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Author Topic: What a difference 1dB SNR makes - crosstalk and real-world performance  (Read 1140 times)

Alex Atkin UK

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Noticed my better line has detereorated in SNR a little lately while my second line had improved, so out of curiosity I forced a resync on Line 2.
Here are the results of both lines before and after I rebooted the second line.

Notice how the errors have shot up as clearly they are now causing crosstalk with each other.  My real-world speed has actually reduced.

(you might have to right click, view image before it will load as the forum image proxy seems buggy, you can then press back on the browser to see it in the page properly)

Also Line 2 had an insanely optimistic handback threshold when it first connected (I synced well below it but was fine with that as I knew from Line 1 it was unrealistic), as you can see this is now much more realistic.

I guess this is a good example of how theoretical stats is meaningless, at least without vectoring to offset the crosstalk.
« Last Edit: July 04, 2019, 08:16:11 AM by Alex Atkin UK »
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Modems: 2 x VMG3925-B10B V5.13(AAVF.12)C0-jumbo
Router: i5-7200U appliance running pfSense.
ISP: Zen Unlimited Fibre 2 + Plusnet Unlimited Fibre Extra.

Alex Atkin UK

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Re: What a difference 1dB SNR makes
« Reply #1 on: July 04, 2019, 08:14:26 AM »

Just forced Line 1 to resync and as expected they have swapped places, its now down to 63.3Mbit with 6.2dB SNRM, very similar to where Line 2 was before.

Just goes to show that the SNRM is not a reliable way to measure how a line will perform in the real world.  Hopefully the errored seconds will return to normal.
« Last Edit: July 04, 2019, 08:17:28 AM by Alex Atkin UK »
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IanG

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While I understand the concept of SNR, I have not found an explanation on this site of how the modem (or presumably the exchange, in the case of upstream SNR) measures this parameter in practice. Is there a standard algorithm?

Interference that is very peaky is likely to cause more errors than true random noise with the same RMS voltage.
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Alex Atkin UK

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Looks like re-syncing the Plusnet (Line 2) probably wasn't a great idea:
Code: [Select]
Max:    Upstream rate = 21116 Kbps, Downstream rate = 63375 Kbps
Bearer: 0, Upstream rate = 20000 Kbps, Downstream rate = 65687 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):        5.3             5.5
Attn(dB):        13.5            0.0
Pwr(dBm):        13.7            5.8

                        VDSL2 framing
                        Bearer 0
MSGc:           18              150
B:              239             236
M:              1               1
T:              21              5
R:              0               16
S:              0.1163          0.3771
L:              16512           5410
D:              1               1
I:              240             255
N:              240             255

                        Counters
                        Bearer 0
OHF:            86147814                25863676
OHFErr:         306             238
RS:             0               1680308085
RSCorr:         0               1307
RSUnCorr:       0               0

                        Bearer 0
HEC:            1101            0
OCD:            2               0
LCD:            2               0
Total Cells:    2827627485              0
Data Cells:     517082722               0
Drop Cells:     381
Bit Errors:     0               0

ES:             215             11442
SES:            0               10
UAS:            31              223987
AS:             158415

                        Bearer 0
INP:            0.00            0.00
INPRein:        0.00            0.00
delay:          0               0
PER:            1.83            6.15
OR:             104.43          202.87
AgR:            65791.00        20203.27

Bitswap:        3365/3366               4867/4867

Total time = 1 days 20 hours 46 sec
FEC:            0               1307
CRC:            306             238
ES:             215             11442
SES:            0               10
UAS:            31              223987
LOS:            0               0
LOF:            0               0
LOM:            0               0
Retr:           0
HostInitRetr:   0
FailedRetr:     0

I've noticed in the past things always start to go a bit pear shaped under load when upstream drops below 6dB SNR, although in this case its a bit odd seeing as I'm synced below attainable.
« Last Edit: July 06, 2019, 12:33:13 AM by Alex Atkin UK »
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Router: i5-7200U appliance running pfSense.
ISP: Zen Unlimited Fibre 2 + Plusnet Unlimited Fibre Extra.

PhilipD

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Hi

I think having two VDSL lines in the same property is just about the worse thing for cross-talk, and you will get more odd results than perhaps someone with a single line.

Also keep an eye on your power levels as these could be changing.  SNR and sync speed is only two parts of the equation, the other is power levels, these can fluctuate and reduce in order to help avoid cross talk to other lines or two reduce a reflected signal, i.e. a kind of good neighbour policy. 

Regards

Phil
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Alex Atkin UK

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Hi

I think having two VDSL lines in the same property is just about the worse thing for cross-talk, and you will get more odd results than perhaps someone with a single line.

Also keep an eye on your power levels as these could be changing.  SNR and sync speed is only two parts of the equation, the other is power levels, these can fluctuate and reduce in order to help avoid cross talk to other lines or two reduce a reflected signal, i.e. a kind of good neighbour policy. 

Regards

Phil

Honestly I was expecting it to cause issues, but when the second line was connected it made ZERO difference to the first one, the line stats stayed the same.

I think this is some issue developing further along the line as the SNR is changing at a very specific time of day it seems.  Unlikely anything I can do about it, just worth monitoring.

Does anyone know what happens if you try to order a new line but there are no spare pairs in the drop wire?  Do you fit a second drop wire, just replace the original with one that has more pairs, or refuse the order entirely?
« Last Edit: July 09, 2019, 02:48:32 AM by Alex Atkin UK »
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burakkucat

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Does anyone know what happens if you try to order a new line but there are no spare pairs in the drop wire?  Do you fit a second drop wire, just replace the original with one that has more pairs, or refuse the order entirely?

I can explain that, using our friend Weaver as an example. First and second lines used both pairs in the aerial drop. When line three was ordered, a second Dropwire 10B (CW1411 specification) was deployed. The fourth line used the second pair in the second aerial drop.

At some point it will be more cost effective to install a multi-pair aerial drop cable and recover the existing Dropwire 10Bs.
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Weaver

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Burrakucat is quite correct of course. Two drop wires, four pairs total, running at high level from pole to upper floor.
« Last Edit: July 10, 2019, 12:12:59 AM by Weaver »
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Alex Atkin UK

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Good to know, I'm considering ordering a third line, let them add the drop wire then cancel the second line once the contract runs out.

The only catch is the engineer took ages to fit the second line, which makes me wonder if there is an issue with the remaining pairs from the pole to the cabinet.  Although he never left the pole, so I'm not sure what he was doing.  Did he perhaps find issues with other lines (would be be expected to make-good the junctions if he saw issues, or would that just be raised for someone to do later?0 or just took him ages to root out the right pair back to the cainet?
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burakkucat

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Did he perhaps find issues with other lines (would be be expected to make-good the junctions if he saw issues, or would that just be raised for someone to do later?0 or just took him ages to root out the right pair back to the cainet?

 :shrug2:  If he discovered defects that were not fixable within the time-frame allocated to him for your line-provide task he should have created an A1024 report and attached a so-labelled tag to the defective plant.
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