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Author Topic: Loss of 1.5dB margin on upstream  (Read 395 times)

Jon21

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Loss of 1.5dB margin on upstream
« on: September 25, 2019, 02:43:22 PM »

Noticed that the upstream SNR has dropped by 1.5dB today in one quick burst. Initially it went down to 3dB then recovered. This has knocked ~1.5Mb off the upstream attainable. Was just wondering what might of caused it? If I hadn't of looked at DSLStats, I wouldn't of known so it's more out of curiosity.

Code: [Select]
adsl info --stats
adsl: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason:    0
Last initialization procedure status:   0
Max:    Upstream rate = 14438 Kbps, Downstream rate = 76234 Kbps
Bearer: 0, Upstream rate = 16076 Kbps, Downstream rate = 76226 Kbps

Link Power State:       L0
Mode:                   VDSL2 Annex B
VDSL2 Profile:          Profile 17a
TPS-TC:                 PTM Mode(0x0)
Trellis:                U:ON /D:ON
Line Status:            No Defect
Training Status:        Showtime
                Down            Up
SNR (dB):        5.9             4.6
Attn(dB):        19.4            0.0
Pwr(dBm):        13.4            6.7

                        VDSL2 framing
                        Bearer 0
MSGc:           18              16
B:              239             238
M:              1               1
T:              22              32
R:              0               16
S:              0.1002          0.4732
L:              19160           4311
D:              1               1
I:              240             255
N:              240             255

                        Counters
                        Bearer 0
OHF:            2038505318              1587068
OHFErr:         24591           96
RS:             0               3367190
RSCorr:         0               22474
RSUnCorr:       0               0

                        Bearer 0
HEC:            68483           0
OCD:            886             0
LCD:            886             0
Total Cells:    2094020773              0
Data Cells:     1382770312              0
Drop Cells:     0
Bit Errors:     0               0

ES:             8321            3327
SES:            13              3
UAS:            28              14498
AS:             3384237

                        Bearer 0
INP:            0.00            0.00
INPRein:        0.00            0.00
delay:          0               0
PER:            1.65            7.60
OR:             115.66          23.15
AgR:            76341.78        16099.13

Bitswap:        67073/67294             7150/7159

Total time = 39 days 4 hours 4 min 25 sec
FEC:            0               22474
CRC:            24591           96
ES:             8321            3327
SES:            13              3
UAS:            28              14498
LOS:            0               0
LOF:            0               0
LOM:            0               0
Latest 15 minutes time = 4 min 25 sec
FEC:            0               0
CRC:            0               0
ES:             0               0
SES:            0               0
UAS:            0               0
LOS:            0               0
LOF:            0               0
LOM:            0               0
Previous 15 minutes time = 15 min 0 sec
FEC:            0               0
CRC:            4               0
ES:             4               0
SES:            0               0
UAS:            0               0
LOS:            0               0
LOF:            0               0
LOM:            0               0
Latest 1 day time = 4 hours 4 min 25 sec
FEC:            0               20861
CRC:            81              33
ES:             58              13
SES:            0               0
UAS:            0               0
LOS:            0               0
LOF:            0               0
LOM:            0               0
Previous 1 day time = 24 hours 0 sec
FEC:            0               31
CRC:            4373            1
ES:             245             1
SES:            1               0
UAS:            0               0
LOS:            0               0
LOF:            0               0
LOM:            0               0
Since Link time = 39 days 4 hours 3 min 55 sec
FEC:            0               22474
CRC:            24591           96
ES:             8321            3327
SES:            13              3
UAS:            0               14470
LOS:            0               0
LOF:            0               0
LOM:            0               0
NTR: mipsCntAtNtr=0 ncoCntAtNtr=0
 >
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burakkucat

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Re: Loss of 1.5dB margin on upstream
« Reply #1 on: September 25, 2019, 04:17:55 PM »

Was just wondering what might of caused it? If I hadn't of looked at DSLStats, I wouldn't of known so it's more out of curiosity.

Nothing stands out, to me. I would just put it down to "one of those things".
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Jon21

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Re: Loss of 1.5dB margin on upstream
« Reply #2 on: October 09, 2019, 03:06:07 PM »

Noticed that the SNR margin has increased now. I did resync the day after my previous post. Possibly down to crosstalk?
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burakkucat

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Re: Loss of 1.5dB margin on upstream
« Reply #3 on: October 09, 2019, 03:10:13 PM »

Noticed that the SNR margin has increased now. I did resync the day after my previous post. Possibly down to crosstalk?

