Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Telephone wiring questions

  1. Aug 11, 2012 #1
    Hey everyone... Not sure if this is the right section, but here goes..

    In studying the telephone network, I've been wondering.. I know that connected to the land line telephone in your house, is a twisted pair cable that leads from your house, meets up with a few lines from the neighboring houses and then into a big wide shielded pipe filled with every one else's telephone cables. This large pipe is usually underground, and leads to the central switching office for your town. My question is, what happens if the direct line from your house to the central office breaks somewhere in the thick bundle of telephone piping. How could the telephone company even find the broken wires amongst thousands of others? And how can they give you a new one? they just can't open up the pipe that probably goes for a few miles, and slip a new wire in.

    Also anyone think it's illegal to go into the manholes marked BELL TELEPHONE? I've always wanted to see whats down there!!
     
  2. jcsd
  3. Aug 11, 2012 #2

    chiro

    User Avatar
    Science Advisor

    Hey Newtons Apple.

    It's going to depend where the break happens. Also be aware that the standards of cabling is different along different parts of the connection: from your house to the actual exchange.

    Also if your connection ends up being so damaged to one of the hubs outside or connections near your house, then typically what will happen is that a new pair will be used and your phone will be based on that pair. There are so many of these available (and this is why you can get say a 2nd line if there are enough pairs to 'go around').

    If for some reason the connection screws up in the protected cabling area, there is going to be a lot to worry about (like the situation where you have a major power outage as a result of something).

    Also I don't know about opening up a hub or getting into pipe access for an actual network, but even if you get in there and don't get caught, one thing is for sure: you won't want to screw things up! (You'll be in deep for that).
     
  4. Aug 11, 2012 #3
    Time Domain Reflectometry (TDR).
     
  5. Aug 11, 2012 #4

    AlephZero

    User Avatar
    Science Advisor
    Homework Helper

    Wires don't often spontaneously break somewhere in the middle (though they sometimes get chewed by rodents). Most likely the break would be a faulty connection somewhere, and of course the connections are accessible otherwise they couldn't have been made in the first place.

    As pantaz said, TDR is used to measure the distance to the break, though that can still leave the problem of where the wiring actually goes. I had a fault on my phone line a few years back and the engineer was scratching his head as to why there should be a fault 300m away when the junction box was 100m away and looked perfectly OK. The problem was that the wires actually went to a different box (in the other direction down the road) from where his plans said they were supposed to go!
     
  6. Aug 21, 2012 #5
    I've been fascinated by phones and phone wiring for years, and have worked on them many times as a facility engineer. Everyone above have pretty well answered your questions--there are spare pairs in the bundle if needed, and normally the problems occur at junction points. What I always thought was cool is the color code used with phone wiring, which has probably been around for 75 years. There are 10 colors that need to be memorized, and with those 10 colors one can find the proper pair in amongst 100's or 1000's of other wires. There are 5 wire (tracer) colors, and 5 field (background) colors--all in a specific order. The wire colors are: blue, orange, green, brown, and slate (gray). The field colors are white, red, black, yellow, and violet. Each pair of wires use a wire and a field color, alternately, like this--the first wire of the first pair is white with a blue tracer, and the second wire is blue with a white tracer. This is the color code for Line 1 of your house. The next pair is white/orange, orange/white (Line 2). Then white/green, green/white...etc. 25 pairs of wires or 50 total wires can be uniquely labeled this way in one bundle.
    What if you have a cable with 100 pairs? Then each group of 25 pair is wrapped with a thin ribbon of tape using the same color codes. The first 25 pair group is wrapped with a white/blue ribbon. The next with white/orange, and the next with white/green, then white/brown. If the cable has a 1000 pairs, the same system applies, repeating as needed. This means that, without having to 'tone out' the wires, you can easily find each end of a unique pair seperated by miles. It's a really cool and simple system.
     
    Last edited: Aug 21, 2012
  7. Sep 1, 2012 #6

    Bobbywhy

    User Avatar
    Gold Member

    NeuronsAtWork, thank you for the great description of the phone wiring color code!
     
  8. Sep 1, 2012 #7
    Bobbywhy--it was my pleasure. Something else I found out--the reason they chose those particular colors was because those were the most common, longest lasting, and highest contrast fabric dye colors that existed at the time. All of the wire insulation was fabric, not plastic like today, so they needed to find dyes that would survive the harsh conditions and long service life. Again, pretty neat!
     
  9. Oct 13, 2012 #8
    25-pair color code

    25-pair color code
     
  10. Apr 26, 2013 #9
    In the UK the telephone company places in lots of spares for just this reason. Laying new cable is expensive compared to swapping over pairs at each end.

    The cable has a careful lap that minimises crosstalk between any two pairs. Damage seems to be crackling on the line (from dry joint or possibly damp) more often than breaks.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook