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Downstream SNRM Drops

#1
Hi guys,

Every so often, my line's downstream SNRM drops by between ~1 - 2 dB for a few minutes and then resumes back to normal. Any ideas what could be causing this? I'm unaware of anything in the house that turns on and off in correspondence with the drops. I'm almost certain that it's not being caused by a physical line fault, because the upstream SNRM isn't affected.

Thanks,

Will
 

Mark.J

Administrator
Staff member
ISPreview Team
#2
SNR does move as interference can be caused by all sorts of things, but the real question is does it affect your performance (service speed) or connection stability?

Some context might help too, such as what connection you have and a paste of your router stats?

I'd also be interested to know if the drop corresponds to high data load on the broadband connection or your local network, such as if somebody is doing a big transfer at the same time or lots of users/devices are active. Make sure to keep your router well away from other potential sources of interference too.
 
#3
It normally doesn't, but CRC errors can occur, sometimes causing the connection to drop, but not normally. I have a VDSL2 connection. I don't think it's related to do with network load, as it happens at night, too.

Unfortunately, moving the router away from other electrical sources isn't feasible, as the master socket is next to them.

Here's a copy of the line stats, as you can see there are large amounts of FECs, but turning off everything electrical and leaving the router on doesn't reduce these much if at all:
Code:
adsl info --stats
adsl: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason:    0
Last initialization procedure status:   0
Max:    Upstream rate = 7034 Kbps, Downstream rate = 56187 Kbps
Bearer: 0, Upstream rate = 7297 Kbps, Downstream rate = 54997 Kbps
Bearer: 1, Upstream rate = 0 Kbps, Downstream rate = 0 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):        3.3             6.4
Attn(dB):        26.4            0.0
Pwr(dBm):        12.0            4.8

                        VDSL2 framing
                        Bearer 0
MSGc:           -6              26
B:              227             223
M:              1               1
T:              0               40
R:              10              0
S:              0.1319          0.9750
L:              14440           1838
D:              4               1
I:              238             112
N:              238             224
Q:              4               0
V:              0               0
RxQueue:                130             0
TxQueue:                26              0
G.INP Framing:          18              0
G.INP lookback:         26              0
RRC bits:               0               24
                        Bearer 1
MSGc:           122             -6
B:              0               0
M:              2               0
T:              2               0
R:              16              0
S:              8.0000          0.0000
L:              32              0
D:              1               0
I:              32              0
N:              32              0
Q:              0               0
V:              0               0
RxQueue:                0               0
TxQueue:                0               0
G.INP Framing:          0               0
G.INP lookback:         0               0
RRC bits:               0               0

                        Counters
                        Bearer 0
OHF:            0               1463070
OHFErr:         0               21
RS:             430819380               58491316
RSCorr:         3561000         0
RSUnCorr:       0               0
                        Bearer 1
OHF:            887657          0
OHFErr:         0               0
RS:             7100762         0
RSCorr:         3593            0
RSUnCorr:       0               0

                        Retransmit Counters
rtx_tx:         35291           0
rtx_c:          35190           0
rtx_uc:         0               0

                        G.INP Counters
LEFTRS:         0               0
minEFTR:        54967           0
errFreeBits:    11954029                0

                        Bearer 0
HEC:            0               0
OCD:            0               0
LCD:            0               0
Total Cells:    1507479103              0
Data Cells:     7950795         0
Drop Cells:     0
Bit Errors:     0               0

                        Bearer 1
HEC:            0               0
OCD:            0               0
LCD:            0               0
Total Cells:    0               0
Data Cells:     0               0
Drop Cells:     0
Bit Errors:     0               0

ES:             0               19
SES:            0               0
UAS:            126             126
AS:             14261

                        Bearer 0
INP:            54.00           0.00
INPRein:        1.00            0.00
delay:          0               0
PER:            0.00            9.78
OR:             0.01            26.15
AgR:            55117.80        7323.39

                        Bearer 1
INP:            2.00            0.00
INPRein:        2.00            0.00
delay:          0               0
PER:            16.06           0.01
OR:             63.75           0.01
AgR:            63.75   0.01