Yes, that could be possible.
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kitz

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Re: Loss of 1.5dB margin on upstream
« Reply #4 on: October 12, 2019, 06:18:54 PM »

From the shape of the drops in the first graph, could possibly be DSM (Dynamic Spectral Management) which has now replaced the bog standard power cutback.   Openreach now uses many different spectral masks to shape bitload and manage upstream power back off   

Which mask you get allocated depends on various factors:-
Primarily cab distance from the exchange (Eside) and distance from cab to the home (D side) but the third element which makes it dynamic is that it adjusts power profile to current crosstalk conditions based on whether neighbouring cross-talker is on or offline at the time of sync then you may get a bit more power (dBm) than if they were not.   It's aim is to distribute power fairer and manage crosstalk more effectively.     eg if your disturber is offline then it can give you a bit more power than if they weren't. 


I've managed to sync before my disturber and the DSM will allocate me a higher upstream power profile than if my disturber is online.  Next time I sync it remembers the previous power profile, but then it starts cutting back in stages.. which in turn affects the SNRM and thus I see the step like effect very similar to that displayed in your graph from 13:11 to 13:36.

Based on the dip at around 13:05 I'd hazard a good guess that someone just got connected to the cab and DSM is adjusting your power profile accordingly.   
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Weaver

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Re: Loss of 1.5dB margin on upstream
« Reply #5 on: October 13, 2019, 02:58:11 AM »

@jon21 I get regular 4 dB - 5 dB sharp drops/rises in upstream SNRM. I have never been able to work out why. I need to scrutinise the situation to see if attenuation is changing. Otherwise, clearly it is noise itself that is going on/off as the power isnít changing. A guess would be some interference noise source coming on/off and that may or may not not be crosstalk but for me it would seem to be some other general noise source not crosstalk because of the timing and because downstream is unaffected.

If itís crosstalk would one expect downstream and upstream both to be affected ? - even though the amounts may not be the same.

Your graph shape is much more complex than my line 3; you have an irregular pattern and I have a simple square wave for line 3, whereas one of the other lines is more complex and irregular. My line 3 has a straight 24 hour period and no more than two levels.

For my line 3, the fact that it is a sharp jump suggests switching something on/off or less likely gating some noise source. I would expect a gentle change from other causes such as temperatures unless a joint were to expand and suddenly become partially broken - in semi-conducting partial or point contact. If the times of day are wrong then that rules out temperature. And if the times of day are some significant points linked to human activity, such as on the hour then that rules out temperature.

Just illustrating the thinking behind my story; your circumstances are of course different.
« Last Edit: October 13, 2019, 03:02:19 AM by Weaver »
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kitz

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Re: Loss of 1.5dB margin on upstream
« Reply #6 on: October 14, 2019, 01:38:59 AM »

Quote
If itís crosstalk would one expect downstream and upstream both to be affected ? - even though the amounts may not be the same.

Valid point .....  which is why I mentioned DSM.   It does some weird and wonderful things when it comes to the upstream effects on the SNRM. 
I promise you I have seen that stepping on my line several times which only affects the upstream. 

Trying to look for the relevant graphs now is like trying to look for a needle in a haystack and this is where I miss MDWS.
Easier for me to find is the effect on my upstream SNRM is when one of my disturbers does a reboot of their modem because I see a distinct spike when scanning through thumbnails.... so I'll post those. 

As expected both up and down are affected when they go off line.   But look what will happen when they come back online - they have a slightly different mask which affects just my upstream SNRM.  Sometimes it goes up and sometimes it goes down.      Currently I'm sitting at 9.3dB.    I'm quite used to seeing different upstream SNRM values now due to DSM changing power profiles.     
I must admit though from my own past experiences, I am more likely to see the stepping effect if it is me who has performed the reboot.... but it is at its most obvious if we have both been knocked out of sync at the same time and my modem comes baak up before theirs.    I see DSM trying to take some of my 'surplus / unused' SNRM in those steps.   I'd need to do a lot more research into DSM to understand properly how it works...  but even then I wouldn't have a clue what power profile openreach use...  so for now I am just aware that its there and that Openreach are using it..  but since I have no control over it I see no point fretting.

Although saying that it has crossed my mind on more than one occasion if it would be possible to get into "power battles" with your neighbour :D :D
I'm not sure if BTw use DSM with their adsl MSANs -  They are likely capable but I can't specifically recall anything being said about them using it for adsl.   
« Last Edit: October 14, 2019, 02:02:03 AM by kitz »
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Weaver

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Re: Loss of 1.5dB margin on upstream
« Reply #7 on: October 14, 2019, 02:27:05 AM »

> it would be possible to get into "power battles" with your neighbour :D :D

That has to be true, surely.
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