Low SNR downstream (@2180mhz?) 100mbps vs 1gbps :(

[soylent]

HI everyone, 

I've had super-stable service for 3yrs now, but in the last 10 days service has become super unstable with download speeds fluctuating from a normal of 800-900mbps to lows of 100-200mbps, multiple times a day. Upload speeds are fine (110-120mbps) and latency is great (10-14ms)

I've had two engineer visits, where they moved the tap (which increased power levels from c.0-1db to c.4-6db (I have a 6gb attenuator fitted)), and they checked for ingress by re-terminating the cable and replaced the isolator.

Looking at the stats, it looks like i have an issue on 3 channels at 2100-2260mhz with 500 million+ Post-RS errors?

3.0 Downstream channels

Channel Frequency (Hz) Power (dBmV) SNR (dB) Modulation Channel ID

1	330000000	6.5	36	QAM 256	25
2	210000000	5.9	27	QAM 256	10
3	218000000	5.8	27	QAM 256	11
4	226000000	5.6	28	QAM 256	12
5	234000000	5.4	28	QAM 256	13
6	242000000	5.2	29	QAM 256	14
7	250000000	4.7	29	QAM 256	15
8	258000000	4.5	30	QAM 256	16
9	266000000	4.8	31	QAM 256	17
10	274000000	5.2	32	QAM 256	18
11	282000000	5.2	33	QAM 256	19
12	290000000	5.2	34	QAM 256	20
13	298000000	5.8	34	QAM 256	21
14	306000000	6.5	35	QAM 256	22
15	314000000	6.5	35	QAM 256	23
16	322000000	6.5	35	QAM 256	24
17	338000000	6.7	36	QAM 256	26
18	346000000	6.7	36	QAM 256	27
19	354000000	6.6	36	QAM 256	28
20	362000000	6.6	36	QAM 256	29
21	370000000	6.5	36	QAM 256	30
22	378000000	6.4	36	QAM 256	31
23	386000000	6.3	36	QAM 256	32
24	394000000	6.1	37	QAM 256	33
25	402000000	5.7	36	QAM 256	34
26	410000000	5.5	36	QAM 256	35
27	418000000	5.5	36	QAM 256	36
28	522000000	5.9	37	QAM 256	37
29	530000000	6	37	QAM 256	38
30	538000000	6	37	QAM 256	39
31	546000000	6	37	QAM 256	40

3.0 Downstream channels

Channel Locked Status RxMER (dB) Pre RS Errors Post RS Errors

1	Locked	36	3648918	0
2	Locked	27	2893225144	573150937
3	Locked	27	3111544092	59228791
4	Locked	28	2139420914	1477331
5	Locked	28	462717082	16647
6	Locked	29	2725584720	42
7	Locked	29	1672364275	3
8	Locked	30	803050519	1
9	Locked	31	161034016	0
10	Locked	32	31494651	0
11	Locked	33	15852872	0
12	Locked	34	8961512	0
13	Locked	34	6323775	0
14	Locked	35	5191454	0
15	Locked	35	4642475	0
16	Locked	35	4125783	0
17	Locked	36	3228336	0
18	Locked	36	2830903	0
19	Locked	36	2900566	0
20	Locked	36	3251102	0
21	Locked	36	3616827	0
22	Locked	36	4025770	0
23	Locked	36	4596840	0
24	Locked	37	5662513	0
25	Locked	36	8394624	0
26	Locked	36	9973161	0
27	Locked	36	9048652	0
28	Locked	37	8715626	0
29	Locked	37	9023826	0
30	Locked	37	8797834	0
31	Locked	37	9649771	0

3.1 Downstream channels

Channel Channel Width (MHz) FFT Type Number of Active Subcarriers Modulation (Active Profile) First Active Subcarrier (Hz)

159

92

4K

1800

QAM 4096

1128

3.1 Downstream channels

Channel ID Locked Status RxMER Data (dB) PLC Power (dBmV) Corrected errors (Active Profile) Uncorrectable errors (Active Profile)

159

Locked

38

6.5

2982152492

0

3.0 Upstream channels

Channel Frequency (Hz) Power (dBmV) Symbol Rate (ksps) Modulation Channel ID

0	49600000	42.8	5120	QAM 64	9
1	43100000	42.5	5120	QAM 64	10
2	36600000	42.5	5120	QAM 64	11
3	30100000	42.3	5120	QAM 64	12
4	23600000	42	5120	QAM 64	13

3.0 Upstream channels

Channel Channel Type T1 Timeouts T2 Timeouts T3 Timeouts T4 Timeouts

0	ATDMA	0	0	0	0
1	ATDMA	0	0	0	0
2	ATDMA	0	0	0	0
3	ATDMA	0	0	2	0
4	ATDMA	0	0	0	0

3.1 Upstream channels

Channel Channel Width (MHz) Power (dBmV) FFT Type Modulation

14

10

36.7

2K

QAM 128

3.1 Upstream channels

Channel Channel Type Number of Active Subcarriers First Active Subcarrier (Hz) T3 Timeouts T4 Timeouts

14

OFDMA

200

74000000

15

1

[Client62]

I see the burst of PostRS errors, the real question is that still ramping up i.e. the fault continues. Or is it a past event that is over and now stable, E.g. some cable work in the street cabinet or the dog from next door has finished chewing on your coax cables ?

