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Author Topic: Crosstalk question  (Read 4628 times)

sevenlayermuddle

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Crosstalk question
« on: July 09, 2009, 11:33:38 AM »

I've been pondering a question...

Myself and quite a lot of neighbours all have phones wired through a duct that runs under my lawn to my own underground feed, and then feeds various poles at the top of my garden from whence it fans out overhead to the other houses.  I think it's a fair assumption that the cables are in close proximity all the way from the exchange as well.

My own line, with carefully optimised wiring setup, sustains just over 4mbps these days, getting me a 3.5 IP profile.  Most (maybe all) of my neighbours get a lot less, typically 1.5 to 2.0.  The BT speed estimator shows only 0.5mbps for some of their phone numbers, despite the fact that - unsurprisingly - the kitz line estimator confirms there's precious little difference between their line lengths and mine.  This leads me to ponder...

So my neighbours' poor performance could be down to poor internal wiring, maybe combined with the fact they mostly have a flying wire feed, whereas mine's underground.  But two questions still niggle me....

1) Could it be the case that my own highly optimised 4 mbps is generating an anti-social level of crosstalk that is penalising my neighbours?

2) If I wake up feeling generous one Saturdau morning, and spend my weekend helping the neighbours to get up to speed, am I likely to see my own speed decrease as a result of increased crosstalk from all these faster lines?

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kitz

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Re: Crosstalk question
« Reply #1 on: July 09, 2009, 12:44:14 PM »

1) No shouldnt do. -  certainly not in the way you mention. Most commonly cross-talk occurs at the dslam/msan (NeXT) and spectral masks are applied to ensure that power output doesnt go above 'x' level.  Of course there will always be a certain amount of crosstalk that cannot be avoided..  and the more lines there are on the dslam then the more likely it is to occur.

AFAIK theres very little info about the spectral masks applied at the dslam, but these (together with power cut back) are the key to ensuring that crosstalk is kept to a minimum.  
What I do know is that the UK has 3 types of spectral masks (short, medium & long lines) and this is applied somewhere in your line profile.  Masks ensure the Signal strength (in the real SNR) never gets too high and reduces the power output accordingly.
You will quite often see this occurring - particularly on short lines...  and its also one of the reasons why I didn't use SNRM in the calculations for estimating maximum line speed.  

Short/Medium lines that are say on fixed 2Mb or even 8Mb will often have power cutback applied and therefore you cant estimate anything from the 'surplus SNRM'.
For example its entirely possible to have say a 7dB atten line syncing at 8Mb with a 12dB SNRM and say a 25dB atten line syncing at 8Mb with a 12dB SNRM.

You also see power cutback occur if a line has had a particularly bad sync and then recovers.  Power cutback kicks in to ensure that the SNR doesnt go above a certain level that will drown out the neighbouring lines.

I'm totally guessing here... but I suppose where it could go wrong is it could be possible there may be an instance of where a phone no from say a long line is re-allocated to a short line and the profile isnt updated correctly/in time that this may cause a problem of NeXT on the line cards neighbouring lines. Power cutback should still kick in though and limit some of the damage?

2) Not really...  the higher the speed.. the more frequencies you use across the bins. ADSL sends data across any of the tones it is able to use, DMT splits data across the tones kinda like a pile of 56k modems all working in tandem.  Enabling you neighbour to use those higher tones should have negligable impact on your own line. Masks and power cutback will ensure that their signal strength doesnt get too much.

In all likelihood their signal strength sent out from the dslam is very likely to be similar to yours anyhow...  its just that because of some local noise in their own property and their line isnt optimised that they are just not using what is available efficiently.

Crosstalk when it occurs can show across all frequencies.  I strongly suspect that my own line is starting to suffer from crosstalk.. not down to what speeds my neighbours get... but just because the MSAN is filling up with 100's/1000's of active lines.  It may (or may not) also have something to do with the fact that the older DSLAMs only used to have say 8/16/32 lines per linecard.   The newer dslams such as the Fujitsu or MSANs have much larger line cards capable of holding many more users.
I see tone reduction across all frequencies right up to around 8Mbish... and then it starts to improve.

