How do OR know which white wire is which?

Having recently moved from the well controlled environment of coaxial cable provided by VM to the rather more variable nature of the copper pair, I was wondering how OR recognise and correctly connect the balance pairs?

I've had been with VM (Nynex -> NTL->VM) for 20 years before being pushed over the edge to end up in VDSL land.

Like anybody contemplating moving to the relatively unknown (and unsafe) environment of the twisted pair, I decided to take a look at my own (twenty year old) 'wet string'.

Having a look inside my BT66 box (where my cable operator 20 years previous had taken great pleasure in cutting wires to my mater socket), I noticed that the old BT cable, coming in from an underground conduit, had ten cables. I also noticed that five of these were colour coded and the remain five were all white.

Five Pair

Luckily, these five pairs were connected to a (rather corroded) screw terminal so took the opportunity to twist the colour/white pairs and disconnect and check for conductor metal type.

In twisting these pairs I started thinking about how OR (or twenty+ years ago, BT) knew which white wire was paired with each colour.

As it's rather important to maintain an impedance balance between the pairs to reduce cross talk and noise induction, how do OR ensure the correct white/colour pair is maintained?

Cheers,

Ian

They put a tone generator on one end of the pair and listen on the other end

Tone generators and failing that seeing which other wire it was twisted with.

I think I know what you're asking.

Well you can just tell, the wires are loosely twisted. So when the insulation was originally stripped back the pairs just sort of fall together, as they are twisted that way in the factory. You've not twisted all of the whites together have you?!

If the pairs become un-twisted and you're unable to cut the cable back any further then you can use a tone at the DP end so that you can find the pair again. It makes a beeping noise on the pair that stops when you put the two wires together.

Using a tone is one way ....... but usually if you have around 250ml of pair out of the sheathing, the associated A leg (which is the white leg for the first 5 pairs (then red)) will come away with it's associated other leg together.

BTW those are the current colour codes, prior to that it was concentric cable.






(Can't open that link BTW)

In reply to a post by Icaras:

I think I know what you're asking.

Well you can just tell, the wires are loosely twisted. So when the insulation was originally stripped back the pairs just sort of fall together, as they are twisted that way in the factory. You've not twisted all of the whites together have you?!

If the pairs become un-twisted and you're unable to cut the cable back any further then you can use a tone at the DP end so that you can find the pair again. It makes a beeping noise on the pair that stops when you put the two wires together.

Thanks for the reply. Yes, I understand what you mean and that explains what the OR guy said when he took a look at the BT66 box - 'Looks like it's never been connected'. I guess if you cut the outer and just pull it up oved the cores you just have five pairs looking at you.

I'm trying to get the link in the previous post working but I twisted the cables in their original pairs (afaik).

I have a great connection (10dB down stream attenuation and 19dB SN margin) but I expect that some poor connections are caused but picking up the wrong white wire.

It must be essential to get the matched pairs together; especially for VDSL.

Anyway, the petroleum jelly seemed to be good after 20 years.

Cheers

Ian

It must be essential to get the matched pairs together; especially for VDSL.

Never a good thing, a split pair.Not quite so bad if it's just in the short length between your 66 and the UG DP joint as there is nothing else in that cable for it to pick up signal from.

If split over a long length in a larger cable, overhearing, syncing on the pair its split with signal, etc etc..... And all impossible to measure.

Andrew

I used the "sound system" out of a musical Christmas Card, for such purposes.

Caused great interest to visitors whenever I was working on the junction box in Reception.

I very much doubt there are many split-pair installations. It's pretty obvious at both ends what the pairs are because of the way they are twisted together. Any telephone technician getting it wrong would have to do so at both ends in the same way. I'd never say it could never happen, but I would think it rather rare.

In reply to a post by MrSaffron:

Tone generators and failing that seeing which other wire it was twisted with.

Hopefully this link will work..

Five Pair Twists

I can understand how a tone generator might confirm a particular wire end to end but can't see how it can be used to confirm the pairing.

Is there any field equipment that can measure crosstalk between pairs?