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Author Topic: FTTC Crosstalk  (Read 6440 times)

tickmike

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FTTC Crosstalk
« on: September 18, 2014, 08:08:04 PM »

FTTC Crosstalk.
Can you just explain a bit more about it please.
Eg Does it come from all the pairs of wires in the 'last Mile' from the Fibre cabinet all interference with one another.?
 
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Black Sheep

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Re: FTTC Crosstalk
« Reply #1 on: September 18, 2014, 08:12:42 PM »

Crosstalk is the interference/noise in the DSLAM (Fibre Cabinet). 'Vectoring' aims to reduce this when finally implemented. :)
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Black Sheep

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Re: FTTC Crosstalk
« Reply #2 on: September 18, 2014, 08:20:48 PM »

Taken from our 'Field Engineering News' (FEN) ..........

'Vectoring
One of the ways we are looking to improve speed on the broadband network is through ‘vectoring’. Vectoring dynamically calculates a “pre-compensation” factor for each line and the crosstalk can be cancelled in the DSLAM. The best example is the noise cancelling headphones that you see people wearing attached to their mobile device on every train, plane and bus journey'.


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NewtronStar

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Re: FTTC Crosstalk
« Reply #3 on: September 18, 2014, 08:46:35 PM »

You can always turn up the volume on the headset to drown out the background noise to the ears the (receiver), but it's different with broadband signals as the background noise is also mixed into the signal, so it's going to need noise reduction algorithm software built into the hardware some how  :-\
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tickmike

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Re: FTTC Crosstalk
« Reply #4 on: September 18, 2014, 10:58:36 PM »

Crosstalk is the interference/noise in the DSLAM (Fibre Cabinet).
It's in the cables  :hmm: 'crosstalk interference depends on the position of the pairs carrying VDSL2 signals with respect to one another' I think , this would be the pairs from the cabinet to the end user.
 
Quote
'Vectoring' aims to reduce this when finally implemented. :)
Found this video it explains about it.
http://www.youtube.com/watch?feature=player_embedded&v=zkVmej4urx4
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Ronski

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Re: FTTC Crosstalk
« Reply #5 on: September 19, 2014, 06:27:44 AM »

Yes,  it's the signal in one cable affecting another. This can happen anywhere between the  DSLAM and the end users equipment. The proximity of one cable to another alters the effect, as does the strength and type of signal.
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Black Sheep

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Re: FTTC Crosstalk
« Reply #6 on: September 19, 2014, 10:37:23 AM »

Vectoring only reduces cross-talk at the DSLAM.
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adslmax

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Re: FTTC Crosstalk
« Reply #7 on: September 19, 2014, 02:46:45 PM »

Don't think BT will not going to do vectoring nationwide until 2016/17 as more likely BT wait until FTTC/FTTP had completed roll out first.
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Ronski

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Re: FTTC Crosstalk
« Reply #8 on: September 19, 2014, 04:06:47 PM »

Vectoring only reduces cross-talk at the DSLAM.

But does it not take into account, and counter cross talk affecting all the line?

The OP asked where cross talk occurs, which is anywhere where there is more than one line carrying a signal in close proximity.
« Last Edit: September 19, 2014, 07:01:33 PM by Ronski »
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Black Sheep

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Re: FTTC Crosstalk
« Reply #9 on: September 19, 2014, 04:22:11 PM »

The information I've personally seen, of which part of it is posted above, states that vectoring of VDSL circuits will only be at the DSLAM. This is called NEXT (Near-end cross-talk).

If the 'noise' is then effectively wiped out at source through the application of pre-compensation factors, maybe the effects of FEXT (Far end) is minimal or negligible ?? I don't know, I'm not clued-up in the slightest with vectoring technology. I'm just re-posting part of what has come from our Chief Engineers office. 
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Bald_Eagle1

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Re: FTTC Crosstalk
« Reply #10 on: September 19, 2014, 06:26:27 PM »

"An access node of a communication system is configured to control crosstalk between channels of the system.

Vectoring circuitry in the access node estimates crosstalk between channels of the system, generates a compensation matrix based on the crosstalk estimates, and generates compensated signals based on the compensation matrix.

The compensation matrix, which may be a precoder matrix or a postcoder matrix, is generated using a multiplicative update process in which a previous version of the compensation matrix comprising one or more non- zero off-diagonal elements is updated by at least one of pre-multiplying by a first auxiliary matrix and post-multiplying by a second auxiliary matrix, with a given one of the auxiliary matrices also comprising one or more non-zero off-diagonal elements.

The compensated signals may be pre-compensated signals or post-compensated signals."



The above extract is from patent WO 2012102917 A1, available for viewing here:-

http://www.google.com/patents/WO2012102917A1?cl=en


Further reading of the patent suggests that it may well be that vectoring pre-compensation is applied at the DSLAM to deal with the effects of crosstalk from many 'disturber' pairs within the whole length of the binder cable(s) (which I presume is commonly known as the D-side bundle).

« Last Edit: September 19, 2014, 06:31:27 PM by Bald_Eagle1 »
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Black Sheep

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Re: FTTC Crosstalk
« Reply #11 on: September 19, 2014, 06:30:29 PM »

Thanks for that reference, BE.

I used to have more info on this, even a thermal image of a 'Before and after' vectored DSLAM. I think I may have posted it on here before, but can't fully remember ? The results were dramatic.   
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Bald_Eagle1

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Re: FTTC Crosstalk
« Reply #12 on: September 19, 2014, 06:33:47 PM »

I presume "access node" is just another term for DSLAM ???

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Black Sheep

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Re: FTTC Crosstalk
« Reply #13 on: September 19, 2014, 07:08:17 PM »

Not sure, BE ....... not a term I've heard before ??

Being as the 'Access network nodes' are the cables and associated furniture ............ I can only assume that 'Access node' is indeed the DSLAM ???
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kitz

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Re: FTTC Crosstalk
« Reply #14 on: September 19, 2014, 09:32:01 PM »

I must admit when I read the reply earlier, I did wonder if somewhere down the line there may have been some confusion in the term AT the dslam.

For example impulse noise/ noise bursts / low level background noise etc can all occur and be introduced anywhere en route... But is actually corrected AT the router by use of error correction algorithms and interleaving.
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