Fibre speed vs distance

[Bill]

Somebody posted this over on tbb:

http://www.buckconsult.co.uk/fttx/BT8cProfilesVDSL2.jpg

Makes a nonsense of the BT estimates!

[Technical Ben]

BT should just advertise their speeds "at the speed of light!". As nothing can travel faster, they will officially have the fastest line possible.

[Bill]

That would cause them problems from the pedants (who? me? perish the thought ).

Electricity frequently travels faster down a cable than light does down a fibre optic!

[Rik]

But not a BT cable, Bill.

[Technical Ben]

That would cause them problems from the pedants (who? me? perish the thought ).

Electricity frequently travels faster down a cable than light does down a fibre optic!

Nope. Electricity travels at "close to the speed of light". Light travels at, well the speed of light.



All those iron atoms, and plastic/glass molecules that get in the way may have some effect on the results though. 
http://en.wikipedia.org/wiki/Speed_of_light

[Rik]

 

[Bill]

Nope. Electricity travels at "close to the speed of light". Light travels at, well the speed of light.

'Fraid not. From your link:

Light and all other electromagnetic radiation always travel at this speed in empty space (vacuum)

My bold.

In an isolated conductor, electricity travels at the same speed ("c", approx 3 x 10⁸ m.sec^-1), in a cable it travels slower. If I remember my transmission line theory, typical values vary from about 0.7c in coax to ~0.9c in a flat pair.

In a fibre optic, the light travels at c divided by the refractive index of the glass, so typically about 0.7c

[Rik]

Could I have a translation, Bill?

[Bill]

Could I have a translation, Bill?

Sure- electricity travels faster down a typical cable than light does down a fibre optic

Doesn't matter a damn, as the data rate has nothing to do with the speed of either the electricity or the light!

[Rik]

So what's the difference between an isolated conductor and a cable?

[Steve]

Just to be awkward the speed of electricity is quite slow if we talk about the movement of electrons,it the propagation of the electromagnetic wave that travels at  the speed of light in a vacuum . The propagation is reduced as Bill states by insulation

[Rik]

I'm beginning to feel I need to go back to school.

[Bill]

So what's the difference between an isolated conductor and a cable?

An isolated conductor is a theoretical entity that doesn't exist in practice. It's a single conductor, perfectly straight, infinitely long and infinitely far from anything else. It makes the maths easier to get the basic equations, which then get complicated when you add other conductors and insulators and try it in the real world

[Rik]

And there was me thinking it was the guy at the front of the orchestra.

[Bill]

Just to be awkward the speed of electricity is quite slow if we talk about the movement of electrons

To be further awkward, you don't need electrons to carry electricity

[Rik]

 

[Glenn]

Now you have sorted that, can you explain wave guide theory please?

[Bill]

If it's any consolation, I HATED transmission line theory

[Bill]

Now you have sorted that, can you explain wave guide theory please?

Not a chance, I never understood it too well myself. Waveguide work is generally considered to be one of the black arts

[Glenn]

The radar instructors at Cosford, gave up trying to explain about waveguides to us simple Flight Systems technicians. When I went back for assimilation training (Air Comms and Air Radar cross training), we got, "It just works, that's all you need to know."

[Rik]

Sounds like a BOSE ad, Glenn.

[Bill]

Our microwave lecturer at uni was an ex-radar man. I remember one of the waveguide experiments where we spent ages getting the VSWR (a measure of mismatch, for Rik's benefit ) down to about 1.05... he looked at what we'd done, said "Yup, that's right", then mentioned that when he was doing it for real, anything below about 4 was reckoned to be quite good

[Rik]

My measure of mismatch, Bill, is when I can't get my trousers on.

[cavillas]

Light and all other electromagnetic radiation always travel at this speed in empty space (vacuum)

Is that why when my wife shines a torch through my left ear it shows up on the wall through my right ear?

[Rik]

Does she pump the air out of the room first, Alf? It's enough to make your blood boil.