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What limits the speed of information through a copper wire?

  1. Mar 16, 2015 #1
    What limits the speed of information through a copper wire? What is the speed of signal in optic fibres? Is it faster and why?
     
  2. jcsd
  3. Mar 16, 2015 #2

    sophiecentaur

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    There are two issues here. There is the speed at which an electromagnetic wave can travel along the wire, which depends upon the diameter of the wire and the materials surrounding it (insulation). The upper limit for this is, of course, c. But in most situations, the wave speed will be more like 2/3c. In nearly every case you will be using a pair of wires (or coax, with inner and outer conductors and not a single wire.
    But there is another important factor when you are actually talking about the transfer of information. If you send a short pulse (flick a very fast switch on and off, for instance) then the bandwidth of the system becomes relevant. The short pulse will be dispersed into a wider, 'soggy' pulse and the receiver will need to decide on when the pulse has actually arrived, as its amplitude starts to build up. Over a short distance, where the propagation delay is short, the widening of the pulse will be the factor that limits the delay in actual information transfer. Also, the background noise (always present in all systems) will affect just when the (on / off) information can be received with certainty.

    Amazing how such a short and succinct question can open such a can of worms . . . .
     
  4. Mar 16, 2015 #3
  5. Mar 16, 2015 #4

    sophiecentaur

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    Which bit of that link were you referring to?
     
  6. Mar 16, 2015 #5
    The bandwidth distance product... followed by record speeds over several recent years
     
    Last edited: Mar 16, 2015
  7. Mar 16, 2015 #6
    The speed of transfer is also intrinsically related to the capacitance of the transmitting cable, Before the final expected voltage shows up at the end it must charge the cable.
     
  8. Mar 16, 2015 #7

    sophiecentaur

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    For a long line, it's normal to use the transmission line equation, which involves the characteristic impedance, rather than just considering the Capacitance. Depending on the terminating load, the line may look like a resistance, Capacitance or Inductance or, in the general case, an Impedance.
     
  9. Mar 16, 2015 #8
    Makes me wonder if the biggest difference between electrical and optic transmission speed or at least density is the proximity effect?
     
  10. Mar 16, 2015 #9

    sophiecentaur

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    I am not sure whether you are talking about speed or data rate here - and I'm not actually sure which the OP really wants to know about.
     
  11. Mar 17, 2015 #10
    I should have mentioned : speed of information transfer.
     
  12. Mar 17, 2015 #11

    jbriggs444

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    A less ambiguous phrasing would be "throughput" versus "latency". You appear to care about latency.
     
  13. Mar 17, 2015 #12

    sophiecentaur

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    I think you must mean 'rate' of transfer. That is Bits per second.
    "Speed" is assumed to be Distance per unit time. Two entirely different ideas. Delay and rate can both be of importance in data systems.
     
  14. Mar 22, 2015 #13
    Haha Good pun!
     
  15. Mar 22, 2015 #14
    Interesting thread! Most here have their engineering hats on. I come from linguistics and A.I., so the limits of the "information" transfer would depend on the local "neural" configurations much more than the medium - unless that medium gave more "information" about its relevance to the content.
    Is this relevant?
     
  16. Mar 22, 2015 #15
    According to shannon's theorem http://en.wikipedia.org/wiki/Shannon–Hartley_theorem it is bandwidth and noise the main factors that affect the data throughput (bits per second) transmitted through a medium. If we care about latency , then the speed of the electromagnetic waves in the medium also plays a role. Optic fibers are better in all those 3 factors over copper wires or coaxial cables. Optic fibers offer enormous bandwidth, zerolike noise and speed of electromagnetic wave almost at theoretical maximum near c=3*10^8m/s^2. The usual twisted copper pair of wires has limited bandwidth because the signal frequencies attenuate the longer the wire is due to electromagnetic radiation and due to conversion of signal energy to heat. Coaxial cable is much better on these two factors as well as shielding the signal from external noise . Speed of the signal in twisted pair and coaxial cable is about 0.70c. As you see only optic fiber offer the absolut best in all factors.

    Now SophieCentaur can you tell me how the diameter of wire affects the speed of electromagnetic wave in it? at least using classical physics i dont see how one can arrive at this result.
     
  17. Mar 22, 2015 #16

    sophiecentaur

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    Data rate and signal delay, due to path length are two different things - and you mention both. I am still not sure which the OP was asking, and that demonstrates how important the correct vocabulary is in these matters. Actual delay time between the transmission and reception of the information can sometimes be relevant but, very often, a delay is quite acceptable and can allow suitable coding that will actually increase the useful data rate. The word "bandwidth" is often mis-used when information rate is in fact what is meant (as in "broadband data"). Bandwidth is strictly an analogue term because there is no one-to-one relationship between the two. The channel noise level is equally important.

    The wave does not travel "in" the wire but is directed by the wire. Also, there are only a very few systems for signal transmission that use only a single conductor. In a practical transmission line, the wires are supported by or even encased in insulators. That will affect the propagation speed (delay).
     
  18. Mar 22, 2015 #17

    Doug Huffman

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  19. Mar 22, 2015 #18

    tech99

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    Regarding waves not travelling in the wire, an alternative view might be that a longitudinal wave of compression propagates through the electron plasma which fills the wire. This then creates the fields surrounding the wire, and in some cases produces EM radiation.
     
  20. Mar 22, 2015 #19

    sophiecentaur

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    I cannot imagine any model based on that would produce a propagation speed of more than a small fraction of c. More like the speed of sound, I should have thought. Do you have any reference to that idea or is it one of your own?
     
  21. Mar 22, 2015 #20

    davenn

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    Are you really serious ??
    I'm not familiar with any case where an EM field isn't created by the movement of electrons in a wire/conductor

    D
     
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