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?

In reply to a post by Eeeps:

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.

Because you put it across a pair

Any telephone technician getting it wrong would have to do so at both ends in the same way.

The definition of a rectified split pair. Also colloquially known as 'leg and leg'.

I'd never say it could never happen, but I would think it rather rare.

I've come across it maybe three times in 20 years.

In reply to a post by Eeeps:
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.
Because you put it across a pair

But he's right, just using a tone doesn't ensure the two legs are a pair.

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

Not as such, you have to have a live circuit down the pair and see how that circuit 'looks'.

The 'pair quality' tests Openreach runs check for A/C balance, if this is low it may be an indicator of a split pair .... but can be other things as well, an HR, a non twisted (but still a) pair.

It does if it's a known pair at one end. As already posted by someone, you short them together at the other end and the tone stops.

It does if it's a known pair at one end.

IF

In the OP's link That's not how the pairs look in the real world, he has added those 'cat5' twists himself.

Even then he may have made a mistake and split the legs as he has stripped the cable back and twisted the pairs. (Which he should not have been tampering with.)

As already posted by someone, you short them together at the other end and the tone stops.

Yes, I know.

In reply to a post by Zarjaz:

In the OP's link That's not how the pairs look in the real world, he has added those 'cat5' twists himself.

Yes. I was hoping the other end has known pairs

>> And all impossible to measure.

If you know what you are looking for, a mole will pick them up.

In reply to a post by Eeeps:

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?

Well because you'd hope that at one end the pair would be intact. Bear in mind that as Zarjaz mentioned if you cut the outer back and grab a white you should find its corresponding colour will come out with it as they'll still be twisted together.

If you know what you are looking for, a mole will pick them up.

Still tricky though, and worse still using the TDR function on the JDSU/Exfo ..... you need a proper 'old one'.

In the case of both the tone generator and my Christmas Card system, if they are not connected to the paired colour and white at the "near" end, the signal level at the far end would generally be obviously low, to anyone experienced in using such systems.

Almost absolute confirmation can then be obtained by twisting the "pair's" bared copper at one end; and going to the far end with a continuity tester, preferably one of the older, analogue meter variety such as an (ancient) AVO 8, as they actually rely upon a significant current flowing through the wires.

Additionally as others have said, if the outer-most sheath/insulation is removed at the remote ends, the twisting of the paired colour and white can be seen.

Edited by deleted (Fri 28-Oct-16 18:55:42)

Also, that depends if you come across either end of the split, if your POI is in the middle of the split you can be stuffed.

In the case of both the tone generator and my Christmas Card system, if they are not connected to the paired colour and white at the "near" end, the signal level at the far end would generally be obviously low, to anyone experienced in using such systems.

I'm sorry, I disagree ..... But then it should be noted that often where I'm listening is in a noisy environment, a roadside joint in a town for instance.

Oh, and despite ordering, they still don't issue musical christmas card guts as standard tool items in my line of business.

Fortunately, I was not normally working in such a noisy environment (and I sympathise with you and the many others I note at the PCPs etc), so my comment is valid generally for indoor and quiet environments, from several years of doing it, over substantial distances, between different buildings up to .5 mile apart, at heights up to 25 feet.

As the OP is apparently in a similar quiet environment ...

When you are faulting the few remaining unused pairs in a local main side, you get good at interpreting the mole or you get good at needlessly sweating off plumbs.

My tip for finding splits is the reflection will repeat on a mole. So if you get a split at 200m, you will get a reflection at 200m and also get a reflection at 400, 600 and 800m. Try it on a drum on the back of the van.

BTW, Only got old tools, if the JDSU and EXFO start graphing pressure, or detecting hydrogen, gimme a ring.

Edited by deleted (Fri 28-Oct-16 22:51:43)

Thank you, preaching to the converted somewhat.

I have the original mole I was issued with twenty years ago, still got the original leads too ! On it's third cover though.

By "mole", do you mean what I know as "Time Domain Reflectometer" or "my Pocket Radar Set"?

This would appear to be the case, given the multiple reflections at "constant pitches" of 200 M; and probably reducing signal/echo strengths.

With my TDR and an oscilloscope, I was able to demonstrate the in-phase reflections from open circuits; and the out-of-phase reflections from short-circuits, to others in my department, who had no experience of the phenomena.

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When preparing to extend thin Ethernet to about 1,000 Metres through U/G ducts, I first tested it up to 1,800 M, using both the TDR initially and also the DECServer that was going to be connected.

So I could confidently arrange the actual shorter working run, also having two cables pulled through - "just in case".

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Otherwise "mole" reminds me of the "mole-skin" cloths used for wiping the larger diameter joints, particularly in underground chambers, back in the 1950s.