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Author Topic: Line 2 downstream slow  (Read 1697 times)

Weaver

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Line 2 downstream slow
« on: March 12, 2020, 12:04:25 PM »

My line 2 is suddenly ~0.5Mbps slower downstream than it should be; it’s normally around 2.9Mbps d/s and now 2.44 Mbps at 0.9 dB SNRM. I disconnected all the modems from the wall sockets last night because of approaching lightning storms. When I started the modems up again line 2 alone was slow. Reported this to AA to see if they can think of anything. The d/s SNRM is actually quite low, not high as you might expect for a cause behind this condition. Any thoughts?

xdslctl: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason:   8000
Last initialization procedure status:   0
Max:   Upstream rate = 430 Kbps, Downstream rate = 2276 Kbps
Bearer:   0, Upstream rate = 515 Kbps, Downstream rate = 2440 Kbps

Link Power State:   L0
Mode:         ADSL2 Annex A
TPS-TC:         ATM Mode(0x0)
Trellis:      U:ON /D:ON
Line Status:      No Defect
Training Status:   Showtime
      Down      Up
SNR (dB):    0.9       5.8
Attn(dB):    64.5       40.7
Pwr(dBm):    18.4       12.4

         ADSL2 framing
         Bearer 0
MSGc:      51      11
B:      28      62
M:      8      1
T:      3      1
R:      14      12
S:      2.9954      3.8462
L:      657      156
D:      1      8

         Counters
         Bearer 0
SF:      55996      56718
SFErr:      1782      0
RS:      1196933      964081
RSCorr:      23224      173
RSUnCorr:   1983      0

ReXmt:      36962      0
ReXmtCorr:   36756      0
ReXmtUnCorr:   2155      0

         Bearer 0
HEC:      2583      0
OCD:      0      0
LCD:      0      0
Total Cells:   5179025      1123115
Data Cells:   31382      11650
Drop Cells:   0
Bit Errors:   116158      0

ES:      41497      200
SES:      23767      0
UAS:      23764      23655
AS:      929

         Bearer 0
INP:      26.00      2.00
INPRein:   0.00      0.00
delay:      8      8
PER:      16.00      16.34
OR:      28.48      8.32
AgR:      2458.15   522.12

Bitswap:   255/258      2/2

Total time = 18 days 8 min 10 sec
FEC:      999726      74912
CRC:      156810      225
ES:      41497      200
SES:      23767      0
UAS:      23764      23655
LOS:      7      0
LOF:      63      0
LOM:      690      0
Latest 15 minutes time = 8 min 10 sec
FEC:      18614      66
CRC:      1679      0
ES:      413      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:      4610      107
CRC:      103      0
ES:      78      0
SES:      0      0
UAS:      461      461
LOS:      0      0
LOF:      0      0
LOM:      0      0
Latest 1 day time = 8 min 10 sec
FEC:      18614      66
CRC:      1679      0
ES:      413      0
SES:      0      0
UAS:      0      0
LOS:      0      0
LOF:      0      0
LOM:      0      0
Previous 1 day time = 24 hours 0 sec
FEC:      4610      107
CRC:      103      0
ES:      154      19
SES:      11      0
UAS:      7051      7040
LOS:      1      0
LOF:      9      0
LOM:      0      0
Since Link time = 15 min 27 sec
FEC:      23224      173
CRC:      1782      0
ES:      491      0
SES:      0      0
UAS:      0      0
LOS:      0      0
LOF:      0      0
LOM:      0      0
NTR: mipsCntAtNtr=0 ncoCntAtNtr=0
« Last Edit: March 12, 2020, 02:38:47 PM by Weaver »
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Weaver

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Re: Line 2 downstream slow
« Reply #1 on: March 12, 2020, 01:42:40 PM »

AA tested the line and couldn’t see anything wrong with it. I’m at a loss - there should be something visible in the stats I think. Perhaps I should swap out the modem in case that modem has gone weird?
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Chrysalis

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Re: Line 2 downstream slow
« Reply #2 on: March 12, 2020, 02:12:04 PM »

you have a historical plot of the line metrics?
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Weaver

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Re: Line 2 downstream slow
« Reply #3 on: March 12, 2020, 02:38:13 PM »

I have a 24 hour SNRM graph: green (the upper trace) is upstream and red (the lower trace) is downstream. The upstream target SNRM is 6dB, downstream is 3dB.



