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There is another question with this same title, but the question is asked differently than what's troubling me, and the answer is not sufficient.

The most prominent analogies I hear to explain bandwidth are the highway example, and the pipe example. In the highway example, bandwidth is the amount of cars that can drive on the highway in a given amount of time, and in the pipe its an amount of water that can flow through.

My question is - by measuring by cars per second, or liters per second, does that mean that a longer highway, pipe or copper wire has a higher bandwidth than a shorter one? That seems strange to me.

Wouldn't it make more sense to give the highway bandwidth as the amount of lanes it has - irrespective of a unit of time? It just makes more sense to me and is simpler to say that the pipe is "1 foot in diameter" rather than "it carries 100 litres per second".

Why do we measure bandwidth in bits per second and not just in bits?

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CodyBugstein
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  • It's the nature of the process. You can't just say - I've got *n*-bits of bandwidth. The immediate question is, over what period of time. – ChiefTwoPencils Feb 24 '15 at 10:13
  • I'm voting to close this question as off-topic because I believe it is easily googleable. This is not a place to have your research done for you. – Chuck Vose Feb 26 '15 at 03:15

2 Answers2

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"My question is - by measuring by cars per second, or liters per second, does that mean that a longer highway, pipe or copper wire has a higher bandwidth than a shorter one?"

No!

Bandwidth is not about how many cars can fit on the road. It's about how many cars can pass a point on the road during a certain time. How many cars per second can pass under a bridge, for example.

Martin G
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  • Oh I see... so bandwidth is how many bits could be put on the line in one second? If it takes 3 seconds for the signal to reach from one end to the other, can there be `3 x bandwidth` data on the line? – CodyBugstein Feb 24 '15 at 11:29
  • It doesn't matter how long it takes for a car to go from A to B (signal to pass through the signal channel). What matters is how many cars per second passes a POINT. Measuring over a second gives you an average over a second. It doesn't say anything whatsoever about how long the road is. – Martin G Feb 24 '15 at 13:08
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No, it wouldn't. You quote a highway in terms of lanes, because it's more understandable, and a reasonable approximation to assume 4 lanes = 4x as much traffic. But even then, you might have a traffic jam, and then 4 lanes is 'transmitting' fewer cars per minute than it would otherwise.

With a hose pipe, the width of the pipe is the speed of transmission if you assume the same water pressure.

These assumptions don't apply to communications - when I'm transmitting 'a bit' nothing physical is moving *. A 'bit' is the smallest piece that 'information' can be broken down into, and in order to transmit it, something needs to change.

If I turn on my torch and shine it at you, I've sent one 'message' (my torch is on). To send you anything more detailed, I would need to turn it off and on again - morse code is an example of doing this. The pattern of switching it off and on gives you some letters. How fast I can switch it off and on again, is how fast I can send a message.

So it is with bandwidth. I need to change things to communicate. If I can change things faster, I can communicate faster.

"bits" would be a measure of the number of torches I own. Bits per second is how fast I can flick them on and off to send a message.

* Electrons and photons do move, as does air to carry sound. But the signal isn't the thing moving - I don't have to move an atom of air from my mouth to your ear to 'talk' to you, the wave propagates through the medium.

Sobrique
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  • This is really insightful, thanks. If the destination is two seconds distance away (I guess `2 x 186,000`km) can there be `2 x bandwidth` of data on the medium at once? – CodyBugstein Feb 24 '15 at 11:31
  • Are you taking about using a transmission channel as storage? But yes, that's about the size of it. Just bear in mind that when they come out the other end, you'd have to 'send' them again, so it's a bit like juggling. You can juggle 8 balls when you could 'hold' 2, but not many people use it as a way to carry their shopping. – Sobrique Feb 24 '15 at 13:31
  • No. Rather, when you put water in the pipe, it takes some time until it reaches the other end. In that time, you can still add more water into the pipe. So the pipe can hold many "batches" of water travelling to the destination – CodyBugstein Feb 24 '15 at 13:35
  • It's a bit of a flawed analogy, because with a pipe, you can stop the water flow. You can't stop an electron or photon from moving. But technically, yes. There will be 'signal waves' in transit on your channel - but not very many, given the length of cable you'd need! – Sobrique Feb 24 '15 at 13:39