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Someone explain how the UK telephone system works


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Old 12-02-2003, 12:52
*MikeB*
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Just a few things I want to know really. Is it correct that from my home to the exchange the cables are copper? How are the exchanges linked to each other, or to the main BT centre? Is it by fibre optic or some other type of link? My exchange has some things that look like transmitters on the side of it, they are white, and there are two of them, you see a similar type of thing on TV transmitters.

Oh and one other thing, how the HELL is it possible to have 2 voice phone lines, and adsl running down the one cable from the pole to my house? I only have one thin cable running from the pole, yet I have all 3 of these services.
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Old 12-02-2003, 13:02
Rog
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Yes, in general you are connected to the exchange by copper pairs. For connections between exchanges AFAIK BT use a variety of links, including copper, fibre-optic, co-ax and microwave (these are the white things you talk about). It all depends on the circumstances and what is most suitable for the location and terrain.

Its possible to have ADSL and voice on one line by FDM (Frequency Division Multiplexing). Voice occupies the lower frquency range and ADSL sits much higher up the spectrum, so the two dont intefere. Thats not stricktly true as you do need a filter to get rid of ADSL noises on the voice line.
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Old 12-02-2003, 13:17
GaryB
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How Stuff Works have an explanation of how ADSL works.
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Old 12-02-2003, 17:09
*MikeB*
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Originally posted by Rog
Its possible to have ADSL and voice on one line by FDM (Frequency Division Multiplexing). Voice occupies the lower frquency range and ADSL sits much higher up the spectrum, so the two dont intefere. Thats not stricktly true as you do need a filter to get rid of ADSL noises on the voice line.
Yes but I have 2 voice lines in addition to adsl. Thanks for the link BTW Gary, but I understand how ADSL works. It was more the cabling between exchanges etc that I wanted to find out about.



Mike
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Old 12-02-2003, 18:34
Rog
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The thing about two voice lines puzzles me. A colleague had 2 voice lines on one cable pair and he had to agree to give one up when ADSL was installed. I wonder if you have 2 cable pairs - i.e. 4 wires coming into your house.
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Old 12-02-2003, 20:23
Orbitalzone
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BT do sometimes fit a cable which has 4 wires inside it... presumably either becuase they feel the customer may require another line in the future or perhaps as a backup if cable failure happens with a cable in an exposed area or maybe the BT engineer had lots of that sort of cable in his van!
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Old 12-02-2003, 20:25
garryk
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Originally posted by Rog
The thing about two voice lines puzzles me. A colleague had 2 voice lines on one cable pair and he had to agree to give one up when ADSL was installed. I wonder if you have 2 cable pairs - i.e. 4 wires coming into your house.
Doubtful. Your colleague probably had the second line provided over the same copper pair because BT used a DACS (Digital Access Carrier System) box on the lines....

For more info see http://hybrid.dtmf.org/files/hybrid-files/dacs.txt

Garry
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Old 13-02-2003, 00:02
sceptre
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Garry's comment makes the most sense. It would also explain why he had to give up the second line - DSL won't work on a DACSed line.
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Old 13-02-2003, 17:38
*MikeB*
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Yeah but when we had the DSL installed, he didnt touch any cabling, I asked him will we need a second cable because we have dacs (I thought we did), he said, no we dont have dacs and quickly fitted the new socket.
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Old 13-02-2003, 20:47
media guy
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If you want some (overly) technical information about how telephone signals are multiplexed together to be sent around the world you can see my last semesters lecture notes.

http://staff.dc.lincoln.ac.uk/~ppack...ures_notes.htm

It's not greatly fun reading though.

Graham
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Old 16-02-2003, 00:40
lincsat
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We used to have 2 voice lines down one cable from the pole to the house. I checked the cable, it's 4 pair.
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