--
Note the above was blurbed completely off the top of my head..  so if anyone knows anything more about NeXT and FeXT ... or wants to research spectral masks.. then please be my guest to add to the convo.
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sevenlayermuddle

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Re: Crosstalk question
« Reply #2 on: July 09, 2009, 02:05:57 PM »

Note the above was blurbed completely off the top of my head..
...which makes it all the more impressive  :)

I'd had it at the back of my mind for a while, and mention of crosstalk in a few recent posts had got me thinking about it again.

Many thanks, as always.
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scottiesmum

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Re: Crosstalk question
« Reply #3 on: July 09, 2009, 02:21:37 PM »

Kitz  ....  you leave me speechless !  :)    It's like a foreign language to me - your knowledge appears to be boundless and that  "off the top of your head " is amazing ....  I do envy you your technical knowledge and 'chapeau' to you   !    I didn't even understand the question  ;D    I LOVE reading all the techy stuff on here, even if I don't understand more than a morsel of it. :)
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silversurfer44

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Re: Crosstalk question
« Reply #4 on: July 09, 2009, 02:58:08 PM »

Strangely enough I have just been reading about NeXT and FeXT so at least I know what they mean now. Apparently FeXT is less serious as it means Near End Crosstalk, i.e. crosstalk on the upstream from the users end and vis versa for NeXT. Wow I'm starting to understand some of this. :thumbs:
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sevenlayermuddle

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Re: Crosstalk question
« Reply #5 on: July 09, 2009, 03:26:13 PM »

Short/Medium lines that are say on fixed 2Mb or even 8Mb will often have power cutback applied and therefore you cant estimate anything from the 'surplus SNRM'.

BTW - that's something that else I've learned from you today, and a trap I may well have fallen into in the past.  :-[
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kitz

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Re: Crosstalk question
« Reply #6 on: July 09, 2009, 03:54:17 PM »

lol...  you've just witnessed what I call one of my 'off the top of head blurbs' which I will on occasion do on the forum... you will notice that I say 'guess' or suspect. I usually put a warning up that what Ive written is 'afaik'.. simply because this would be an area that requires much more research and some areas are stretching my limits simply because theres little info about to be absolutely certain.
Its often something that Ive come across amongst some real techo stuff and its stored somewhere in the back of my head and doesnt come out again until asked on occasion such as this.
 
Stuff on the main site is much more carefully presented.. but on occasion I do like to discuss and the warning is there simply because if anyone else has anything or more knowledge to add then its appreciated.

-----------


Something I did forget to mention in the above post - but it is something that I have mentioned previously which youve probably seen... is that routers/modems also have their own masks which are applied depending on the type of adsl (eg adsl1 or adsl2+ or annex_M) to also ensure that the Signal doesnt get too strong within each tone.
 
You may see these referred to as PSD (Power Spectral Density) Masks, and the router will apply different PSD masks depending upon which type of adsl you configure - eg g.dmt, adsl 2, adsl2+, gdmt.lite, annex_M etc.  These masks tell the router which bins it can use and which bins should transmit a signal no more than 'x'dB.

Not all routers contain all masks - hence why some routers may not be adsl2+ compatible... or in the case of the 2wire I recently bought it (disappointingly) doesnt contain an Annex_M mask...  therefore it steadfastly refuses to transmit data at the required frequencies and over-rides anything set on the dslam :(

Ive just done a quick google and theres a little bit more info on PSD masks from Texas Instruments who are one of the major router and dslam chipset manufacturers here.
The opening paragraph says it all really.

"The purpose of the limitation of the PSD mask is to reduce cross-talk noise and to limit power consumption."