Bitswap:        6521/7221               12/12

Total time = 3 hours 59 min 47 sec
FEC:            3561000         0
CRC:            0               21
ES:             0               19
SES:            0               0
UAS:            126             126
LOS:            0               0
LOF:            0               0
LOM:            0               0
Retr:           0
FailedRetr:     0
FailedFastRetr: 0
Latest 15 minutes time = 14 min 47 sec
FEC:            401766          0
CRC:            0               3
ES:             0               3
SES:            0               0
UAS:            0               0
LOS:            0               0
LOF:            0               0
LOM:            0               0
Retr:           0
FailedRetr:     0
FailedFastRetr: 0
Previous 15 minutes time = 15 min 0 sec
FEC:            698827          0
CRC:            0               3
ES:             0               3
SES:            0               0
UAS:            0               0
LOS:            0               0
LOF:            0               0
LOM:            0               0
Retr:           N/A
FailedRetr:     N/A
FailedFastRetr: N/A
Latest 1 day time = 3 hours 59 min 47 sec
FEC:            3561000         0
CRC:            0               21
ES:             0               19
SES:            0               0
UAS:            126             126
LOS:            0               0
LOF:            0               0
LOM:            0               0
Retr:           0
FailedRetr:     0
FailedFastRetr: 0
Previous 1 day time = 0 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
Retr:           0
FailedRetr:     0
FailedFastRetr: 0
Since Link time = 3 hours 57 min 39 sec
FEC:            3561000         0
CRC:            0               21
ES:             0               19
SES:            0               0
UAS:            0               0
LOS:            0               0
LOF:            0               0
LOM:            0               0
Retr:           0
FailedRetr:     0
FailedFastRetr: 0
NTR: mipsCntAtNtr=0 ncoCntAtNtr=0
>
Oh, and just remembered you're based in Dorset, not far away from me near sunny Sidmouth, Devon! :)
 
Last edited:
#4
Bit of a mess: 1542730213226.png
Code:
adsl info --stats
adsl: ADSL driver and PHY status
Status: Showtime
Last Retrain Reason:    0
Last initialization procedure status:   0
Max:    Upstream rate = 7015 Kbps, Downstream rate = 55592 Kbps
Bearer: 0, Upstream rate = 7297 Kbps, Downstream rate = 54997 Kbps
Bearer: 1, Upstream rate = 0 Kbps, Downstream rate = 0 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):        3.1             6.3
Attn(dB):        26.4            0.0
Pwr(dBm):        12.0            4.8

                        VDSL2 framing
                        Bearer 0
MSGc:           -6              26
B:              227             223
M:              1               1
T:              0               40
R:              10              0
S:              0.1319          0.9750
L:              14440           1838
D:              4               1
I:              238             112
N:              238             224
Q:              4               0
V:              0               0
RxQueue:                130             0
TxQueue:                26              0
G.INP Framing:          18              0
G.INP lookback:         26              0
RRC bits:               0               24
                        Bearer 1
MSGc:           122             -6
B:              0               0
M:              2               0
T:              2               0
R:              16              0
S:              8.0000          0.0000
L:              32              0
D:              1               0
I:              32              0
N:              32              0
Q:              0               0
V:              0               0
RxQueue:                0               0
TxQueue:                0               0
G.INP Framing:          0               0
G.INP lookback:         0               0
RRC bits:               0               0

                        Counters
                        Bearer 0
OHF:            0               10157016
OHFErr:         68              532
RS:             2990866172              406062054
RSCorr:         54102839                0
RSUnCorr:       0               0
                        Bearer 1
OHF:            6161992         0
OHFErr:         0               0
RS:             49295439                0
RSCorr:         22169           0
RSUnCorr:       0               0

                        Retransmit Counters
rtx_tx:         398409          0
rtx_c:          343633          0
rtx_uc:         89              0

                        G.INP Counters
LEFTRS:         51              0
minEFTR:        54967           0
errFreeBits:    82966891                0

