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Does the transmission medium affect the speed of a signal? For instance does light traveling through a fiber cable get a bit from A => B faster than copper can transmit a bit the over the same distance?

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Yes, but why do you want to know? Light can travel around the entire Earth in 1/10th of a second. The latency of transmission down the cable is often irrelevant compared to processing delays or bandwidth concerns. –  endolith Dec 15 '10 at 22:18
    
I think the speed of signal in copper is related to rms speed of electron that is significantly less than speed of light. –  Santosh Linkha Dec 16 '10 at 5:33
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@explorex: no: the speed of information in cables is the speed of waves inside them, but not the speed of electron, which is order of magnitudes faster. Like the wave of the see can travel much faster than the water below them –  Frédéric Grosshans Dec 16 '10 at 10:59
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@endolith: The speed of light in fiber is ~ 8 in/ns (0.2 m/ns). Certainly the processing cascade is significant (usually the largest part of the ping time, but the transmission delay is enough to measure. That is because 0.1 s is forever in computer clocks. –  dmckee Dec 16 '10 at 17:20
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@explorex: According to en.wikipedia.org/wiki/Electric_current#Drift_speed , the rms speed of elecrons in copper wire is of the order of mm/s . Really much slower than signal speed ! –  Frédéric Grosshans Dec 17 '10 at 9:29

4 Answers 4

Depends on coaxial cable's dielectric RG58 using PVC will have a Velocity of Propagation Factor(VOPF) of .66 Andrew's FSJ1 having foam dielectric has a VOPF of .78 Fiber Optic has a VOPF of .68. A pulse sent down 100' of RG58 will have 154,045pS delay, FSJ1 will have a 130,346pS delay, Fiber will have a 149,515pS delay

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Summary: A good rule of thumb is roughly 2/3 the speed of light in a vacuum for all these transmission media. Optic fiber is slower than some and faster than others.

Details:

Electrical energy normally doesn't flow inside of metals. Poynting-flow diagrams show where the energy flows, and the path(s) traveled by the information carried by that energy, although they are a bit difficult to draw.

As conqenator pointed out,

The refractive index ... Typical values for core and cladding of an optical fiber are 1.48 and 1.46, respectively.

which gives a wave propagation speed of 1/1.48 and 1/1.46 of the speed of light in a vacuum (0.67 c and 0.68 c) in these regions of a typical optical fiber.

Various popular cables have a wave propagation speed that ranges from 42% to 95% of the speed of light. In particular, 100 Ohm Cat5e cable has a typical propagation speed of 0.64 c. The 300 Ohm Twin-lead has a typical propagation speed of 0.82 c.

dmckee reminded us that it's usually adequate to approximate all of these as around 2/3 c.

So if you use Cat5e cable, bits will arrive slightly later than bits sent through fiber optic. If you use 300 Ohm Twin-lead, bits will arrive slightly before bits sent through fiber optic.

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Well, definitely yes, i think. Depending on properties of the cable as well as the Relative permittivity(also measured as dielectric constant) of the material surrounding the copper is a factor.

EDIT: The velocity factor also called wave propagation speed or velocity of propagation (VoP or vP), of a transmission medium is the speed at which a wavefront (of an acoustic signal, for example, or an electromagnetic signal, a radio signal, a light pulse in a fibre channel or a change of the electrical voltage on a copper wire) passes through the medium, relative to the speed of light. For optical signals, refractive index is a similar quantity.

Twisted pair copper cabling are known to transmit signals at speeds anywhere between 40% to 70% of that of light. So clearly, there are a lot of variables that come into play here that vary with material used, methods of cabling etc. See this table for reference.

As for your question, a quick wikipedia lookup for fiber optic cables reveals this..

The typical value for the cladding of an optical fiber is 1.46. The core value is typically 1.48. The larger the index of refraction, the slower light travels in that medium. From this information, a good rule of thumb is that signal using optical fiber for communication will travel at around 200 million meters per second.

Which at first glance, might seem like faster but does not necessarily mean "always faster" than any/all types of copper cables.

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The medium does affect the speed of propagation, but I don't have detailed information on the technologies currently in use.

The rule of thumb is about 2/3 of the speed in vacuum for either coax cable or fiber. No idea for twisted pair.

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+1. In the paper "measuring the speed of light using ping" [arxiv.org/abs/physics/0201053], there is some discussion on the propagation speed for different cables. –  Frédéric Grosshans Dec 15 '10 at 17:51
    
The paper mentioned in the comment above is useful, but the link has a typo. Here is the correct link: arxiv.org/abs/physics/0201053 –  codingoutloud Aug 4 '12 at 17:25

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