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Questions about BT landlines.
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Old 15-02-2019, 11:16   #1
RichardCoulter
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Questions about BT landlines.

I remember as a child we had a Post Office (now BT) phoneline. It consisted of two wires coming into the property, but for many years there only appears to be one.

Is there still two and they are now twisted together, or did they find a way to do it with only the one?

Was/is one for outgoing calls or voices and the other for incoming calls or voices?
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Old 15-02-2019, 12:38   #2
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Re: Questions about BT landlines.

Quote:
Originally Posted by RichardCoulter View Post
I remember as a child we had a Post Office (now BT) phoneline. It consisted of two wires coming into the property, but for many years there only appears to be one.

Is there still two and they are now twisted together, or did they find a way to do it with only the one?

Was/is one for outgoing calls or voices and the other for incoming calls or voices?

No.
Twisted pair.
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Old 15-02-2019, 12:54   #3
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Re: Questions about BT landlines.

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Originally Posted by pip08456 View Post
Twisted pair.
Thanks, I suppose twisting them together makes them more robust from things like bad weather.

Do you know why there needs to be two wires coming in/out of the property?
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Old 15-02-2019, 13:07   #4
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Re: Questions about BT landlines.

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Originally Posted by RichardCoulter View Post
Thanks, I suppose twisting them together makes them more robust from things like bad weather.

Do you know why there needs to be two wires coming in/out of the property?
A electrial circuit needs 2 wires. Twisted pair more resistant to interference not weather. External cable from pole has steel wire in as well.
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Old 15-02-2019, 13:22   #5
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Re: Questions about BT landlines.

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A electrial circuit needs 2 wires. Twisted pair more resistant to interference not weather. External cable from pole has steel wire in as well.
Well, it has to be said that there are seldom 'bad lines' any more. I once stayed in a house in 1990 where you could hear Radio 3 in the background at all times!
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Old 15-02-2019, 14:50   #6
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Re: Questions about BT landlines.

The twisted pair also has a chracteristic impedence, IIRC for telephony this is 600 Ohms. The BT cable usually has two twisted pairs in the run to the property so there's a spare pair if needed.

Older cables were figure of eight single pair and prior to that two individual bare wires from the pole in the street to a pair of insulators on a bracket on the property wall.
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Old 15-02-2019, 15:57   #7
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Re: Questions about BT landlines.

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Originally Posted by heero_yuy View Post
The twisted pair also has a chracteristic impedence, IIRC for telephony this is 600 Ohms. The BT cable usually has two twisted pairs in the run to the property so there's a spare pair if needed.

Older cables were figure of eight single pair and prior to that two individual bare wires from the pole in the street to a pair of insulators on a bracket on the property wall.
Yes, it's the individual bare wires that I remember.
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Old 06-03-2019, 13:19   #8
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Re: Questions about BT landlines.

The original wires were single non-insulated, hence the rows of insulators that used to be seen on poles. Some heritage railways still have them.
https://goo.gl/images/PgMh43

Then BT went to a “shotgun” type cable similar to what Virgin uses now (two insulated single wires). With the advent of dial-up internet & the requirement for more than one line at a property, the twin pair cable was introduced. Cables linked to poles now usually have 2 or 4 pairs.
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