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Standard User allofus
(newbie) Sun 02-Nov-14 10:08:55
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BT Infinity and line length


[link to this post]
 
We live in a village that is getting BT Infinity as part of BDUK. The BT Openreach engineer said that BT Infinity which uses VDSL2 performance drops off dramatically (like a cliff were his words) when the copper line length exceeds around 300Mts. Any Aluminium in the line also degrades speed quickly. Also, as more Infinity customer are in your cabinet / line then cross talk increases and speed drops. BT are now getting requests to go back to ADSL2 after an Infinity 'upgrade'.

I found a chart from apparently Ofcom which shows how VDSL2, VDSL and the ADSLx compare by length of line - http://nbnmyths.wordpress.com/why-not-fttn/.

My question is why does BT persist with VDSL2 when for longer real world line lengths VDSL gives better performance?
Standard User BatBoy
(legend) Sun 02-Nov-14 10:30:51
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Re: BT Infinity and line length


[re: allofus] [link to this post]
 
At 300m you can get 50Mbps down and 18Mbps up. Is that not enough?


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Standard User MHC
(sensei) Sun 02-Nov-14 10:44:16
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Re: BT Infinity and line length


[re: allofus] [link to this post]
 
No idea where that graph came from ... but it not very accurate and does not reflect actual experiences.

My line length from the cabinet, for example, is 600metres using SatNav or 460 metres following the track on Google Earth. Currently I am getting a sync of around 72 Mbps, max achievable of 74 and throughputs at about 70.

According to that graph I should be seeing about 20-25 ...


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M H C


taurus excreta cerebrum vincit


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Standard User tdw42
(newbie) Sun 02-Nov-14 11:24:14
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Re: BT Infinity and line length


[re: allofus] [link to this post]
 
The graph on this page is more representative http://www.increasebroadbandspeed.co.uk/2013/chart-b... but still errs on the conservative side.

When Openreach deploy vectoring technology it should reduce crosstalk and improve performance on cabinets with many subscribers.

Don't overlook the fact that the connection from the cabinet to the premises is only the last leg of a long journey for data packets - the (shared) connections from the cabinet to exchange, exchange to ISP, ISP to the internet are overcommitted and may become congested; the ISP may control the flow of some categories of traffic in an attempt to reduce congestion, so in many cases 'the internet going slowly' is not due to the final part of the journey.

As to switching back, it depends on expectations and what you are prepared to pay - if you only have 1-2Mb ADSL but can get at least an order of magnitude more with FTTC you may well be happy to pay more for the service so you can stream films/catchup TV, but if you have at least 6Mb ADSL and would only get 20-30Mb with FTTC you may not be...
Administrator MrSaffron
(staff) Sun 02-Nov-14 11:35:55
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Re: BT Infinity and line length


[re: tdw42] [link to this post]
 
And then of course there is http://www.thinkbroadband.com/guide/fibre-broadband.... it may look pessimistic but we took account of crosstalk impact from day 1

There is also http://maps.thinkbroadband.com/#!lat=54.628258777628... where we have calculated a speed for every postcode (outside NI) on the assumption every cabinet gets a FTTC service

The author of the above post is a thinkbroadband staff member. It may not constitute an official statement on behalf of thinkbroadband.
Standard User TheEulerID
(learned) Sun 02-Nov-14 11:49:47
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Re: BT Infinity and line length


[re: allofus] [link to this post]
 
VDSL is used because it's about 10-15% of the cost of putting in FTTP.

There are other technologies on the way to push fibre further into the network but basically the closer it gets to the premises, the more expensive things become.

As for "dropping off a cliff", I get 57Mbps at 700 metres (which is just about bang on what the prediction would be). You get decent speeds up to about 1km, but by 1.5km it's not up to much.
Standard User TheEulerID
(learned) Sun 02-Nov-14 11:57:20
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Re: BT Infinity and line length


[re: MrSaffron] [link to this post]
 
I'm on a 700m line and sync at 57mbps (and the cabinet has been enabled for a year or so). SO that downstream speed does seem pessimistic to me. However, upstream is about what I get.

However, I'm not wholly certain you can go with one prediction. Apart from local line conditions, I seem to recall that the SNFP for sun-loop unbundling had different power masking requirements depending on how far the cabinet is from the exchange. I don't know for sure if that affects the VDSL frequency plan used in the UK or if it plays any part in BT's FTTC product.
Standard User MHC
(sensei) Sun 02-Nov-14 12:33:15
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Re: BT Infinity and line length


[re: MrSaffron] [link to this post]
 
Your figures are extremely pessimistic:

Take three ways to measure my distance from the cabinet and your forecast:

Direct straight line, cabinets to pole to house: 380m estimate: 43/16
Measured on Google Earth: 460m estimate: 40/15
By satnav and walking: 600m estimate 35/14

Allow for distortions on GE and innacuracies of the phone based satnav and I am probably around or just under 500m away.

I am now getting 68 to 70 down and around 18 up. Down stream sync is over 70 and max attainable rates are shown as 73/23 In the past I had also seen a slow drop off due to cross talk.


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M H C


taurus excreta cerebrum vincit
Standard User BatBoy
(legend) Sun 02-Nov-14 12:46:43
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Re: BT Infinity and line length


[re: MHC] [link to this post]
 
What's your attenuation?


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Standard User RobertoS
(elder) Sun 02-Nov-14 12:51:06
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Re: BT Infinity and line length


[re: TheEulerID] [link to this post]
 
In reply to a post by TheEulerID:
VDSL is used because it's about 10-15% of the cost of putting in FTTP.
Errm - that might confuse the OP wink.
In reply to a post by allofus:
My question is why does BT persist with VDSL2 when for longer real world line lengths VDSL gives better performance?


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