                        Bearer 0
HEC:            0               0
OCD:            0               0
LCD:            0               0
Total Cells:    1872621845              0
Data Cells:     180812636               0
Drop Cells:     0
Bit Errors:     0               0

                        Bearer 1
HEC:            0               0
OCD:            0               0
LCD:            0               0
Total Cells:    0               0
Data Cells:     0               0
Drop Cells:     0
Bit Errors:     0               0

ES:             12              496
SES:            8               0
UAS:            126             126
AS:             98993

                        Bearer 0
INP:            54.00           0.00
INPRein:        1.00            0.00
delay:          0               0
PER:            0.00            9.78
OR:             0.01            26.15
AgR:            55117.80        7323.39

                        Bearer 1
INP:            2.00            0.00
INPRein:        2.00            0.00
delay:          0               0
PER:            16.06           0.01
OR:             63.75           0.01
AgR:            63.75   0.01

Bitswap:        48713/52952             120/122

Total time = 1 days 3 hours 31 min 59 sec
FEC:            54102839                0
CRC:            68              532
ES:             12              496
SES:            8               0
UAS:            126             126
LOS:            0               0
LOF:            0               0
LOM:            0               0
Retr:           0
FailedRetr:     0
FailedFastRetr: 0
Latest 15 minutes time = 1 min 59 sec
FEC:            15621           0
CRC:            0               1
ES:             0               1
SES:            0               0
UAS:            0               0
LOS:            0               0
LOF:            0               0
LOM:            0               0
Retr:           0
FailedRetr:     0
FailedFastRetr: 0
Previous 15 minutes time = 15 min 0 sec
FEC:            245322          0
CRC:            0               13
ES:             0               13
SES:            0               0
UAS:            0               0
LOS:            0               0
LOF:            0               0
LOM:            0               0
Retr:           N/A
FailedRetr:     N/A
FailedFastRetr: N/A
Latest 1 day time = 3 hours 31 min 59 sec
FEC:            3278493         0
CRC:            0               44
ES:             0               43
SES:            0               0
UAS:            0               0
LOS:            0               0
LOF:            0               0
LOM:            0               0
Retr:           0
FailedRetr:     0
FailedFastRetr: 0
Previous 1 day time = 24 hours 0 sec
FEC:            50824346                0
CRC:            68              488
ES:             12              453
SES:            8               0
UAS:            126             126
LOS:            0               0
LOF:            0               0
LOM:            0               0
Retr:           0
FailedRetr:     0
FailedFastRetr: 0
Since Link time = 1 days 3 hours 29 min 51 sec
FEC:            54102839                0
CRC:            68              532
ES:             12              496
SES:            8               0
UAS:            0               0
LOS:            0               0
LOF:            0               0
LOM:            0               0
Retr:           0
FailedRetr:     0
FailedFastRetr: 0
NTR: mipsCntAtNtr=0 ncoCntAtNtr=0
 >
 

Mark.J

Administrator
Staff member
ISPreview Team
#5
Please excuse the late reply, I've got quite a lot of work to get through ATM. The stats don't look right to me, you should be getting better upstream speeds than that but oddly your downstream SNR is way lower than upstream.

I think I'd start with the basics and first look for a possible problem in your local environment.

How much cable is there between your router and the master socket? What kind of master socket is it? Have you made sure you're using a filter on every phone extension? If you have an older NTE5 master socket then it's possible it may not be filtering out interference from the ring/bell wire and that won't help (in this case you could try plugging into the test socket behind the faceplate and see what happens after a week or two).

Also do you have a backup router that you can test with as different chipsets can be better in certain scenarios.
 
#6
No worries, Mark! Downstream SNR is lower because the target SNRM is 3 dB. Upstream has always been low, I agree it should be higher. My line is mostly overhead and ~1000 m long, but because of vectoring on the downstream, I can achieve very high rates on it. There's a 2m twisted pair CW1308 cable going to a master socket 5c with a MK4 faceplate. All extensions are disconnected, have been for years, and there's no bell wire due to new socket. The backup router (BT Smart Hub) has similar stats (can't monitor the line on that/access in-depth stats on it).
 

