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Standard User kneewax
(learned) Wed 16-Nov-11 09:15:05
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How important is line legnth?

[link to this post]
Before I start I need to say this: I know the odds are stacked against me getting anything other than a mediocre speed on my ADSL line, I know that Rural communities all suffer like this and I know that there is little or no alternative!

BUT I could still do with help/advice.

Here's the setup:

Rural Area on the very end of the exchange (STD codes on opposite side of the road are different!)
Ethernet Modem: Draytek Vigor 120
Router: D-Link DIR685

My Broadband Speed Test

18 months ago I moved for qwork reasons out of a Virgin Media are in Wiltshire (appalling Customer Service but worth it for the relatively stable and super fast fibre connection) to a village about 5 miles outside Poole in Dorset. Not having cable as an option (despite the end of their line only being 3 miles away) here I returned to IDNET who I had previously been with for some years and was always a happy customer.

Since then we have been plagued with speed issues. The line out here was originally a 512kbps line until BT upgraded the exchange earlier this year. When we started to get nearer a heady 1mb. this is the status-quo and while frustratingly slow, if we could actually achieve somewhere near that speed consistently then it would be OK.

The problem is that once every 4-6 weeks or so the speed on our connection drops sharply to between 50 and 120 kbps. Each time a call or email to IDNET support results in them getting BT to reset the loop where upon speed increases and over the course of the usual 72hrs settles down to a manageable 750kbps or so. Until the whole thing goes around again a few weeks later.

The last time we went through this cycle the speed dropped again after about 10days and as a result I've tried to really engage with IDNET as to what the issues underlying causes might be and what solutions we could explore. After nearly engaging a BT engineer to look, they decided against that and now have said botht hat we cannot expect more out the line (not an acceptable option) and that we should get that 750Kbps (Which is contradictory!) They are now saying that the physical line legnth to our property is the problem and there is nothing to be done.

How much of an issue is that line length and while I do understand it to account for some speed drop off, should we really expect to drop to less than a 10th of the exchange capability?

What options do I have to get a service that is not slower than the 56k modem I was using 12 years ago?

Been a while since I've used the forums so hope I've posted in the right area.



I'm not flat - the band's sharp.
Administrator MrSaffron
(staff) Wed 16-Nov-11 09:28:49
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Re: How important is line legnth?

[re: kneewax] [link to this post]
Line length as in the attenuation of the signal is the biggest single factor governing ADSL speeds.

can you dig out the statistics from your ADSL router?

Have you asked IDNet about putting you onto a 'fixed rate half meg profile'...that might give you back the relatively stable 0.5Meg you used to have.

Andrew Ferguson, [email protected] - formerly known as
The author of the above post is a thinkbroadband staff member. It may not constitute an official statement on behalf of thinkbroadband.
Standard User Apprentice
(knowledge is power) Wed 16-Nov-11 09:31:37
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Re: How important is line legnth?

[re: kneewax] [link to this post]
Could you post the line stats for your connection >

and the results from the BT PT >

Does the exchange have any issues? >


What is attenuation? > link



Edited by Apprentice (Wed 16-Nov-11 09:37:25)

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(Unregistered)Wed 16-Nov-11 11:16:35
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Re: How important is line legnth?

[re: kneewax] [link to this post]
Here is a useful graph that shows you what speed ranges to expect with different values of line attenuation.
Your router should tell you the attenuation it is measuring on your line.
This could be more than a line of that length using that poundage of plant should be.(copper cables come in different thicknesses and the thinner the cable the higher the attenuation)
So the first thing to check is wether for the length of your line and the size of the cables should result in the attenuation your router is measuring.
Assuming that it is reasonable, the next thing to check is the frequency response of your line. ADSL signals are represented on the line as carriers at 4kHz spacing and as many bits are encoded in each carrier as possible so as the high frequency response of the line drops, fewer and fewer bits can be encoded in these bins as they are called. You might get 12 bits per bin on a bin at lower frequency but only 1 or 2 bits for a bin at the high frequency end where the line response has dropped almost to zero.
Here are graphs of a good line and a poor line
and here is a graph of a line that has interference which is killing the bins at the frequency of the interference. This is a classic line fault caused by the pairs to two houses being split, so one house used the 2 A legs of two pairs and the other house used the 2 B legs of the 2 pairs. This was for only the last 70m of the exchange lines but cause a drop from 5Mbps to 1 Mbps.

Note that Vigor routers such as the 2600 and 2800 give these helpfull graphs but the data is usually stored in most routers somewhere, but you may have to telnet to them and give telnet commands to make them disgorge the bin info.

Provided there are no actual faults then you should get a speed in the range shown on the wiki graph.

I hope this helps and as a matter of interest it took me 4 years of pushing before BT finally sent out an engineer who knew what he was doing and found the fault.
Here is a picture of him finding it.
Standard User RobertoS
(sensei) Wed 16-Nov-11 12:21:45
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Re: How important is line legnth?

[re: kneewax] [link to this post]
Have you tried using the test socket inside the master, on the wall at the back in this picture, and what colour wires are connected to which terminals on the faceplate when you carefully remove it?

If the ring wire is there, (T3), whatever causes the line drops could be coming through it. In this case not a high probability, but you should certainly make sure it isn't connected.

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