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What limits digital data throughput in a wire?

Discussion in 'Electronic Basics' started by Eric R Snow, Jan 15, 2004.

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  1. Eric R Snow

    Eric R Snow Guest

    I've been thinking about this but haven't come up with a satisfactory
    answer. If a voltage pulse is sent down a wire does it get
    lengethened? E.G. if a one picosecond pulse is sent down a wire will
    it be longer at the recieving end? Is this why digital information
    can't be sent in unlimited quantities? Or is it because the pulses are
    in essence half wave ac? So the current eventually travels on the
    outside of the wire? Just curious.
    Thanks,
    Eric
     
  2. John Larkin

    John Larkin Guest

    Yes. Any real wire or coaxial cable has resistive and maybe dielectric
    and non-TEM mode losses. This causes amplitude loss, dispersion, and
    ultimately, for long runs, Johnson noise that buries the signal.

    Dispersion results in different frequency components traveling at
    different velocities, which spreads out fast pulses and makes adjacent
    pulses start to overlap, which is called "intersymbol interferance".

    A true 1 ps pulse can't be propagated even a foot on high-quality coax
    without severe loss and distortion. HP and Tek could make 200 GHz
    sampling scopes, but anything above the current 70 GHz or so is mostly
    useless, as you can't get a signal that fast to the scope over a
    reasonable length of cable.

    Fiber optics is *much* better, especially for long runs.

    John
     
  3. Roy McCammon

    Roy McCammon Guest

    Signal to noise ratio and bandwidth are the ultimate limits.

    Yes, but that is a linear effect and can be equalized out.
    Its called dispersion.

    In long haul communication it is typical to use a protocol
    that has a net 0 dc component.
     
  4. Another factor is crosstalk, in the case of Cat5 cable it's both NEXT
    and FEXT. Right now, most PCs are sold with Gigiabit Ethernet cards
    which send 1 gigabit per second down a 100 meter or 328 foot cable.
    They are trying to up that to ten GB, maybe someday. But like he says
    below, the solution will probably be fiber to everyones' desktop.
    Well, that's why they use single-mode fiber. ;-)

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