telephone cable

[Stephen]

Hello,

I notice that a lot of "telephone extension kits" as seen in the shops, are supplied with flat cable. Does this mean the pairs are not twisted? And if so, are there any implications to this?

I've just bought some own-brand telephone cable from CPC. It is the white round cable with twisted pairs inside. Following the advice on the earlier thread, I bought the 2-pair, 4-conductor cable. I notice there is a lot of space inside the white insulation. Is it supposed to be like this or is it because I bought a cheap brand? Does the manufacturer use the same outer insulation for the 3-pair and 4-pair cables and is this why there is space in my reel because these "extra" pairs are missing?

Thanks.

[Roger Mills]

Probably.

Yes. It's *not* suitable for extensions which will have broadband kit (ADSL modem or router) at the far end.

Dunno. CPC stuff *should* be ok. I'm sure it will work. Is the fresh air a problem?

[Peter Andrews]

If the pairs aren't twisted it can upset your ADSL signal, but of course the extension kits work fine for the audio telephone service.

Telephone cable isn't normally that tight in it's outer, don't know why though!

Peter

[Harry Bloomfield]

Stephen formulated on Wednesday :

They are not twisted and because of this are much more susceptible to external interference. You probably have broadband and this type of extension could well cause interference to the broadband signal, reducing your bandwidth.

It is always like that and has always been like that. Even the several thousand pair cable used to be fairly loosely packed.

[John Rumm]

It will pick up more noise. Probably ok on voice, might be a problem on ADSL, depending on length (a couple of metres won't hurt too much since the router itself usually has a flat lead that long!)

That is fairly normal IME.

Don't think so - there is a similar space in the 3 pair version. It allows the cable to be abused on corners and otherwise flattened a little in places.

[Dave Liquorice]

I'd say it's not suitable for use a on a line that is used for ADSL full stop. The filter filters the POTS side not the ADLS side, if this flat, untwisted, cable is used pre filter then any interference it picks up is connected directly to the modem. Depending on the level of this interference and the common mode rejection of the modems line circuitry it could have a very bad affect on the rates the ADSL will run at.

Even use post filter, on the POTS side, I wouldn't be happy about.

[Roger Mills]

Fair comment. To summarise, lets says that it's ok for extending an analog-only setup, but best avoided *anywhere* in a setup which uses ADSL broadband.

[Owain]

Yes, they're cheap shit.

Also some of the cheaper stuff has puny thin wires that don't work in the proper IDC connectors.

Owain

[Andrew Gabriel]

If packed tight, it would end up with the twists concentrating themselves into kinks, with untwisted lengths between the kinks, and also adjacent pairs locking twists against each other (each pair should have different number of twists per metre to stop that from happening over a long cable run).

Phone cable only needs a very few twists per metre in theory, but is almost always made with very much more because it's easier to make sure you don't end up with parallel runs where the twists ran off into little clumps. If you go back to hundreds of pair paper spaced cables where that's not an issue, they're something like 1.5 or 2.5 twists per metre (I can't recall the exact figure).

[John Weston]

Phone cable was two and a half turns per foot AFAICR. We didn't have metres in those days :-)

[Bill Wright]

I would think a cable with a large number of small diameter individual conductors would have to be loosely packed, or the conductors on the outside of a bend would be severely stressed.

Bill

[Stephen]

Thanks for all the reassurance. I had not thought about all the twists knotting together and flexibility for corners etc.

I don't know if there was any confusion but I'm not using the flat cable; that's why I bought this reel to replace the flat cable supplied.

Thanks again.