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[frogfoot]

Hi,
Just a quick post to ask the learned fellows on this forum to comment on this phenomenon which has been occurring regularly on my FTTC line.
Personally I put it down to a firmware bug in the Billion 8800axl.  I don't see how the total SNR can have a single spike when the individual SNR values can have a step change. 

[kitz]

I have seen similar on my own line in the past.  It looks like youve had a noise burst.  The separation is something to do with bitswap and gain.   There's usually a bit of tx power left in reserve for the bitswap process, which can increase gain to raise the SNR a little bit sufficient to cover the bit loading.

When one of the subchannels has a reduction in SNR then it's not unusual for one of the other subchannels to see an increase.  The increase in U2 never seems to be as much as the decrease in U0 or U1 though.*  When I had a line fault last year they would even cross over each other and then back again.

The first graph is the mean average of all the tones across all three subchannels. The second graph is the mean average for each of the subchannels. 

*U2 carries far more tones (@810) than U1 (@334) than U0 (@25), so because the mean average is used, then it will take less tones in U0 or U1 than in U2 to skew the difference.   I know what Im trying to say, but its late and Im not doing a very good job of explaining it, other than say it would only take a couple of tones to be below par in U0 to make it show a greater difference than it would for the corresponding increase same number of tones in U2 to affect the mean average.. which will in turn correct the total mean average.

ahhhhh.. sorry can some one explain it better re the mean averages and why it graphs like that, yet still be correct?   

[frogfoot]

Thanks Kitz, actually that makes perfect sense, if U2 has more influence than the others then a small increase in U2 can offset a much larger decrease in the others. I still need to track down the cause of the impulse noise if I can as they can be a couple of dB or more, they are not causing any problems at the moment, but as more people are added to the cab I get closer and closer to 6dB, then the spikes may really be something to worry about.

[plexy]

I get nearly those exact graphs when my phone goes off hook, or a I receive an incoming call. Only the upstream SNR changes, not the DS. The overall US spikes down, sometimes by 1dB, other times by 5.5dB. The 'per band' US SNR graph shows some bands going lower than others.

Ive also had for about half year now, changes in the US SNRM - some days its 6.5, other days its 15. It stays at one for a few days, then switches to the other and vice versa. 

Try a 17070 option 1 ring back test and see if you get the same downward trend when line is ringing in/answered. If you do, then welcome to the club If not, then ignore me!

edit, like so;

[frogfoot]

Add me to the club, taking the phone off the hook, or an incoming call seems to be what is causing some of the spikes.  Is this likely to be a filter problem, I have and Openreach V2 filtered faceplate. I'm wondering if a V3 with its slightly better filtering (allegedly) may help matters.

[tommy45]

I get nearly those exact graphs when my phone goes off hook, or a I receive an incoming call. Only the upstream SNR changes, not the DS. The overall US spikes down, sometimes by 1dB, other times by 5.5dB. The 'per band' US SNR graph shows some bands going lower than others.

Ive also had for about half year now, changes in the US SNRM - some days its 6.5, other days its 15. It stays at one for a few days, then switches to the other and vice versa. 

Try a 17070 option 1 ring back test and see if you get the same downward trend when line is ringing in/answered. If you do, then welcome to the club If not, then ignore me!

edit, like so;

There shouldn't be any interaction between PSTN and Xdsl the fact that there is could be a poor contact somewhere  even a failing NTE master socket

[frogfoot]

It's fairly easy to rule out the internal wiring etc. but then you have the NTE socket, wiring to the Street Cabinet, all the way back to the exchange as possible sources of problems.  I imagine phoning up BT and saying 'I get a slight decrease in SNR when the phone rings' is not going to get an engineer rushing to check the line out. It may be indicative of a problem but until it gets much worse I can't see there is anything that can be done other than change the faceplate filter.

[burakkucat]

The classic physical defect in the metallic pathway which can give rise to such a phenomenon is a HR or semiconducting joint.

[ip75]

I have something very similar with my line, except in my case the SNRM drops immediately *after* the call has ended, and climbs back to where it was over the next 30 seconds or so. I can't really imagine why it behaves like that - most descriptions of this phenomenon have the drop happening during the call, or even when the phone rings.

[burakkucat]

That is, indeed, rather peculiar. At the moment I am unable to think of an underlying issue that fits your observation.