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twisted pairs + line resistance

djelite
Grafter
Posts: 65
Registered: 15-06-2011

twisted pairs + line resistance

Just a thought all homes usually have 2 sets of phone wires that can be used to create 2 seperate phone lines.
In areas where poor quality low gauge wires are used like here in milton keynes would it not be possible for these 2 pairs to be connected together by openreach at each junction to create a lower resistance resulting in a better line capable of a better connection?
Just a thought  Roll eyes
18 REPLIES
pierre_pierre
Grafter
Posts: 19,757
Registered: 30-07-2007

Re: twisted pairs + line resistance

most people have copper wires, a lot of MK have aluminium, which gives poor quality transmission at BB speed, I think that is why they are trialling Fibre to the house FTTP in those locations
Community Veteran
Posts: 2,106
Registered: 06-02-2008

Re: twisted pairs + line resistance

A lot of the problem with decrease in speeds is an increase in reactance and capacitance, not resistance. Both of these could actually be increased by using two pairs Smiley
Edit: Clarification
djelite
Grafter
Posts: 65
Registered: 15-06-2011

Re: twisted pairs + line resistance

Pierre most of mk will not have fibre untill 2015 or later. The estate the exchange is on don`t even have it.  The first estate trialling had problems as you say (my brother could only achieve 1-2 meg), but also not sure if it was due to alluminium or low gauge copper and distance as walnut tree a newish estate.
Had a quick word with an openreach engineer this morning and he said in some cases they have actually joined the pairs to achieve a better connection.
So suppose its one of those things that can be a bit hit and miss. Although a survey/trial into this would be interesting.
itsme
Grafter
Posts: 5,924
Thanks: 1
Registered: 07-04-2007

Re: twisted pairs + line resistance

Quote from: djelite
Just a thought all homes usually have 2 sets of phone wires that can be used to create 2 seperate phone lines.

Don't know why you believe the above is true for all houses. But if it is then a twin pair cable is only used from the house to the telegraph pole/street cabinet and as this is a short distance then paralleling the pairs will have minimum affect.
djelite
Grafter
Posts: 65
Registered: 15-06-2011

Re: twisted pairs + line resistance

Back in the days of dial up i had to have 2 seperate phone numbers one for data and one for telephone. During severall moves in totaly different counties this was achieved using the 2 seperate pairs,this and google is what led me to believe this is how it works in most homes.
Any distance can be affected and openreach have found this to work on the occasions they have tried (when there has been extreme problems), I am only going on what the openreach engineer has told me this morning.
does anything like a reactance matching unit exist?
As i said though just a thought.
Community Veteran
Posts: 6,735
Thanks: 12
Registered: 02-02-2008

Re: twisted pairs + line resistance

You misunderstand.
Many modern "drop-wires" do indeed have 2 pairs in them to allow a 2nd line to be added without running new cable, though older ones (such as the one replaced at my house a while back) only have 1.
But from the telegraph pole or whatever point back into the system you just get 1 pair allocated to you.
To enable the 2nd pair (for a separate line) the engineer will connect to a spare pair in the "bundle" terminating on your pole/cabinet.
If there isn't one it's lots of work.....
djelite
Grafter
Posts: 65
Registered: 15-06-2011

Re: twisted pairs + line resistance

thanks for that hp. i understand that its only as far as the cabinet but even that distance can have some effect surely as you will see most providers reckon that an internal extension can even effect the line and ok maybe not everyone does have dual pairs.
I am wondering if  because i had two phone lines at one point if thats effecting my connection speed  9.5 mb/s which is not too bad at all but compaired to neighbours who are getting closer to 20-21 mb/s during off peak and most of us are on same model of dslam. .
If you have un-connected wires going to exchange (i presume when i cancelled second line it was done at exchange not cabinet) can they in effect act like an arial picking up interference?  
Community Veteran
Posts: 2,106
Registered: 06-02-2008