The wide gap from 07:40 - 09:30 is where Janet unplugged the modems because of lightning. On the right various discontinuities are due to me resetting the SNRM target or forcing a retrain.
« Last Edit: March 12, 2020, 02:43:56 PM by Weaver »
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Weaver

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Re: Line 2 downstream slow
« Reply #4 on: March 12, 2020, 02:46:35 PM »

I can post up the full stats for one of the other lines for comparison. I have briefly looked at such a comparison and I can’t see any radical differences which would seem to be the cause of such such slowness.
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PhilipD

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Re: Line 2 downstream slow
« Reply #5 on: March 12, 2020, 03:02:13 PM »

Check the power levels, these can change for various reasons, not a lot you can do about it, but if they've been reduced then you will sync at a slower speed and it will explain the loss of speed. 

Don't forget it is all changing goal posts and anytime you've been off line you shuffle the pack again when you resync and you aren't necessarily in the same position as when you started.  For example someone else might have resync'd whilst you were offline and without your cross-talk because you are disconnected they get a better sync and higher power level, when you join again after this event you are now seeing more crosstalk from them than you had before.  It might even itself out after some time.
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Weaver

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Re: Line 2 downstream slow
« Reply #6 on: March 13, 2020, 12:50:11 PM »

The power levels are the same as for line 1.
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PhilipD

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Re: Line 2 downstream slow
« Reply #7 on: March 13, 2020, 01:42:21 PM »

The power levels are the same as for line 1.

Yes but what were the power levels on this line before, no good comparing it to another line, you want to see what's changed on the line with the problem.

More than likely others got a higher sync when you were off line, they are using more tones and/or higher power and now when you sync you are seeing a bit more cross-talk, just luck of the draw.  xDSL as you know is a mess of overlapping frequencies, crosstalk, work arounds, round robin adjustments, tricks and magic to work over old telephone cable, there is nothing static about it, each time you resync it's pot luck where you sit in the pecking order, it's a best efforts service with no guarantees on sync speed. At least with A&A they are charging you enough to still be interested in following up issues to micro-manage a few long lines, all you can do is see if they can get something happening at Openreach, perhaps a pair swap somewhere at the exchange or port swap, it might give you back what you've lost, it might be worse.

Regards

Phil
« Last Edit: March 13, 2020, 01:45:49 PM by PhilipD »
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Weaver

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Re: Line 2 downstream slow
« Reply #8 on: March 14, 2020, 01:58:31 AM »

The answer: I swapped out the modem and got all the speed back. So why would a modem suddenly become slow ?
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PhilipD

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Re: Line 2 downstream slow
« Reply #9 on: March 14, 2020, 10:02:27 AM »

Hi

If you put that other modem back and resync again is it constantly down by the same amount or does it go back up? Perhaps you have just regained your position in the cross-talk rankings.

You also need to remember these are cheap consumer devices, they make many measurements to decide on sync-rate etc, yet aren't scientifically calibrated and no two are going to operate exactly the same.  Rounding plus rounding errors in analogue to digital converters and software on the chips can result in the very small differences in tolerances of components ending up being noticeable in the end result and two seemingly identical modems behaving differently.  Why did it change after being off, could be any number or reasons, temperature of components at the time of resync, difference in noise profile on the line and this modem due to different tolerances is slightly more conservative or noisy internally than another one.  As it cooled down a decoupling ceramic capacitor could have cracked causing more noise or an electrolytic capacitor is close to degrading and the cooling then warming cycle due to being turned off pushed it further towards a fault condition. Even dust and the moisture levels on the PCB can change it's characteristics.

This other modem might have always been able to sync ~500K higher.

You also would need to do several tests with each modem to know for sure if one is consistently syncing higher, there is always an element of variably in sync speed on each resync.  I've been there before, tried a different modem, surprised at how much better it was, set it all up properly to use permanently, and by the next day it had resync lower and ended up being worse, I put the original one back.

The short answer, no one can tell you why it is a bit slower with any absolute certainty.

Regards

Phil
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Weaver

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Re: Line 2 downstream slow
« Reply #10 on: March 14, 2020, 11:44:29 AM »

Good point about electrical internals. Some of these devices are second hand and I don’t know how old they are.

The change in d/s sync rate is quite a significant one - 2.4Mbps down from 2.9Mbps is 17% for example. It never happens in the normal way of things.
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