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Author Topic: Is this a problem ?  (Read 1026 times)


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Is this a problem ?
« on: June 26, 2019, 11:10:05 AM »

my FTTC (VDSL) connection on the upload, had a funny yesterday morning....   the SNR margin on U2  ( I have a ZyXEL VMG3925-B10B )  shows the signal went down to around 6db ( around 8:55am  then rose abovce the previous normal of just over 9db and has been sitting up at around 11db.  ( see pixs )

And now, since early this morning ( 26th,.  the other two are changing. )  Is this something I need to worry about ?

my line is capable of over the 40/10 service I subscribe to ( from post office broadband here in UK ) and is still showing the same capped speeds.

thoughts by experts please ?



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Re: Is this a problem ?
« Reply #1 on: June 26, 2019, 07:04:34 PM »

Hmm . . . Whenever I see "peculiar" behaviour of an xDSL circuit (and the source of the peculiarity is not obvious), I advise powering down the modem, disconnecting it from the incoming line and allowing the DSLAM's line-card port "time to relax". Overnight would be a good period.

Something odd has/is occurred/occurring with your circuit but nothing else can be deduced from the plots that you have shown.
« Last Edit: July 30, 2019, 08:58:38 PM by burakkucat »
:cat:  100% Linux and, previously, Unix. Co-founder of the ELRepo Project.

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Re: Is this a problem ?
« Reply #2 on: June 29, 2019, 05:09:03 AM »

It looks like its just power management coming into play.  It's not unusual if for some reason the overall SNR/m changes, then power management can increase the gain in a channel to maintain connectivity by ensuring that there is sufficient total SNR to support the overall bit-load.

Increasing the gain is a normal part of the bitswap process which has an inbuilt allowance for these type of circumstances. 
Adjusting the power level in U2 to maintain the overall bit rate is quite logical, as they are the tones most likely to be marked out of service due to bitswap because they are higher frequency and more susceptible to noise.  Although it should be said that the process can ramp up the power in whatever channel it is needed most as long as it doesn't exceed the overall maximum power level.

If you look at the 2nd graph which is more magnified, you can quite clearly see that at around 08:50ish SNRM dipped across all channels.  Very shortly after SNRM rose in U2 indicating that either there was a miraculous recovery in U2 only or that the power was ramped up for U2.  The latter is the most likely in your case because it rose above its normal level.

In circumstances such as the above, unless it's service affecting and/or you are getting lots of errors it's best just letting the modem do it's thing.  A resync when some of the tones are below 6dB would likely mean you get a lower sync speed as a fresh sync won't have the benefit of the power gain.   Yes you will recover tones that may have previously been marked as unusable, but they will not be able to load as many bits as at the time of your last sync.
It's difficult to say with certainty on a line which usually has natural surplus, but the fact that at one point U0 was running at an average of ~5.5dB then you may not quite have got 10Mbps.   

What caused the original dip in SNRM I have no idea, although judging from recovery in the other channels whatever was causing the noise appears to be abating, so it should be now safe to resync to get a fresh bit allocation table.
Please do not PM me with queries for broadband help as I may not be able to respond.
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Re: Is this a problem ?
« Reply #3 on: July 30, 2019, 08:44:53 PM »

I'm interested to learn more about how the cabinet can hike up the transmit power, as I had this result yesterday:

This occured after I shot for a downstream SNRM of 1.6dB. My default target would have been 6dB with an ECI cab (with Vodafone as ISP).

The measured SNRM worked its way up aggressively at first and then settled down around 3.7dB. If I get a free TX power boost as a result of asking for a silly-low SNRM, then this feels like a free lunch to me.