know.

Given the time it was down you could well be correct.
 :)
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tubaman

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Re: Larger than usual SNR swing - weather related?
« Reply #5 on: January 12, 2021, 07:11:44 PM »

This is really bizarre. I've been watching this as it's been getting dark and it's now as below, with ES not very good.
Code: [Select]
====================================================================================
    VDSL Training Status:   Showtime
                    Mode:   VDSL2 Annex B
            VDSL Profile:   Profile 17a
                G.Vector:   Disable
            Traffic Type:   PTM Mode
             Link Uptime:   0 day: 15 hours: 46 minutes
====================================================================================
       VDSL Port Details       Upstream         Downstream
               Line Rate:      7.737 Mbps       43.951 Mbps
    Actual Net Data Rate:      7.712 Mbps       43.952 Mbps
          Trellis Coding:         ON                ON
              SNR Margin:        5.5 dB            1.5 dB
            Actual Delay:          0 ms              0 ms
          Transmit Power:        6.4 dBm          12.0 dBm
           Receive Power:      -15.2 dBm         -22.8 dBm
              Actual INP:        0.0 symbols      50.0 symbols
       Total Attenuation:        0.0 dB          26.3 dB
Attainable Net Data Rate:      7.712 Mbps       41.474 Mbps
====================================================================================
  VDSL Band Status        U0      U1      U2      U3      U4      D1      D2      D3
  Line Attenuation(dB): 11.3 49.9   N/A   N/A   N/A 21.1 59.5 89.4
Signal Attenuation(dB): 11.3 49.7   N/A   N/A   N/A 32.8 59.1   N/A
        SNR Margin(dB): 5.1 5.5   N/A   N/A   N/A 1.4 1.5   N/A
         TX Power(dBm): -4.0 6.0   N/A   N/A   N/A 9.9 7.8   N/A
====================================================================================

            VDSL Counters

           Downstream        Upstream
Since Link time = 46 min 39 sec
FEC: 62877338 0
CRC: 1373 17
ES: 343 17
SES: 27 0
UAS: 0 0
LOS: 0 0
LOF: 0 0
LOM: 0 0
Latest 15 minutes time = 10 min 38 sec
FEC: 5102599 0
CRC: 362 0
ES: 133 0
SES: 9 0
UAS: 0 0
LOS: 0 0
LOF: 0 0
LOM: 0 0
Previous 15 minutes time = 15 min 0 sec
FEC: 7169012 0
CRC: 285 0
ES: 143 0
SES: 3 0
UAS: 0 0
LOS: 0 0
LOF: 0 0
LOM: 0 0
Latest 1 day time = 8 hours 25 min 38 sec
FEC: 40344006 0
CRC: 1372 10
ES: 342 10
SES: 27 0
UAS: 0 0
LOS: 0 0
LOF: 0 0
LOM: 0 0
Previous 1 day time = 24 hours 0 sec
FEC: 44459171 0
CRC: 1024 13
ES: 50 13
SES: 20 0
UAS: 397 387
LOS: 1 0
LOF: 6 0
LOM: 0 0
Total time = 1 days 8 hours 25 min 38 sec
FEC: 355824682 13292
CRC: 13390 3973
ES: 3100 3523
SES: 186 1
UAS: 485 453
LOS: 2 0
LOF: 13 0
LOM: 3 0
====================================================================================

It gradually dropped from about 3.2dB to 2.6 and that was ok, but the drop down to below 2dB was quite sudden. What I don't understand is why only the D1 and D2 bands are seeing this issue, with U0 and U1 holding out fine.
What is going on here?
 ???
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burakkucat

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Re: Larger than usual SNR swing - weather related?
« Reply #6 on: January 12, 2021, 08:02:55 PM »

What I don't understand is why only the D1 and D2 bands are seeing this issue, with U0 and U1 holding out fine.
What is going on here?
 ???