By all means continue to read it as it does contain some useful info, but Im afraid my eyes often glaze over when I start to see algorithms/complicated maths and/or lots of figures.. and thats when my brain refuses to store any more and I start just using 'x' :(
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kitz

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Re: Crosstalk question
« Reply #7 on: July 09, 2009, 03:57:49 PM »

Strangely enough I have just been reading about NeXT and FeXT so at least I know what they mean now. Apparently FeXT is less serious as it means Near End Crosstalk, i.e. crosstalk on the upstream from the users end and vis versa for NeXT. Wow I'm starting to understand some of this. :thumbs:

AIUI
FeXT = Far End CrossTalk = at/from the router
NeXT = Near End CrossTalk = at/from the dslam
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kitz

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Re: Crosstalk question
« Reply #8 on: July 09, 2009, 04:11:05 PM »

Short/Medium lines that are say on fixed 2Mb or even 8Mb will often have power cutback applied and therefore you cant estimate anything from the 'surplus SNRM'.

BTW - that's something that else I've learned from you today, and a trap I may well have fallen into in the past.  :-[

I must admit... You me both, it was something I didnt discover until about 2005 when I wondered why my then 7dB atten line only had the same SNRM as lines that were longer and researched further.
It was only during the advent of rate adaptive dsl that you began to spot such things...  and its only after that when I also started looking at the power output.  AFAIK no-one before then really paid any attention to output power...  and it was only then when looking into it that I found out about power cutback - something that seemed to have never been mentioned elsewhere.

AFAIK though as long as the line is running at about the usual 18/19 dBm power then you can use the average 800 kbps per 3dB SNRM.

If a line though is on fixed rate 2Mb.. or if its a good/short 8Mb line not running at full power then each 3dB SNRM is going to be a whole lot more than the 800 kbps average.

BTW downstream full power is around 19dBm.. varying 1dBm either way.
and upstream possibly 12/13dB?
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sevenlayermuddle

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Re: Crosstalk question
« Reply #9 on: July 09, 2009, 05:56:38 PM »

and thats when my brain refuses to store any more and I start just using 'x' :(

I often find these days that I'm faced with a dilemma.  I can, if I must, memorise something new.  But I know that in order to make space for it in my memory I'll have to forget something else, and I never know what it'll be.  :D

Thanks again for all the info.  I do intend some further reading as per your TI link and anything else I may find, partly to make sure I've understood it all, and partly just because it's fun.  But I can't guarantee I'll remember it for long.

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kitz

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Re: Crosstalk question
« Reply #10 on: July 09, 2009, 06:47:07 PM »

Ooops just found out my info is a tad dated about the 3 categories  :-[ - theres now 4 classifications - the new one being extra short.

According to the ANFP Specification for BT Access
Quote
The categorization of customers’ locations is in terms of ‘electrical distance’ from the MDF, defined by loss values at 100 kHz, for a nominal line to the MDF.

Category name   Is applied to interfaces at these locations
up extra shortthe customer NTP where electrical distance from the MDF is 21 dB or less
up shortthe customer NTP where electrical distance is 26 dB or less but over 21 dB
up mediumthe customer NTP where electrical distance is 29 dB or less but over 26 dB
up longthe customer NTP where electrical distance from the MDF is over 29 dB

On page 35 of the ANFP it actually lists the mask values for each tone. - linky
Have fun - because I dont even intend to try and remember anything about it  ???


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silversurfer44

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Re: Crosstalk question
« Reply #11 on: July 09, 2009, 07:16:15 PM »

I stand corrected, simply typos. I got my NeXT & FeXT the wrong way round. :blush: I will try and be more careful. :)
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Ezzer

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Re: Crosstalk question
« Reply #12 on: July 09, 2009, 08:13:21 PM »

Vagely related to NeXT & FeXT one thing I've noticed is how quiet the dsl is as picked up on my radio in the frames room in an exchange compaired to say a dp at a customers end. (I use my radio for REIN to listen to radio's 1,4&5 in an exchange during lunch times)

I can clearly hear broadband on the radios as I drive under dropwires from poles to end users properties. In an exchange I can only pick up the broadband when the radio passes right by the idf, curious
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