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[lemzip]

Quick question all.

I have dropped from 79Mbit to 65Mbit and a loss of 4db in SNR. The loss is showing on the downstream where the upstream is showing the same pre loss.

I have only recently upgraded to 80Mbit and all was stable for a year before any change in SNR.

No obvious things my side i can find.

Thanks

[ejs]

What's the question?

[lemzip]

As i have no means of showing crosstalk as i have a sky hub 3, does crosstalk introduce itself on the downstream and not the upstream or would you expect it to affect upstream snr also?

[ejs]

I think the main aspect is that crosstalk gets worse as the frequency of the signal increases, so for 60-80 Mbps, the highest frequencies in use will be in the downstream D3 band. So the downstream is worst affected by crosstalk because the downstream is using the highest frequencies.

There may be other factors, upstream may generally operate at a lower power output than the downstream. The downstream signals will be strongest at the cabinet, which is where all the lines are close together, although I don't know how that would affect the SNR, highest signal strength vs. highest crosstalk noise.

[lemzip]

Okay got it. Bit of a steep hit to the downstream speed..guess i have to live with it.

Pitty, 79Mbit was nice. Feel slightly peeved at paying the full price. Perhaps if it gets worse and under 55Mbit i can get a cost reduction due to Sky's 55Mbit promise.

[Chrysalis]

I am not sure where this idea of crosstalk worse at higher frequencies comes from, my personal experience on my line is its worse on lower frequencies as thats where the dsl signal is stronger.

To me higher frequencies are more vulnerable to external noise, like lightning and radio, but lower is more vulnerable to crosstalk.

But I guess its possible my line is just weird and breaks the trend.

[lemzip]

Well i had a constant snr of around 18dB @ 16 dB attenuation.

Upgraded to 80Mbit and sync'd at around 75Mbit initially. Gimp was ativated and sync'd at 79Mbit with 6.5 dB snr.

Noticed snr dropped to 2.1 dB after 1 week. Router resync'd at 65Mbit with 6 dB snr. Upload has remained with the same SNR post upgrade @ 15.15 dB @ 20Mbit.

[lee111s]

Certainly sounds like a crosstalker or two

[ejs]

I am not sure where this idea of crosstalk worse at higher frequencies comes from

I thought it came from all the studies of crosstalk that have been done, but then most of the recent ones have been looking at frequencies beyond the VDSL2 range.

Of course crosstalk from ADSL users will only affect ADSL frequencies, and crosstalk from longer VDSL2 lines will only affect the frequencies that those longer VDSL2 lines are using. So if you've got a lot of users on your cabinet with much longer lines than yours, then I suppose there might be more crosstalk on the lower VDSL2 frequencies.

[lemzip]

Hi,

I had a resync this morning and i am unsure but perhaps i have been set on a 5 dB target SNR:

Connection Speed 67147 kbps   19999 kbps
Line Attenuation   16.4 dB   0.0 dB
Noise Margin   5.4 dB   15.15 dB

Gained 3Mbps for 1dB  I hope this is it attempting to move down to a possible 3 dB but could have just been a blip. I was  watching the SNR pre drop and it was static at 6.4dB.

Thanks for the replies all - interesting stuff.

[WWWombat]

I thought it came from all the studies of crosstalk that have been done, but then most of the recent ones have been looking at frequencies beyond the VDSL2 range.

This pair of graphs comes from work in G.Fast, and highlights the key underlying change in understanding for frequencies above 20-30MHz: that crosstalk grows at a higher rate at higher frequencies. That understanding, and the better cable models that came from that understanding, makes G.Fast (with vectoring) effective with all that "cross-shouting" going on.

[Chrysalis]

thanks guys, I will assume its just my line/area been unusual then.

[WWWombat]

I assume that the visible artifacts of crosstalk will vary everywhere.

On Downstream, it will depend on, for example, whether the nearby crosstalkers (ie pairs lying close in the bundle) happen to be very long lines or very short lines. The longer lines won't use the high frequencies at all, so won't induce crosstalk at those frequencies on your line. D3 should show less crosstalk because of usage statistics.

Upstream has similar dependencies, but with UPBO thrown into the mix too.

[lemzip]

Hi,

The Target SNR is moving downwards. From 5.4dB to 4.4dB.

Connection Speed   70619 kbps   19999 kbps
Line Attenuation   16.4 dB   0.0 dB
Noise Margin   4.3 dB   15.15 dB

I was 79500 @ 6dB but if i get to 3dB at 73000 kbps i would be happy and hope one day my crosstalker moves

[lemzip]

and finally on 3dB:

Connection Speed   73666 kbps   19999 kbps
Line Attenuation   16.4 dB   0.0 dB
Noise Margin   3.4 dB   15.15 dB

So while not brilliant its fine. Interesting I average 3Mbps for 1dB snr.

Each step down was around 48 hours +- 30 minutes.

Cheers