Mark.J

Administrator
Staff member
ISPreview Team
#7
Ah you're on one of those rare VDSL2 lines with Vectoring enabled? Ok I've not personally had enough direct experience with those and by the sounds of it your setup should be pretty solid.

At this point I'd probably be hunting out my old AM radio and seeing if it picks up any interference above the normal. This old Zen video is a good one but I think they're only reflecting ADSL (612Khz setting) but that's still a good place to start (VDSL2 goes up to 17MHz).

 
#8
Yes, very lucky! Right, so I simply need to tune a radio between 612KHz - 17MHz? Pretty sure it's not in my property as I turned everything off and the FECs still occur, and I'm not aware of anything that turns on and off at the times the SNRM drops.

Could it simply be because most of my line is overhead so it picks up a lot more noise than buried lines? Just wondering if a lot of it could be being caused by the nearby radio transmitter in Exeter.

I've noticed that at times in the past the FECs have stopped for long periods, so I think it's something external. Trying to track it down would be very difficult. Maybe I may be trying to fix something that I can't? At least it isn't too service affecting.
 
Last edited:

Mark.J

Administrator
Staff member
ISPreview Team
#9
The stats could point to something more local to your property, but I certainly wouldn't rule out an external problem. Overhead lines can be particularly problematic as they may be affected by birds, tree branches (touching the line) and general erosion or water ingress around the joints.

On the radio, I'd first try tuning AM to 612KHz and remember, the interference doesn't have to be coming from within your own home. We've seen cases where several houses on a street were affected because one person had a faulty power adapter.

However the issue is highly unlikely to be caused by the nearby radio transmitter. DSL is designed to work alongside such signals so they're rarely a problem for it.
 
#10
Yes, we definitely have trees touching the line, but I would think the problem would show on the upstream too, if that was the case. I'll keep a monitor, but whatever's causing the issue doesn't correspond with anything in the house, so it's a mystery.
 

Mark.J

Administrator
Staff member
ISPreview Team
#11
Yes that has me confused too, but then it's possible that Vectoring might somehow be changing the normally expected behaviour a bit. But I've not had enough direct experience of how it changes the stats to know for sure.

Have you raised this issue with your ISP at all?
 
#12
Yes, it does make me wonder if it is vectoring. When G.INP is off, the issue doesn't occur, which is weird. I could do, but they may flog me off saying it isn't service affecting/your speed is acceptable etc.
 
#14
Tree branches touching the line is not good. I had to get BT to replace my line twice due to micro fractures caused by branch strikes. They do cause some very strange error patterns, especially if the wind is blowing strongly enough, and from the right direction to cause the branch to rub/hit the already damaged cable.
As here, my upload speed never seemed to be a problem, only download speeds.
 
#15
Hmm, interesting. I've just read that line faults normally effect the upstream (especially high resistance ones), as well. Was your line noisy? Did you notice errors occur on the upstream, too? The part of our cable that was previously ripped from tree branches was replaced nearly 4 years ago, would that make a difference?

One thing I have noticed is that the SNRM is normally more stable/higher when it's wet. Would that suggest that a joint is becoming oxidised leading to a slightly better connection?
 
#20
Hmm, interesting. I've just read that line faults normally effect the upstream (especially high resistance ones), as well. Was your line noisy? Did you notice errors occur on the upstream, too? The part of our cable that was previously ripped from tree branches was replaced nearly 4 years ago, would that make a difference?

One thing I have noticed is that the SNRM is normally more stable/higher when it's wet. Would that suggest that a joint is becoming oxidised leading to a slightly better connection?

Sorry, really bad head cold (compromised immune system), so I havent logged in.

For normal phone use there was no apparent noise; however there were a LOT of CRC errors cropping up when it was windy.
More stable when rainy could suggest the outer insulation is breached and water getting in - it being a conductor.
Short term it might help the signal, but long term....

I understand micro-fractures can be tested for remotely nowadays, back 10 years and they lugged in a box to plug into my test socket.
guy said there were so many, they couldnt be counted - and suggested the number was in the thousands at least.
 
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