Fibre speed vs distance

Bill · Oct 10, 2010, 00:10:26

Somebody posted this over on tbb:

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

Makes a nonsense of the BT estimates!

Technical Ben · Oct 10, 2010, 09:00:09

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 · Oct 10, 2010, 09:15:41

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 · Oct 10, 2010, 12:12:59

But not a BT cable, Bill.

Technical Ben · Oct 10, 2010, 16:04:29

Quote from: Bill on Oct 10, 2010, 09:15:41
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 · Oct 10, 2010, 16:07:42

Bill · Oct 10, 2010, 16:17:17

Quote from: Technical Ben on Oct 10, 2010, 16:04:29
Nope. Electricity travels at "close to the speed of light". Light travels at, well the speed of light.

'Fraid not. From your link:

QuoteLight 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 · Oct 10, 2010, 16:19:17

Could I have a translation, Bill?

Bill · Oct 10, 2010, 16:23:38

Quote from: Rik on Oct 10, 2010, 16:19:17
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 · Oct 10, 2010, 16:26:23

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

Steve · Oct 10, 2010, 16:26:45

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 · Oct 10, 2010, 16:29:20

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

Bill · Oct 10, 2010, 16:35:56

Quote from: Rik on Oct 10, 2010, 16:26:23
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 · Oct 10, 2010, 16:37:11

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

Bill · Oct 10, 2010, 16:37:34

Quote from: Steve on Oct 10, 2010, 16:26:45
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 · Oct 10, 2010, 16:40:54

Glenn · Oct 10, 2010, 17:00:24

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

Bill · Oct 10, 2010, 17:00:52

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

Bill · Oct 10, 2010, 17:02:35

Quote from: Glenn on Oct 10, 2010, 17:00:24
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 · Oct 10, 2010, 17:05:43

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 · Oct 10, 2010, 17:10:58

Sounds like a BOSE ad, Glenn.

Bill · Oct 10, 2010, 17:16:17

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 · Oct 10, 2010, 17:18:18

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

cavillas · Oct 10, 2010, 17:21:10

QuoteLight 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 · Oct 10, 2010, 17:22:57

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

Bill · Oct 10, 2010, 17:24:56

Quote from: Rik on Oct 10, 2010, 17:18:18
My measure of mismatch, Bill, is when I can't get my trousers on.

The usual meaning of VSWR is Voltage Standing Wave Ratio, I suppose it could stand for Very Stretchy Waist Restraint

Rik · Oct 10, 2010, 17:26:16

I like very stretchy.

esh · Oct 11, 2010, 13:15:43

How do I find my nearest green box without hiring a helicopter?

Any suggestions?

Rik · Oct 11, 2010, 13:20:42

Ask a BT engineer if you see one is about the only way I know, esh. I'm lucky, because mine just at the top of the road, and I've seen BT working there.

Technical Ben · Oct 11, 2010, 16:37:58

Quote from: Bill on Oct 10, 2010, 16:17:17
'Fraid not. From your link:
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

Your right.

So, is it the fact that light is refracted (and so has longer to travel) that slows it down?

jm_paulin · Oct 11, 2010, 17:03:02

Quote from: Technical Ben on Oct 11, 2010, 16:37:58
So, is it the fact that light is refracted (and so has longer to travel) that slows it down?

Albert said: c is always c, regardless of your speed or anything else.
The killer was: The distance between A and B, that is in fact close to infinity....

I then switch to a logic course... when they taught me: 1 + 1 = 1

Technical Ben · Oct 11, 2010, 20:19:02

Well, you know we are all moving at C right? The only difference is how directly we move to each point. So if you feel your going round in circles you are! (Well, your atoms are anyhow)

alexwright · Oct 20, 2010, 22:29:06

Quote from: Technical Ben on Oct 11, 2010, 16:37:58
Your right.
So, is it the fact that light is refracted (and so has longer to travel) that slows it down?

No, it is due to light being a wave. The material a wave has to travel through makes a difference to the maximum velocity achievable.

Light can slow down to almost pedestrian speeds in the right material. See this link

There is also a type of radiation given off when electrons move faster than the local speed of light:
Cherenkov Radiation
The energy released slows them down to the local speed of light for the medium they are travelling through.

Rik · Oct 21, 2010, 11:25:04

I feel like I'm sitting in a physics class here. Thanks for all the technical background guys.