That I cannot answer.  :no:  However, given the time-frame in which the symptoms appeared, I would be tempted to "walk the route" of the D-side cable to the "fibre cabinet", looking out for faulty street lighting whilst simultaneously checking for RF-mush in the 300 - 600 kHz range.
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tubaman

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Re: Larger than usual SNR swing - weather related?
« Reply #7 on: January 12, 2021, 08:23:20 PM »

That I cannot answer.  :no:  However, given the time-frame in which the symptoms appeared, I would be tempted to "walk the route" of the D-side cable to the "fibre cabinet", looking out for faulty street lighting whilst simultaneously checking for RF-mush in the 300 - 600 kHz range.
It's not street lights as we don't have them here.
Even stranger, at approx 20:00 the issue abruptly stopped and I have returned to ~3dB. If it was sudden in it's start and stop I would be thinking electrical interference, but the start appears to be more gradual. Whether the stop is usually this abrupt I don't know as this is the first time I have observed it.
This has generated about 700 ES in approx 2.5hrs which isn't great.
It's all very odd.
 ???
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burakkucat

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Re: Larger than usual SNR swing - weather related?
« Reply #8 on: January 12, 2021, 08:25:15 PM »

Very perplexing.  :-\
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tubaman

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Re: Larger than usual SNR swing - weather related?
« Reply #9 on: January 13, 2021, 07:28:28 AM »

A punishment beating from DLM in the early hours - back to 4dB SNR. I told it that moving me back to 3dB the night before was a bad plan, but it knew best as usual! In reality this is actually probably the right place to be right now as it'll allow my line to sustain the ~1.5dB early evening drops without going into ES meltdown.
What is curious is I only had about 800 ES in the last 24 hour period, which I didn't think was enough to upset the DLM gods. I wonder if the very high numbers in a short timeframe is taken into account?  ???
« Last Edit: January 13, 2021, 07:43:48 AM by tubaman »
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burakkucat

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Re: Larger than usual SNR swing - weather related?
« Reply #10 on: January 13, 2021, 05:28:30 PM »

<snip>
What is curious is I only had about 800 ES in the last 24 hour period, which I didn't think was enough to upset the DLM gods. I wonder if the very high numbers in a short timeframe is taken into account?  ???

That is a possibility. We know that the smallest time quantum, upon which both the DSLAM and the modem will use when counting events, is 15 minutes.

[doodles]
1 x 24 hr == 96 x 15 min

1 x 15 min == 900 sec


[pseudo-code]
if (800 ES in one 15 min quantum) then (almost one ES per second is liable to rouse the DLM process); fi
[/pseudo-code]
[/doodles]
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tubaman

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Re: Larger than usual SNR swing - weather related?
« Reply #11 on: January 13, 2021, 08:44:57 PM »

So here is this evening's strangeness. I fired up dslstats so have graphs!
The SNR per tone and QLN in this post are from when SNR had dropped.
I'll put SNR per tone and QLN from when the SNR was restored in the next post.
HLOG remained exactly the same looking at all times.
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tubaman

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Re: Larger than usual SNR swing - weather related?
« Reply #12 on: January 13, 2021, 08:47:40 PM »

QLN and SNR per tone from after the SNR was restored.
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burakkucat

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Re: Larger than usual SNR swing - weather related?
« Reply #13 on: January 13, 2021, 10:41:23 PM »

Hmm . . . I am having a mental block. I think the channel characteristics function, H, is only determined when the xTU-C and xTU-R, at either end of the circuit, train-up. Hence an examination of plots of the logarithmic presentation of the channel characteristics function, Hlog, at different times all post the same one discrete circuit train-up will be identical.

The two QLN plots you have shown are identical (I viewed them in full screen mode in a continuous cycle). My mental block continues . . .

The SNR plots show a definite hit in both the lower frequencies of the DS1 band and the lower frequencies of the DS2 band. Which is very peculiar.  :-\

In the case of the DS1 band, it is almost as if the DSPBO is being increased (the "bad" plot) and then restored to normality (the "good" plot).  :hmm:
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tubaman

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Re: Larger than usual SNR swing - weather related?
« Reply #14 on: January 14, 2021, 02:19:28 PM »

I must admit I couldn't see any difference between the QLN plots either but thought I might be missing something.
As you say burakkucat the SNR band hits are certainly very odd. What is also odd is the slight improvement between approx 17.45 and 18.50 and the large number of spikes where SNR very briefly increased.
Using dslstats allowed me to see that both the start and end of these episodes are pretty sudden, but I'm still no nearer to knowing what is going on.
At least being at 4dB now allows all this to happen with virtually no ES occuring - at least until DLM decides to give 3dB a go again.
 :)
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