BACK TO BASICS: What is “bandwidth?”

Bandwidth. It’s one of those terms people use without really understanding where it came from, or what it really means. We use it to talk about our connections, our devices, and sometimes even ourselves. So, let’s take a moment to understand what this term really means.

Origins of the word

In order to understand bandwidth, we have to understand how the word was originally used. That means a little bit of RF theory.

We use the word “spectrum” to describe the electromagnetic radiation all around us. Electromagnetic radiation can be thought of as different waves. Some of those waves go up and down very fast (like a million times a second) and some are much slower, like 1,000 times a second. We call the number of times the waves happen, “frequency.”

This spectrum includes visible light, X-rays and other forms of destructive radiation, as well as all radio, TV, and cellular broadcasts.

Strike up the bands

We can’t really talk about the entire electromagnetic spectrum as one thing. That’s a limitation of our own human bodies. We perceive part of it as visible light. We perceive another part by the effect it has in exciting the molecules on our skin; it makes us hot. Most of the spectrum is completely invisible to us, although at shorter wavelengths, large amounts of radiation can destroy our cells.

So, in order to separate the good from the bad, the useful from the useless, we organize the spectrum into smaller groups. You can see them in the graphic above. Broadly, we call these “bands.” There’s the visible bands, the infrared band, the broadcast band, and so forth.

But really, even that is too broad a usage. From the point of view of the government, we carve up those much bigger bands into smaller bands. These are very narrow chunks of frequency that are used for a common purpose. For example, you have the FM broadcast band which includes FM radio and television. It’s from about 50-600MHz.

Where bandwidth starts to come in

Within each band, each user gets a “channel.” A channel is a chunk of a band that can be used by a specific broadcaster. For example, a local radio broadcaster may have the right to broadcast at 100.7 megahertz. But really they are broadcasting across a wider range from 100.6 to 100.8. This lets them put a stereo broadcast out and have it look good.

See, you can’t really just broadcast on one frequency. That’s not how it works. In order to send a signal, it has to be on a range of frequencies. The wider the range, the more information can be sent at one time. Radio uses fairly little information because it’s only sound. Therefore it doesn’t need a very wide range of frequencies. However, TV has a lot more information. You know, that old saying that a picture is worth a thousand words is really true. It takes a lot more information to send high-definition video than it takes to send low-quality audio.

So how do we measure how much information we can send? It turns out that in general, it has to do with the size of the frequency range you use. (Yes, there’s fancy digital compression now but let’s not get into that right now.) So a wider channel can hold more information.

We use the term “bandwidth” as the “width” of the channel that’s used to carry that information. If you think it should have been called “channelwidth” well you’re not wrong. But that’s just not how it worked out.

Bandwidth on cables

Bandwidth on cables has the same meaning. There are cases when we are using the entire cable to transmit a signal. We call that “baseband.” Think of how old-school RCA audio cables work. Each wire carries one audio source and it can’t carry anything else.

However, coaxial cables have the capacity to carry many different channels. Your typical cable can carry frequencies from under 2 megahertz all the way to 3 gigahertz. And, if the signals on that cable are “modulated”, meaning limited to a specific channels, they don’t interfere with each other.

Bandwidth in computer networking

Here’s where things begin to get a little less precise. We tend to use words in English in ways that they weren’t always intended for. In the case of computers and networking, we tend to use the word “bandwidth” when we actually use capacity. Wi-Fi, for example, has a fixed bandwidth. Each channel is 20MHz and that’s going to carry all your networking information no matter what your internet speed is. However, when our networks are slow, we tend to call them “congested” and say they need more “bandwidth.” What we really mean is we need more speed.

This comes from the idea that there’s a fixed amount of information that can fit on each fiber line from each neighborhood. If you want faster speeds or more capacity, you add more fiber. This is akin to opening up additional broadcast bands, even though we’re not really doing that.

Bandwidth in humans

When someone says “I don’t have the bandwidth for that” it’s a techy way of saying they’re too busy, or too scattered, or too tired. It’s not a really good way of using that term but people use it anyway.

Anyway, use the word however you would like. But know that if you really want to use it correctly, you should be talking about broadcasts over the air or through a wire. Everything else is just slang.

About the Author

Stuart Sweet
Stuart Sweet is the editor-in-chief of The Solid Signal Blog and a "master plumber" at Signal Group, LLC. He is the author of over 9,000 articles and longform tutorials including many posted here. Reach him by clicking on "Contact the Editor" at the bottom of this page.