Telephony query

I just thought, under conventional telephony technology, i.e. pre-21CN, when you talk on the phone, you are using sound waves - right?

As these travel at 770 mph (in air *), I can't understand why we don't hear a delay when we talk to someone (say) 500 miles away, as this should cause aprox a 45 minute delay and this doesn't seem to be happening

I realise that the 21CN transfers voice digitally and an electronic signal is supposedly faster than sound?

I just happened to think of this query whilst I was reading an article about the speed of light the other day.

Any comments anyone?



* I'm not sure if this speed is the same travelling down a telephone line (see http://en.wikipedia.org/wiki/Speed_of_sound )


p.s. I didn't know where to post this topic as there doesn't seem to be a board category that is suitable.

No they are travelling as electromagnetic waves - 3x10^8 ms-1 or 186000miles per second multiplied by the factor for the cable ... about 0.65 (ish) for copper and 1 for fibre optic. And yes there is a delay - about 0.1 second to the US when switching is allowed for. But on UK internal calls it is not noticeable.

Edited by MHC (Mon 31-Dec-07 08:19:08)

And of course the delay is in both directions so imperceptible..

Sorry, can't resist. Strictly the electromagnetic wave carrying the signal is not travelling in the copper, so it's not the copper that determines the speed. Instead the wave is travelling outside the copper, in the insulation and surrounding medium. So the speed of the electric signal is determined by the shape and materials used around the copper, not the copper itself. The closer to a vacuum the better. Two air spaced wires achieve around 95% of the speed in a vacuum. Coax cable with foam insulation can achieve around 80%. Twisted pair is typically around 65-70%.

Optic fibres are also made of solid material (not a vacuum), and so the speed of light (which of course is also an electromagnetic wave) in the fibre is also slower than in a vacuum. It's typically around 70%, depending on the type of fibre - very similar to twisted pair speeds in fact.

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No they are travelling as electromagnetic waves - 3x10^8 ms-1 or 186000miles per second multiplied by the factor for the cable ... about 0.65 (ish) for copper and 1 for fibre optic. And yes there is a delay - about 0.1 second to the US when switching is allowed for. But on UK internal calls it is not noticeable.

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Thanks for replying.

I wasn't going to post the query because it sounds daft but I'm glad I did now.

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Strictly the electromagnetic wave carrying the signal is not travelling in the copper, so it's not the copper that determines the speed. Instead the wave is travelling outside the copper, in the insulation and surrounding medium.

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I didn't know that.

I assume that broadband signals travel in the same way i.e. travelling outside the copper in the insulation?

p.s. I was obviously mistaken thinking that sound waves are used.

Cheers.

how can that be true, the electromagnetic wave must travel down the copper, if it were to travel down the insulation, every line in the cable would have cross talk on them. also cables perform better in extreme cold than in a vacuum, par se.

Edited by yz450 (Mon 31-Dec-07 17:27:35)

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how can that be true, the electromagnetic wave must travel down the copper, if it were to travel down the insulation, every line in the cable would have cross talk on them

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You're thinking about the electrons rather than the electromagnetic wave (consisting of photons). Electrons are confined to the copper, but they travel extremely slowly and don't carry the electrical energy themselves. The energy travels in the electromagnetic field surrounding the copper. Copper (and metals in general) are very good at absorbing electromagnetic waves rather than conducting them. Any photons entering the copper get absorbed in a very short distance, ending up just heating the metal. That's why metals are very effective for electric shielding for example.

As long as the wires in a pair are close together and the frequency is fairly low then the wave is concentrated very close to (but not in) the copper, the majority travelling within the insulation. As the frequency increases the wave spreads out further. That's actually what causes crosstalk. If the wave travelled within the copper then there wouldn't be any crosstalk at all.

Conversely within a coax cable the wave travels entirely within the insulation between the shield and the central wire (but still not in the copper itself), so crosstalk doesn't occur. The outer copper shielding in the cable prevents the wave from escaping.

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Conversely within a coax cable the wave travels entirely within the insulation between the shield and the central wire (but still not in the copper itself), so crosstalk doesn't occur. The outer copper shielding in the cable prevents the wave from escaping.

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So the electrical signal travels through the insulation and is prevented from leaking out by the conductor. Point well made. I think the electrical engineers around here are going to suffer, now that that one is out.

Your a wealth of knowledge and I do enjoy reading your replies