Re: twisted pairs + line resistance

A cancelled line can pick up as much interference as it likes as this won't be passed onto your existing line as there is no physical connection between them (unless you've done this yourself inside your house)
Spare pairs in the cable from your DP (the box on the pole, close to your property) back through the cabinets to the exchange aren't there for free use, they're there for extra lines to be added in the future. A friendly engineer may well be willing to twist a few together if it provides a better connection, but there's no telling when this would be un-done by future engineers, installing new lines or repairing faults on other cables.
djelite
Grafter
Posts: 65
Registered: 15-06-2011

Re: twisted pairs + line resistance

thanks for that ben.
Community Veteran
Posts: 6,735
Thanks: 12
Registered: 02-02-2008

Re: twisted pairs + line resistance

Not to mention that the unused line SHOULD be a twisted pair, so hopefully picking up not much at all.
maximod
Grafter
Posts: 139
Registered: 15-08-2010

Re: twisted pairs + line resistance

On long lines small amounts of crosstalk (adsl)between bundled lines reduces the snr to some extent.
I have 2 lines in to the house,approx 2 years ago the second line was discontinued,the dial tone was still there up till 2 months ago.
I checked the unused line and it had no continuity between the lines and to earth,so i checked the capacitance between the line at it read 325nf !
I would say this implies the line has probably been disconected at the exchange.
Community Veteran
Posts: 2,106
Registered: 06-02-2008

Re: twisted pairs + line resistance

Lines are disconnected in the exchange normally after 6 months of being ceased by the provider, but this may be delayed for any reason.
We normally see capacitances of lines up to 300-350nF when they start reaching 55-50dB attenuation but can see them up to around 450nF - often caused by aluminium cabling and this causes major problems. I believe that this is because the higher oxidation state of Al causes a greater number of free electrons which increases dielectric polarization which causes higher attenuation in the higher frequencies used by ADSL Smiley
Edit: After further thinking, the atomic radius and Pauling radius of Al is also lower than Cu's, increasing numbers of free electrons for the same volume. The cheaper cost and lower density of Al may also mean that thicker Al cables are used in place of Cu, all increasing capacitance.
maximod
Grafter
Posts: 139
Registered: 15-08-2010

Re: twisted pairs + line resistance

Hi Ben,
I have been monitoring the bitloading of my line for some months now.
I have wound my own common mode choke with twisted pair,made only a small difference,the choke gives approx 1.5mh common mode.
I have reduced bitloading for 198khz and 252khz (Radio 4 and RTE ireland).
This looks like longitudinal imbalance,i have done a few tests lately by compensating with 100k pot across the line and a cap in series between the wiper and ground.
I can improve balance (increase bitloading) on 252khz,but 198khz requires a different balance setting !
Also i have been experiencing local mains bourne interference,that is being coupled underground between telephone line and mains that comes down the lane here.
The local interference shows as harmonic related reduced bitloading,and move frequency with temperature,i have been tracking it with my lw radio,but cannot locate it down to an exact house !
BTW Ben,my profile has been stuck at 1.5mb for a while now,could you please re set this for me ?
Thanks,
Paul.
Community Veteran
Posts: 2,106
Registered: 06-02-2008

Re: twisted pairs + line resistance

@maximod
Some nice investigation you've done there! Sounds like it might be time to get BT involved in a REIN investigation - we now have a new process whereby we can raise a fault and then push this straight to BT's REIN team without it having to be referred by an SFI engineer. Your connection logs don't look like typical REIN problems (characterised by periods of repeated disconnections interspersed with stability) but if you're seeing problems with mains harmonics then this may be more to do with the supply for the area, rather than a faulty device / cable that you could track down to a house. If you do raise a fault, then please include very clear notes to say what you've found as we wouldn't look to REIN being the cause of the issue for the reasons outlined above.
P.S. you're profiled correctly @ 1500kbps.