Thread: Why is it so much faster

1. Electricity doesn't necessarily travel at the speed of light, but it's speed is significant. Not enough to explain bandwidth.

I remember an old friend telling me that a fiber optic cable could basically replace a wall of electric wires.

Why are fiber optics so much faster? Is it because light travels so much faster? Is it because you can switch the signal on and off so much faster? A combination of both?

2.

3. IIRC, electrical signals travel at about half the speed of light. Also, you can pack the bits of information more densely in fiber optics.

4. Originally Posted by mjr150
Electricity doesn't necessarily travel at the speed of light, but it's speed is significant. Not enough to explain bandwidth.

I remember an old friend telling me that a fiber optic cable could basically replace a wall of electric wires.

Why are fiber optics so much faster? Is it because light travels so much faster? Is it because you can switch the signal on and off so much faster? A combination of both?
That depends on what you mean. The electrons in an electrical wire travel at only a few meters per second -- basically at walking speed. The associated electromagnetic wave is much faster -- speed of light.. (Think about a long tube filled with pingpong balls. Push on one end and a ball quickly pops ouy of the other end, even though no single ball moves very quickly or very far.)

What is important in this question is not speed per se, but rather bandwidth. Light, because t is of high frequency is also capable of higher bandwidth than the frequencies usually associated with "electricity". Remember that light is an electromsgnetic wave too.

5. Originally Posted by mjr150
Electricity doesn't necessarily travel at the speed of light, but it's speed is significant. Not enough to explain bandwidth.

I remember an old friend telling me that a fiber optic cable could basically replace a wall of electric wires.

Why are fiber optics so much faster? Is it because light travels so much faster? Is it because you can switch the signal on and off so much faster? A combination of both?
The amount of information a signal carries determines its bandwidth. Bandwidth determines how far apart in frequency different signals have to be in order to not interfere with each other. There are limits to the range of frequencies that normal wiring can carry, so there is a limit to how many different signals a single wire can transfer without cross interference.

The range of frequencies in visible light, and thus the range of the frequencies a fiber optic cable can carry, is magnitudes larger. Thus a single fiber optic cable can carry many many more separate signals at the same bandwidth without cross interference than a wire can and you need fewer of them to carry the same amount of signal traffic.

6. little question....can you find out how frequencies (of electric) will interfere by the same means that light interferes?

7. Dispersion, or the different speeds of different frequencies down the same material, is the killer. Too much dispersion and the high frequency parts of the bandwidth get out of step with the low frequency and the signal turns into mush.

In most materials if you choose a very high base frequency you can reduce the dispersion for a given bandwidth. So a copper coax with a bandwidth of 50MHz, running from 0 to 50MHz, will suffer badly from dispersion. However, the same cable will have a lower dispersion if using 200MHz-250MHz to get that same bandwidth. But you then hit another problem - loss in the cable gets higher with higher frequency, so the signal may not turn into mush so readily, but it is much weaker.

You rebuild the signal every so often with repeaters, but these cost so you want as few as possible.

Modern fibre-optics have low loss and with signals in the 500THz region, the dispersion can be negligible for bandwidths in the near GHz region. You have to watch the optics, though, as if the diameter of the fibre is too great signals bounce from side to side instead of travelling down the middle and you get several different path lengths leading to another form of dispersion and mushing up the signal , so fibre optics are very tightly designed. However, you end up with longer between repeaters or much greater bandwidths.

The exact numbers may be off - I have been out of the business a very long time - but that's the general problem.

8. Good stuff. Thanks

9. Originally Posted by Janus
The amount of information a signal carries determines its bandwidth. Bandwidth determines how far apart in frequency different signals have to be in order to not interfere with each other. There are limits to the range of frequencies that normal wiring can carry, so there is a limit to how many different signals a single wire can transfer without cross interference.
To my knowledge there are a few more factors to the equation, it's the noise/inteferance, that often needs a redundant, best a dynamic redundant.
Also the modems/routers, how fast are they, is the packages compressed or uncompressed? If the packages are small, uncompressed transfer is faster ..etc.

10. IIRC, electrical signals travel at about half the speed of light. Also, you can pack the bits of information more densely in fiber optics.

Electric signals are elctromagnetic waves. They should "travel at lightspeed" (for copper, aluminium etc) and also do this.

The reason they seem not to is that there are two wires for an electric circuit. This doubles the distance as two wires run together packed as one cable. So actually they still do but the distance has to be doubled because of this. The ratio meters/sec otherwise comes out as only halve.

Optic cables don,t run double.

If the wires of an electrical circuit are layed out like a cirkle from one pole to the other (ac or dc), the distance to one pole can be different then to the other pole even though the poles are almost at same position (as part of one battery or dynamo).

It,s basic relativity (speed of light independant from observer) that it makes no difference where you would meassure the speed for the electromagnetic signal/energy, Closer to one pole or closer to the other : the ratio between total length of the cable and time is always the same as C for copper/aluminium etc. For every local observation withing such a system no matter where.

11. IMHO cable impedance is interesting, see http://en.wikipedia.org/wiki/Characteristic_impedance and the lossless line. Start with alternating current in a copper wire, take a step to a fibre optic cable, then to light in free space, and there's not that much difference between them. It's all electromagnetism.

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