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Why do we refer to our internet speeds as "bandwidth" with units of measurement like "10 Mbps"?

As I understand it, Bandwidth is, as the name implies, the width of the frequency band, typically measured in Hertz. So if you had a signal with a spectrum from 2MHz to 10MHz, your bandwidth would be 8MHz.

Granted, bandwidth is a factor of internet speed, but it is itself not a measure of speed. So how did we get to referring to internet speed as "Bandwidth" and giving it units of measure in terms of bits per second (which is a measure of Data Rate, not bandwidth)?

Am I misunderstanding something?

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You might find this answer helpful as regards the engineering terms What is the meaning of the word 'band' in the following terms: broadband, baseband and bandwidth?

But I believe the sociologists' answer is more to the point: because that's what millions of non-technical internet users adopted as their word for this. Also of course widely used as metaphor for "available cognitive capacity", so widely in fact that many would not regard it as metaphor.

With regard to internet speed, I'd have said the linguistic mechanism was the microcosm form of synecdoche: the part standing for the whole.

It's important to remember that specialists often have very restricted meanings for words that non-specialists use in wider or different way: compare for example what "leg" means for surgeons or "fruit" means to botanists.

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Granted, bandwidth is a factor of internet speed,

Not just a factor but the single most important factor. The maximum possible capacity on a channel of limited bandwidth and subject to additive white Gaussian noise is.

C = B log2(1+S/N)

Notice the logarithm in there, which means even a large improvement in SNR will only make a relatively small difference in channel capacity. Also note that that is a theoretical maximum, real modulation schemes, especially simpler ones will achieve lower capacities.

So for the most part historically data rate was roughly equal to bandwidth, maybe a bit higher if you were lucky enough to have a very clean channel. I believe this is why the concepts of bandwidth and data rate came to be conflated.

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