50M : Post RS Errors

[Sephiroth]

Yes - an unnecessary attenuator or splitter does remove components that can break down and cause noise, though your SNR is just about tolerable and not the standard that VM generally want to see, IMO.

[Jon22]

Originally Posted by Sephiroth

Yes - an unnecessary attenuator or splitter does remove components that can break down and cause noise, though your SNR is just about tolerable and not the standard that VM generally want to see, IMO.

Just been reading your power level and snr sticky thread on the VM forum. If the SNR is above 32dB I'm guessing VM won't do anything? Hence the technician saying everything was ok? Not a great deal I can do I suppose other than monitor it.

[Sephiroth]

The VM Forum Team usually send an engineer in when it's down at the 33 dBmv mark, which they did. Above 34.5 they'll say it's within tolerance especially if you have no post-RS errors.

Post-RS errors must not happen (except during the first part of modem reset when there is no Sync). VM may respond that 1% is tolerable, but that shouldn't be accepted and must be pushed back.

If VM wanted to move to 1024QAM for higher speeds then 34 would be no good. So they don't want low SNR.

[Jon22]

Seems to be sorted, not had any post RS errors since last Thursday.

DS-1 DS-2 DS-3 DS-4 DS-5 DS-6 DS-7 DS-8
Frequency (Hz) 299000000 267000000 275000000 283000000 291000000 307000000 315000000 323000000
Lock Status(QAM Lock/FEC Sync/MPEG Lock) Locked Locked Locked Locked Locked Locked Locked Locked
Channel ID 5 1 2 3 4 6 7 8
Modulation 256QAM 256QAM 256QAM 256QAM 256QAM 256QAM 256QAM 256QAM
Symbol Rate (Msym/sec) 6.952000 6.952000 6.952000 6.952000 6.952000 6.952000 6.952000 6.952000
Interleave Depth I=12
J=17 I=12
J=17 I=12
J=17 I=12
J=17 I=12
J=17 I=12
J=17 I=12
J=17 I=12
J=17
Power Level (dBmV) 3.94 3.74 3.72 3.97 4.07 4.31 4.23 4.43
RxMER (dB) 34.48 34.63 34.35 34.35 34.63 34.77 34.77 34.48
Pre RS Errors 132852 115394 182001 150531 65819 34871 41659 84211
Post RS Errors 0 0 0 0 0 0 0 0

[alanbjames]

i have the same issue, i had a SNR issue which made my SNR drop to 31dBmv across all channels.

Now its between 37-38dBmv across the board but still Pre RS Errors continue.

[Sephiroth]

Pre-RS errors are nothing to worry about because the data is corrected in flight.

[Eeeps]

Just a note that MER (Modulation Error Ratio) as a measure includes more than just plan SNR (Signal to Noise Ratio).

MER will also reduce for other signal path problems such as amp linearity, filter phase distortions, laser clipping etc.

As Seph indicates this figure is very much borderline. Given the cold weather I wouldn't be surprised if you start to have issues as temperatures rise.

Ian

[Ignitionnet]

As I understand it both MER and SNR take account of all signal issues by the time things arrive at the modem, they're just calculated differently. MER is based on the distance each decoded QAM symbol comes out at from perfect centre of that square in the constellation. SNR is a simpler measure of the power of the noise floor versus the wanted signal.

MER can only be done on digital signals - it is a measure of perfect demodulation versus the actual demodulation.

[Jon22]

Getting post RS errors again.

Downstream
DS-1 DS-2 DS-3 DS-4 DS-5 DS-6 DS-7 DS-8
Frequency (Hz) 299000000 267000000 275000000 283000000 291000000 307000000 315000000 323000000
Lock Status(QAM Lock/FEC Sync/MPEG Lock) Locked Locked Locked Locked Locked Locked Locked Locked
Channel ID 13 9 10 11 12 14 15 16
Modulation 256QAM 256QAM 256QAM 256QAM 256QAM 256QAM 256QAM 256QAM
Symbol Rate (Msym/sec) 6.952000 6.952000 6.952000 6.952000 6.952000 6.952000 6.952000 6.952000
Interleave Depth I=12
J=17 I=12
J=17 I=12
J=17 I=12
J=17 I=12
J=17 I=12
J=17 I=12
J=17 I=12
J=17
Power Level (dBmV) 2.81 2.81 2.75 2.94 3.00 3.23 3.26 3.49
RxMER (dB) 32.41 27.97 28.87 29.71 30.95 33.60 34.63 34.63
Pre RS Errors 58368 31994881 16094512 5805880 743797 7286 4690 4686
Post RS Errors 865 1306 931 879 704 753 879 981

The SNR is low on some of the channels, going to guess that something is failing. Weirdly, everything still seems to be working fine for now.

[qasdfdsaq]

Looks like a lot of noise on the line. That'll cause low SNR and errors, but it's VM's problem to deal with.

That SNR, especially on the middle channels, is definitely low enough to cause issues in itself.

[Jon22]

Originally Posted by qasdfdsaq

Looks like a lot of noise on the line. That'll cause low SNR and errors, but it's VM's problem to deal with.

That SNR, especially on the middle channels, is definitely low enough to cause issues in itself.

Is that something that'll be affecting multiple users? Would VM be able to see/monitor it and pass it on to the network engineers?

[Kushan]

Originally Posted by Jon22

Is that something that'll be affecting multiple users? Would VM be able to see/monitor it and pass it on to the network engineers?

Give them a ring and find out.

[qasdfdsaq]

Originally Posted by Jon22

Is that something that'll be affecting multiple users? Would VM be able to see/monitor it and pass it on to the network engineers?

Could do. It all depends on the source of the noise. Could be a dodgy amp, could be something the neighbours plugged in that they shouldn't have, could be a dangling wire somewhere.

[Jon22]

Thanks Kushan and qasdfdsaq. I've bumped the thread I had going on the VM forum. Have to wait and see what they say.

[Kushan]

Originally Posted by Jon22

Thanks Kushan and qasdfdsaq. I've bumped the thread I had going on the VM forum. Have to wait and see what they say.

You would be much better off calling them, they can check for issues there and then, as well as book an engineer if need be.