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Speed of signals in digital phone line?

  1. May 17, 2008 #1
    [SOLVED] Speed of signals in digital phone line?

    I know that the rate of information transfer in a phone line is 64 kbit/s. I originally thought that this rate of info transfer was the same as the speed of transmission but I now know that this is not true: the speed = distance / time for signal propagation whereas the rate of info transfer = no. bits per page / time to send one page.

    Is the speed at which a signal travels down a phone line a set value which I should know?

    I'd really appreciate any help! Thanks
     
  2. jcsd
  3. May 17, 2008 #2
    sorry! got the wrong sub-forum...
     
  4. May 17, 2008 #3
    [SOLVED] Speed of signals in digital phone line?

    1. The problem statement, all variables and given/known data

    Context: an example of signal transmission - fax and the use of phone lines.
    Question: State the approx. speed at which the signals travel.

    I know that the rate of information transfer in a phone line is 64 kbit/s. I originally thought that this rate of info transfer was the same as the speed of the signal but I now know that this is not true: the speed = distance / time for signal propagation whereas the rate of info transfer = no. bits per page / time to send one page.

    Is the speed at which a signal travels down a phone line a set value which I should know?

    I'd really appreciate any help! Thanks


    2. Relevant equations

    included above

    3. The attempt at a solution
     
  5. May 17, 2008 #4

    dst

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    It's an electrical signal so it travels at the speed of light.
     
  6. May 17, 2008 #5
    Electricity at the speed of light? what about resistance, propagation delay, and/or bandwidth? Dont these things factor into the equation? If you really think about it... telephone travels thru copper til it either reaches fiber/microwave or a switch.. there has to be some loss there but you would have to measure with Time Domain Reflectometry or some such. I never assumed electricity to travel at the speed of light because of the medium thru which she travels has resistance. (ie. copper).. fiber may be a different story, however the end to end encoding/decoding of the signal is surely slower than light itself.

    Regards,

    -map
     
  7. May 17, 2008 #6

    dst

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    Spherical chickens in vacuums, spherical chickens in vacuums.
     
  8. May 17, 2008 #7
    When you talk to someone, you say a certain number of words per minute (analogous to 64 kB/s), but that has nothing to do with the speed of sound - which is what is actually transferring those words.
    We have sound in the air, and in phone lines we electromagnetic waves; how fast do they travel?
     
  9. May 17, 2008 #8

    Chi Meson

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    It's not electromagnetic waves, per se, traveling through copper phone lines. It is a pulsing electric field. Now there's a connection between these things, but electromagnetic waves are made up of photons, and phtons do not travel through phone lines (optic fiber, yes, copper, no).

    Yet it is the same limitation. The electric field does not establish itself through the copper wires instantaneously, and so the modulation (oscillation of the strength) of the field takes time to get from one end to the other.

    The speed at which this takes place is nearly the speed at which those electromagnetic waves travel.
     
  10. May 17, 2008 #9
    dst how can an electron travel at speed of light? As far as I know, thats impossible even if we ignore stuff like resistance...
     
  11. May 17, 2008 #10

    Redbelly98

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    I would say that electric field pulses traveling along the wiring are electromagnetic waves, and they travel at the speed of light in the surrounding medium.
     
  12. May 17, 2008 #11
    The electrons drift quite slowly, on the order of millimeters per second, but the electromagnetic wave propagates at a significant fraction of c in copper wire.
     
  13. May 18, 2008 #12
    thanks v. much!! so the answer is: roughly (a bit under) the speed of light
     
  14. May 18, 2008 #13

    cristo

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    This probably doesn't read well, since I've merged two threads on the same questions together, so don't blame me!!
     
  15. May 18, 2008 #14

    Chi Meson

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    I was wondering;"What the ..... where did those posts come from?"
     
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