General Discussion
  >> Fibre Broadband


Register (or login) on our website and you will not see this ad.


Pages in this thread: 1 | 2 | (show all)   Print Thread
Standard User fibreperson
(newbie) Wed 04-Sep-13 22:48:41
Print Post

FTTC speed issue - copper cable problem?


[link to this post]
 
Just had fibre broadband fitted (talktalk), however the resultant speed has been similar to pre-fibre installation.

Complaints about the low speed resulted in BT engineers being called out and they identified a fault in the cable between the FTTC cabinet and our nearby distribution cabinet.

The engineers' readings showed 40mb at the FTTC and 8mb at the distribution cabinet - they said that the figure should have been nearer 20mb. The cable, which is presumably made of copper, is 0.9 miles in length.

With regard to a drop in speed over a length of copper cable, does anyone know what the figures should be in practice or in theory?

Following the engineers' reports, rather than fix the cable, BT have now said that the best speed they can offer is 5mb.
Administrator MrSaffron
(staff) Wed 04-Sep-13 22:58:23
Print Post

Re: FTTC speed issue - copper cable problem?


[re: fibreperson] [link to this post]
 
Half way down this page http://www.thinkbroadband.com/guide/fibre-broadband....

.9 miles is a long way for vdsl2

Andrew Ferguson, andrew@thinkbroadband.com
www.thinkbroadband.com - formerly known as ADSLguide.org.uk
The author of the above post is a thinkbroadband staff member. It may not constitute an official statement on behalf of thinkbroadband.
Standard User fibreperson
(newbie) Wed 04-Sep-13 23:27:14
Print Post

Re: FTTC speed issue - copper cable problem?


[re: MrSaffron] [link to this post]
 
Thanks for the information.

Surely the effect of the fibre optic through FTTC is effectively to move the exchange closer to the property. The FTTC cabinet here is about 1 mile from the exchange and I would have presumed replacing the copper wire with the fibre to this point would have produced a higher speed, but it hasn't. Hence this apparent speed loss to the distribution cabinet seems strange.


Register (or login) on our website and you will not see this ad.

Standard User kasg
(fountain of knowledge) Wed 04-Sep-13 23:39:09
Print Post

Re: FTTC speed issue - copper cable problem?


[re: fibreperson] [link to this post]
 
In reply to a post by fibreperson:
The FTTC cabinet here is about 1 mile from the exchange and I would have presumed replacing the copper wire with the fibre to this point would have produced a higher speed, but it hasn't..

What you are forgetting is that VDSL speeds deteriorate more rapidly over distance than ADSL.

Kevin

plusnet Unlimited Fibre - BQM
Using OpenDNS
Domains and web hosting with TSOHOST
Standard User mr_mojo
(knowledge is power) Thu 05-Sep-13 00:04:02
Print Post

Re: FTTC speed issue - copper cable problem?


[re: fibreperson] [link to this post]
 
The problem with VDSL is that to prevent interference with normal broadband connections a lot of frequencies are 'blacklisted' for use on FTTC. Much like on a normal radio set FM uses different frequencies to AM radio to prevent interference.

So imagine at 0.9 miles if you had all the frequency range you might of got 22mbit (from the ADSL2+ bands) + 5-10mbit (from the 'post ADSL2+' bands) for say 35mbit total on a 0.9 mile line if such a thing existed.

However, because you don't, you don't get any of the bands that ADSL2+ can use, which are the ones that propagate longer distances the most.

(Simplistically) this is why VDSL deteriorates faster than ADSL. It's not like they designed it to not reach further, it's more to do with the fact that the frequencies that go the furthest aren't used in conjunction with VDSL, causing this very bad distance performance.
Standard User eckiedoo
(member) Thu 05-Sep-13 07:10:22
Print Post

Re: FTTC speed issue - copper cable problem?


[re: MrSaffron] [link to this post]
 
Morning Andrew

Is it normal for the FTTC Cabinet to be ".9 mile" (about 1,450 metres) from the "distribution cabinet"?

My impression to date, is that they are generally very much closer together, say 5 to 20 metres, otherwise BT Openreach is doing what amounts to a complete, new line, in this case.

Would the BE BRAS Tester still be of use, to get the basic copper length to the exchange, for comparison?
Standard User WWWombat
(fountain of knowledge) Thu 05-Sep-13 08:58:25
Print Post

Re: FTTC speed issue - copper cable problem?


[re: mr_mojo] [link to this post]
 
In reply to a post by mr_mojo:
However, because you don't, you don't get any of the bands that ADSL2+ can use, which are the ones that propagate longer distances the most.

(Simplistically) this is why VDSL deteriorates faster than ADSL. It's not like they designed it to not reach further, it's more to do with the fact that the frequencies that go the furthest aren't used in conjunction with VDSL, causing this very bad distance performance.

That's wrong.

VDSL2 in general, and specifically within BT, continues to use the frequencies that ADSL and ADSL2+ use. Those frequencies are NOT blacklisted.

However, the power of those frequencies (as transmitted by the FTTC cabinet) is reduced from the maximum, to ensure that the ADSL/2+ signals from the exchange are not swamped.

The amount the power is reduced varies from cabinet to cabinet, along with the exact frequencies the reduction happens over - the PSD power masks that control this are designed so that the FTTC cabinet transmits at the same power level that ADSL signals arrive at the PCP, and covers the subset of ADSL frequencies that make it over the exchange-PCP distance.

A PCP close to the exchange doesn't reduce power much, but does so over a wide frequency band. PCP's far from the exchange reduce power much more, but over a smaller band.

The reduction in power certainly results in less data being transferred in the lower frequency bands than theoretically possible, but it is by no means zero.
Standard User Chrysalis
(legend) Thu 05-Sep-13 09:11:35
Print Post

Re: FTTC speed issue - copper cable problem?


[re: WWWombat] [link to this post]
 
the power seems to be lower than equivelent adsl as I think BT are been very cautious.

eg. on my tones shared with adsl the bitloading is under 5 bits per tone at the 125 tone, the highest bitloading is 9bits in the range. My adsl however bad it was had various tones up to max 15 bitloading or at least 13-14 (on bad days), when far from the exchange the power cutback is pretty brutal. You right its not 0, but its not much above 0. I am estimating my power cutback is around 15-20mbit lost sync speed. Even when my line had no crosstalk/fault the adsl tones had very low bitloading. Those tones have been unaffected by my high QLN.

BT Infinity 2 Since Dec 2012 - BQM

Edited by Chrysalis (Thu 05-Sep-13 09:11:45)

Administrator MrSaffron
(staff) Thu 05-Sep-13 09:18:40
Print Post

Re: FTTC speed issue - copper cable problem?


[re: eckiedoo] [link to this post]
 
If by distribution cabinet you mean the standard green PSTN telephone cab, NO

BT has a within 50m aim, 100m at the very most rule. So I suspect the poster is not referring to their propert cabinet, but rather a smaller sub cabinet that some people have.

BE BRAS tester will only give range to the exchange. Openreach will have line length info and also data from the VDSL2 modem to look at.

Andrew Ferguson, andrew@thinkbroadband.com
www.thinkbroadband.com - formerly known as ADSLguide.org.uk
The author of the above post is a thinkbroadband staff member. It may not constitute an official statement on behalf of thinkbroadband.
Standard User eckiedoo
(member) Fri 06-Sep-13 07:19:34
Print Post

Re: FTTC speed issue - copper cable problem?


[re: Chrysalis] [link to this post]
 
Morning Chrysalis

Would you clarify please, if my interpretation of your comments in conjunction with WWWombat's, imply the following.



At the customer's NTE, the signals present consist of-

1) Audio-frequency for the land-line phone, say 50 Hz to 5 KHz (plus odds and ends, Bell, CLI etc, possibly Fax), over the existing copper route from from the Exchange, via the green cabinet


2a) ADSLx equivalent, say 10 KHZ to 1 MHz, over the fibre to the new FTTC Cabinet, then the new, short link to the existing green cabinet and onward over the old local customer line to the customer NTE, commoned up with the Audio above

2b the extra, higher frequency BB signals or tones, say 1 MHz to 2 MHz, over the fibre etc as in 2a.


That is, the VDSL consists of 2a +2b.

----------------------------


The VDSL combination is set to an Attenuation level equivalent to what the original ADSL coming over the copper from the Exchange to the green cabinet had been.

Is that an on-site adjustment at the FTTC cabinet, during VDSL installation specific to that customer, or is it a general setting in the FTTC cabinet equipment, based on the general distance to the Exchange, eg a Look-up table.?

So there is no apparent distance-based advantage of installing fibre/VDSL, only that from the extra higher frequency of 2b above, which disappears more rapidly over the final copper to the customer, due to the greater attenuation characteristics of higher frequencies?

-------------------

When VDSL is installed, it looks as though there should be an HF Rejection filter installed in the green cabinet, on the Exchange side of the existing copper, if there is space. Is that correct or does the VDSL radiate back towards the Exchange, "unrestricted"?

Thanks in advance.
Pages in this thread: 1 | 2 | (show all)   Print Thread

Jump to