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Author Topic: FTTC Crosstalk  (Read 4270 times)


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Re: FTTC Crosstalk
« Reply #15 on: September 20, 2014, 12:28:55 AM »

MSAN -- Multi-Service Access Node
MSAM -- Multi-Service Access Module
DSLAM -- Digital Subscriber Line Access Multiplexer

The current deployment of FTTC with VDSL2 to the end-user uses any one of the above devices which, in the case of the first two, are configured as a basic DSLAM.
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Re: FTTC Crosstalk
« Reply #16 on: September 20, 2014, 01:25:29 AM »

Useful read I found
I have a set of 8 fixed IP's From my Eclipse isp.
BT ADSL2 (G992.3) line>HG612 set as a Modem, Bridge, WAN not Bound to LAN1 or 2 >Smoothwall (Hardware Firewall and routing) > Ethernet LAN, DMZ,WiFI LAN and Spare LAN .
DSLstats LAN2  linked Ethernet

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Re: FTTC Crosstalk
« Reply #17 on: September 20, 2014, 08:47:42 AM »

Thanks Cat for the clarification. As mooted, vectoring technology only cancels cross-talk at the DSLAM, where it as it's peak.

The theory behind it appears complex and is waaaay above what I need to understand, but if any resident scientists can put it into layman's terms reading from the link tickmike has posted up, I'd be more than happy to try and get my napper round it ?  :)

We know that to minimise magnetic-field induction, wires should be as near to 90 degree's to one another as possible. Obviously, that is unachievable in the D-side network, so in my tiny mind I'm wondering if the DSL signals are generated 'out of phase' with each and every other circuit in the bundle ?? Which would be similar to physically running cables at 90 degrees ?

If this is totally wrong go easy with me, it's early morning and I haven't had my brekkie yet.  ;D 



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Re: FTTC Crosstalk
« Reply #18 on: September 21, 2014, 10:15:36 AM »

To see the effects of increasing crosstalk on a live connection, see the attached 730 day montage of the stats harvested for my connection.

In a nutshell, DS Attainable rate has reduced from around 35 Mbps to around 21 Mbps.
Actual DS Sync speed has reduced from around 30 Mbps (Interleaved) to around 21 Mbps (whenever DLM decides my connection can handle fastpath - it is usually around 18.5 Mbps these days when Interleaved).

There have also been reductions in US Attainable & actual sync speed rates, but by smaller amounts.

Due to the huge amount of data between October 2012 & the present time (around 1 million samples), the graphs' horizontal scale doesn't depict minor fluctuations too well, but I think the overall picture is quite clear.

Don't be misled by some of the 'sudden' changes in some of the graphs though.

Some of theose sudden changes were due to gradually adding the plotting of more stats data. i.e. the data simply wasn't plotted previously.

Some of the other sudden changes came with the HG612's firmware update in October 2013. i.e. data is now reported differently by the modem (more realistically??)
e.g. DS Bitswapping now seems far more effective & DS Signal attenuation is probably more realistic than as reported by the original firmware version.

Other things to take into consideration when trying to determine the effects of crosstalk are general seasonal changes in line/signal attenuation, particularly evident in the higher frequency D2 & U1 bands (I don't have enough data from a connection capable of using all the frequency bands for comparison purposes, but I imagine a similar or more pronounced effect on attenuation would be seen).

Attenuation on my connection is at its lowest during January/February than it is during August.

It appears that ecah time my connection speeds are reduced by crosstalk, I see a few days of reduced SNRM before the connection resyncs back around the target 6dB (at lower speeds).

We do experience occasional power cuts where I live.
As the HG612 modem resyncs/boots up quite quickly, I do often see higher sync speeds (up to around 25 Mbps) for a few days, but I do also see SNRM reduce dramatically & fairly quickly (down to around 1 dB) before the connection resyncs back at 'usual' speeds.

Presumably the reduced SNRM is the effect of other modems booting up again when power is restored.

DLM does appear to allow a few days of grace in these circumstances, where attainable rate is significantly lower than sync speed.

Some users see much more daily SNRM fluctuation than I see on my connection.
I live in a fairly rural location where electrical 'noise' interference is probably more static than say a heavily populated area where thousands of lights, TVs, central heating etc. will be switched on in the evenings, along with the natural daily SNRM fluctuations.

I live around 1100m from the cabinet & imagine my signal attenuation is much worse than line attenuation due to some sort of power restriction at the DSLAM to avoid my connection swamping other connections with crosstalk.
Likewise, other connections living closer to me than the cabinet will no doubt also have the same power restrictions & increased signal attenuation.

Those living closer to the cabinet won't need as much power per Mbps as I need, so they can still obtain higher speeds, even if their power has been reduced in order to reduce the effects of crosstalk.

I am not aware that our modems can provide a simple measure of crosstalk as such from snapsot data.
My observations are based on really long term monitoring trends which generally even out peaks/troughs from users going on holiday & switching their modems off etc. (to some extent).

I have been able to see this gradual deterioration of attainable & sync speeds as I was possibly the first or one of the first to be connected to FTTC from my cabinet (Plusnet got me connected & working before BT officially reported that FTTC was even available).

In summary then, it appears that we end users have to take the time of year (ambient temperature), time of day/night, possibility of other modems being switched off, rural/urban location etc. into account, using 'reasonably' sized sets of ongoing data before categorically stating how much/little we are being affected by crosstalk.

It may even be worth considering the general age group of residents in any given area as I 'imagine' that younger people are more likely to have FTTC connections than older people, partlly due to cost & partly due to less general internet interest/need from retired people living off their pensions.

As mentioned above, my comments are based on empirical data from my connection, along with more limited data that I have seen from other users' connections.

Please feel free to discuss/discredit any of the above and/or provide any factual technical details accordingly.



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Re: FTTC Crosstalk
« Reply #19 on: September 21, 2014, 11:24:26 AM »

Published alcatel test results showed that crosstalk was inconsistent and its effectively a pair lottery however telco's can still mitigate crosstalk by high quality cabling such as bigger diameter copper and tighter twists, avoiding cross/split pairs, using low density cable bundles etc.  Of course one mitigation we know openreach are doing is power cutback on the upstream, almost everyone with short FTTC distance has their U1 cutback to aid longer lines who need U1.

Based on that I sort of guess the cabling in my area is poor given the insane levels of possible crosstalk I have seen on my original pair, which went from 110 attainable down to 50.  Thats over 50%, although I will never know if its all crosstalk or a fault, as the engineer found it easier to swap the pair than to diagnose further.  He only managed to get 50mbit at 120m from the cab.  My new pair is syncing at 73mbit roughly in line with my estimate but my belief is BT set the estimates to include predicted worst case crosstalk, which is why in early days many people get speeds of 20+ mbit above their estimate.  If we assume my new pair could also get 110mbit without crosstalk then this pair is suffering about 40% loss as well.
« Last Edit: September 21, 2014, 11:31:32 AM by Chrysalis »
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