Kitz Forum
Broadband Related => Broadband Technology => Topic started by: Weaver on October 18, 2016, 12:06:42 PM
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How much of BT’s copper access network is twisted pair? (As opposed to not twisted.) Dumb question, I know.
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All of it is twisted pair ...... :)
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Said it was a dumb question. ;D
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Except for old drop wire, and 40lb copper open wires for that matter, but not much of the latter still about. ;)
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Yeah, good point.
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Hi
Of course there is twisted pair and there is better twisted pair, as I understand it the BT normal stuff isn't twisted enough for the sort of frequencies pushed through it for ADSL/VDSL :'( It's all a hack for data over POTs. If you strip back telephone cable it isn't always obvious that it is twisted as the twists are far apart (okay for voice but not good enough for higher frequencies), whereas data cables like Cat 5, Cat 6 etc have many more twists per metre to better prevent crosstalk and interference.
(https://forum.kitz.co.uk/proxy.php?request=http%3A%2F%2Fwww.hometoys.com%2Fhtinews%2Ffeb97%2Farticles%2Fhomewire%2Ftp_wire.jpg&hash=41325a7082ca550bc31579a584eeda4e8fe3d926)
Regards
Phil
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Perhaps BT stands for barely twisted ;D
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The dropwire looks twisted CW1417 2 Pair BT ref Dropwire 10B Cable and link is
http://www.netshop.co.uk/product/820/cw1417-2pr (http://www.netshop.co.uk/product/820/cw1417-2pr)
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It has been for the last circa 30yrs, NS. Prior to that, DW3, DW6 were parallel wires. :)
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It is know wonder the drop wire 10B is very steady during 70 Mph wind gusts with those three steel strainer wires and the tension bar at the premises
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b*cat remembers when the aerial drop to his home was two separate, uninsulated, cadmium-copper wires each with its own porcelain insulator. ;)
And there is a vague memory of aerial junction (or would they have been trunk?) circuits, where a "twist" was created for each pair by swapping insulators used at either end of each pole-pole section.
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Was looking on the net as one does this link comes from a so called BT engineer says most drop wires become corroded at the top of DP (Pole)
http://www.andywatts.co.uk/work-bt.html (http://www.andywatts.co.uk/work-bt.html)
is this a true engineer experience
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b*cat remembers when the aerial drop to his home was two separate, uninsulated, cadmium-copper wires each with its own porcelain insulator. ;)
And there is a vague memory of aerial junction (or would they have been trunk?) circuits, where a "twist" was created for each pair by swapping insulators used at either end of each pole-pole section.
Hence my reference to 40lb copper wires above. ;) It wasn't at every pole, they were called transposition poles where the pairs were not only reversed but also moved around within the nest of wires. There were complicated transposition rules governing what went where. I couldn't find any UK stufff on the web but found this http://the-electric-orphanage.com/wp-open-wire-transpositioning-systems/
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I have an old but full and detailed pdf spec from B3 cable solutions for external 2 pair drop wire compliant with BT spec CW1411.
The 'lay' ie distance between twists of the cable pairs is as follows
pair 1 Orange/white 89mm
Pair 2 Green/black 95mm
The breaking load is quoted as being between a minimum of 1350 Newtons and an max of 1550 Newtons
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I couldn't find any UK stufff on the web but found this http://the-electric-orphanage.com/wp-open-wire-transpositioning-systems/
Thank you. That was an interesting read. :)
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I have an old but full and detailed pdf spec from B3 cable solutions for external 2 pair drop wire compliant with BT spec CW1411.
Snap! :D
I see that the electrical characteristics of CW1411 cable are stated to be --
- Maximum Conductor Resistance: 95 Ohms per km
- Maximum Mutual Capacitance: 56 nF per km
- Maximum Capacitance Unbalance: 275 pF per 500 m
- Minimum Insulation Resistance: 10,000 Mega Ohms per km
I suppose there is some logical reason why the maximum capacitive unbalance is stated as 275 pF per 500 metres rather than 550 pF per km . . . :-\