[soylent]

thanks - fault continues, but at stable rate - increments by a stable c.100k errors an hour

[soylent]

here's the time plot of download vs upload. until 10days ago, used to be reliably 900mbs down, 100+ up

[Tudor]

I would say that there are 9 channels that have the SNR too low. You definitely need a technician again, I suspect either a faulty hub or cabinet amplifier. 

[soylent]

Tudor, thanks for the quick reply!

In another thread, you also commented that there's potential interference from DAB at 2180mhz? do you think that's relevant here?

Should I wait for the virgin team to reply here? Hard to give Virgin troubleshooting info over the phone as the generic support scripts goes until it ends in: "send engineer"

[Reece_MH]

Hi soylent 👋

Thanks for posting, and welcome to the Forums.

I'm glad to see that some of our Community have been helping you in trying to diagnose/resolve these issues. I've taken a look on our side, and these issues would need another Technician to come out, and they potentially refer this to our Networks Team, depending on any issues they identify.

I can, however, see that you have been able to book a Technician directly. Please do let us know if you need anything else, or how things are after the visit, and we'll be more than happy to assist.

Thanks,

[soylent]

Thanks Reece, and everyone else. Will update after the technician visit

[soylent]

Technician visited, tried changing the hub, re-terminated the cable at both ends (my premises and at cabinet), removed the attenuator, but this has totally killed three channels (see below). He said it's impossible it's the cabinet amplifier, so he's recommended pulling new cable. That's a big/expensive job - is that right, or is there another thing to diagnose/troubleshoot first?

3.0 Downstream channels

Channel Frequency (Hz) Power (dBmV) SNR (dB) Modulation Channel ID

1	330000000	7.9	35	QAM 256	25
2	194000000	7.4	0	QAM 256	8
3	202000000	7.1	0	QAM 256	9
4	210000000	7.2	0	QAM 256	10
5	218000000	7.1	27	QAM 256	11
6	226000000	6.9	28	QAM 256	12
7	234000000	6.7	28	QAM 256	13
8	242000000	6.6	29	QAM 256	14
9	250000000	6.2	29	QAM 256	15
10	258000000	6	30	QAM 256	16
11	266000000	6.2	31	QAM 256	17
12	274000000	6.5	32	QAM 256	18
13	282000000	6.4	33	QAM 256	19
14	290000000	6.4	34	QAM 256	20
15	298000000	7	34	QAM 256	21
16	306000000	7.9	34	QAM 256	22
17	314000000	8	35	QAM 256	23
18	322000000	7.9	35	QAM 256	24
19	338000000	8	36	QAM 256	26
20	346000000	8.1	36	QAM 256	27
21	354000000	8	36	QAM 256	28
22	362000000	7.9	36	QAM 256	29
23	370000000	7.9	36	QAM 256	30
24	378000000	7.8	36	QAM 256	31

3.0 Downstream channels

Channel Locked Status RxMER (dB) Pre RS Errors Post RS Errors

1	Locked	35	5327328	0
2	Locked	0	5630286	11328015
3	Locked	0	282032	422099
4	Locked	0	2716875742	509644010
5	Locked	27	3636260409	129466068
6	Locked	28	3444471585	6922012
7	Locked	28	2775257723	181792
8	Locked	29	1752962663	1050
9	Locked	29	1106187315	21
10	Locked	30	521262929	5
11	Locked	31	157683122	4
12	Locked	32	48762184	0
13	Locked	33	24083842	0
14	Locked	34	13306384	0
15	Locked	34	9082072	0
16	Locked	34	7391460	0
17	Locked	35	6643464	0
18	Locked	35	5981438	0
19	Locked	36	4761059	0
20	Locked	36	4241844	0
21	Locked	36	4364298	0
22	Locked	36	4960314	0
23	Locked	36	5525451	0
24	Locked	36	6110791	0

3.1 Downstream channels

Channel Channel Width (MHz) FFT Type Number of Active Subcarriers Modulation (Active Profile) First Active Subcarrier (Hz)

159

92

4K

1800

QAM 4096

1128

3.1 Downstream channels

Channel ID Locked Status RxMER Data (dB) PLC Power (dBmV) Corrected errors (Active Profile) Uncorrectable errors (Active Profile)

159

Locked

38

8.1

1373359823

0

[John_GS]

Hi @soylent 

Thanks for the update. The tech has noted it as a damaged cable so will be booking in the repull with the relevant team. I've reached out to the field manager for an update on this for you and will be in touch when I know more.

Best wishes.