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by MarkusWandel 1650 days ago
Light is slow, not exactly 1 foot per nanosecond but it's a reasonable mnemonic. Think about that, if you had an optical cable running a reasonable (these days) 40 Gbits/second link and it's 3 feet long, that's 120 bits in transit from end to end at any given moment. And that's if it is optical and ignoring the slightly slower speed of lighit in glass; any sort of actual wire is slower.

For a transmission line i.e. one limited by distributed inductance and capacitance, the standard practice is to drive a powerful transient into one end, which then travels along the wire, switching any receivers along the way, and then absorb it in a termination resistor at the other end so it doesn't bounce back and glitch the receivers on the return trip. Again you have to consider an actual traveling wave; there's no such thing as "instant" on a wire of any length.
3 comments
throw0101a 1650 days ago
> Light is slow, not exactly 1 foot per nanosecond but it's a reasonable mnemonic.

"Admiral Grace Hopper Explains the Nanosecond" is a good illustration of this (segment is only 2m):

* https://www.youtube.com/watch?v=9eyFDBPk4Yw

* https://en.wikipedia.org/wiki/Grace_Hopper

Full lecture:

* https://www.youtube.com/watch?v=ZR0ujwlvbkQ

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perlgeek 1649 days ago
> Think about that, if you had an optical cable running a reasonable (these days) 40 Gbits/second link and it's 3 feet long, that's 120 bits in transit from end to end at any given moment. And

Since glass[1] has an refractive index of roughly 1.5, speed of light in a glass fiber is roughly 2/3 foot per nanosecond, so you actually have 180 bits in transit.

[1]: at least most kinds of glass used in optical fibers and windows, at the wavelenths typically used in optical communications, which tends to be roughly in the range of 800nm to 1500nm

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m348e912 1650 days ago
>>Light is slow

I am going to have to go ahead and disagree with you on that

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layer8 1650 days ago
Most stuff in the universe is moving apart faster than light, so yes it’s quite slow. ;)
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userbinator 1650 days ago
...and from the perspective of a modern computer and high-speed digital circuits, the speed of light doesn't seem that fast --- light moves only 30cm between each cycle of a 1GHz clock.
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