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101 posts

Master Geek

#139545 12-Feb-2014 13:59
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I upgraded my connection to Telecom VDSL a few months ago and was pretty happy to get (Up/Down) of about 9/45Mbps. The link has been very stable and consistently sitting at this rate for months.
A week or so ago, I connected to the router to make a change and noticed that the link had only been up for 2 days and my connection was now 10/37Mbps.  I figured someone had been playing around in the cabinet and this had resulted in the new connection, but was a little surprised at the loss of 8Mbps.
I checked the line details and they didn't seem to have changed ...


Line Attenuation (Up/Down) [dB]: 21.4 / 10.2


SN Margin (Up/Down) [dB]:  16.2 / 12.2

... and a reboot of the router re-connected at about the same rate.... so I rang Telecom to see whether they could shed any light on it.

After confirming that 'Yes... indeed.. I had lost 8Mbps compared to the previous few months and that indeed my Line Attenuation and Noise levels were about the same, they asked Chorus about it. The response from Chorus was...

"This customer's sync rates have decreased with the addition of new VDSL connections off the same cable terminal - this is a design aspect and the line is performing to specifications - more connections add more noise and the average sync rates will decrease as a result. We can confirm that the sync rates your customer is presently getting are in line with neighbouring VDSL connections and the line is testing good/is error free."


So ... as I read this, they are basically saying that as they connect more VDSL users, (and nothing to do with traffic volumes) the line speeds will keep reducing (even though my line noise levels haven't increased) due to 'more noise' somewhere in their part of the system. As long as the results are still above some benchmark then that is just too bad.

It feels as though as more users are added I am likely to continue to see reduced performance and, as long as I am still getting better than about 15Mbps then that is just tough.

Is this a correct interpretation of this or is there some limit to the number of additional connections that can be added to the 'cable terminal' ??

I realise that this definitely counts as a 'First World Problem' and that I should probably be happy with 37Mbps but it is a little frustrating when you know it can be better and don't know whether it might keep dropping as they add connections.

Any of the "telco wizards" on Geekzone able to shed further light on this?


Many Thanks

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19282 posts

Uber Geek
Inactive user

  #985085 12-Feb-2014 14:03
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That is correct and good explanation and be pleased you can get VDSL and not connected to a Conklin cabinet


28688 posts

Uber Geek

Biddle Corp
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  #985086 12-Feb-2014 14:04
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What you're seeing is the consequences of crosstalk - everything is basically working as designed.

It shows the limitations of xDSL and why fibre is a superior solution to copper.

The effects of crosstalk will depend on a large number of variables - but certainly isn't helped by ISPs such as Big Pipe who have decided that they won't force master filters on their VDSL2 customers, meaning that a poor connection has a much greater chance of causing impact to customers closers to the ISAM.

If you want to understand more have a bit of a read on google of FEXT (far end cross talk)


6615 posts

Uber Geek
Inactive user

  #985092 12-Feb-2014 14:07
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The more EMI or interference from neighboring connections will cause it to deteriorate. It cant stay at the same rate as it will cause your connection to become unstable or have negative effects. Mine is 20M from a Cabinet but i get 45Mbp/s. Due to EMI

101 posts

Master Geek

  #985101 12-Feb-2014 14:13
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Thanks for the feedback from all above.

Oh well.... it was nice while it lasted! :-)

It will be interesting to see how much further it degrades over the coming months. I guess eventually fibre will hit our neighbourhood and then we can look at whole different set of issues :-)


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Uber Geek

  #986334 12-Feb-2014 19:18
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Bear in mind that the modem doesn't normally report a noise figure, it reports noise margin.

Noise margin is basically the difference between the background noise and the transmitted signal on the line, and is normally set at a target of 12dB. If the background noise increases (in this case due to additional nearby connections) then the noise margin will drop, and eventually the connection will adjust the sync rate to maintain the noise margin. If the noise margin drops too low, then reliability suffers.

So, looking at the figure reported by your modem, it shouldn't have changed other than by a small fraction because it is a set target the connection works to - the background noise will have increased, and the sync rate dropped in order to maintain that margin.

To take it to the very extreme, if the noise margin dropped to 0dB, then the transmitted signal would be indistinguishable from the background noise and the connection would fail completely.

EDIT: The same happens to ADSL connections - they are significantly impacted by how many nearby connections there are. I've seen a connection off a rural cabinet with 30 something dB attenuation syncing at about 14,000kb/s yet connections in an urban area get about the same sync rate with under 10dB attenuation, all because of higher background noise levels from crosstalk in the feeder cables.

1948 posts

Uber Geek
Inactive user

  #986355 12-Feb-2014 19:57
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You could always ring up the broadband helpdesk and ask if they could kick off the dlm process again. I may be able to do something about it. Flick me an email at "pl at" and I'll see what I can do. But I am pretty busy right now so email me again in a few days if I don't get back to you.

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Uber Geek

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  #986382 12-Feb-2014 20:52
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I started off on 42mbit, now I'm 36mbit, still completely happy since I like the upload speed but I know this is far end crosstalk and something you can't prevent.

This is why Kirdog will never get his Annex-M


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Uber Geek


  #986424 12-Feb-2014 21:33
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Wait till a powercut and then make it resync when everyone else without UPS's are offline. That will get you good speeds, and it will maintain them when they come back online so long as the noise margin keeps above about 4.

Works great with ADSL, but VDSL has DLM which comes along and ruins it after a while and dumps you onto a crap profile because of the high error rate.


4025 posts

Uber Geek


  #986486 12-Feb-2014 22:25
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The quality, condition and age of the wiring in the street could fix this. Noise is very much what it sounds like - if you want to compare it to audio noise that's not a bad analogy. How hard is it to talk in a loud room full of people? its going to get worse the more people talk.
And just like that, you could tell everyone to shut up so you can talk, but i have a feeling that doing that in the loud room (aka, telling your neighbors to downgrade/get off broadband) wouldn't go down so well !

101 posts

Master Geek

  #986571 13-Feb-2014 08:40
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Thanks for the offer PL. From what people have said and further reading I've done it sounds as though this is just the way it is.

Perhaps I'll start a 'NO VDSL' campaign on my street and try and discourage anyone else from taking it up. This ... combined with a forced power outage and richms's idea about synching while everyone else is offline might keep me going till fibre arrives in 2099 :-)


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