What is bandwidth?

Summary: Bandwidth is a term you hear frequently but it can be confusing and it's easy to gloss over exactly what it means.

Can you explain bandwidth to me in layman's terms? I have looked it up on the internet, but I get the standard mathematical explanation. My brain doesn't really work mathematically so I need something a little more tangible, or some examples of what is FAST and what is SLOW. For example, according to bandwidth.com, my download speed is 17237 kbps and my upload speed is 1615 kbps. I understand that means 17.237 mbps and 1.615 mbps respectively. But what does that mean? Is that fast? Slow? What do I compare it to?

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That's fast. Compared to me, anyway, that's fast. Given your ISP (from your email address) and the speeds you're seeing I'd guess you probably have cable internet.

I'm going to bring out the oldest metaphor I have to try and put a handle on how fast is fast. No math, but first just a teeny, tiny bit of computerese.

That part's inevitable.

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First, let's define the term: bandwidth simply is the speed at which data is transferred. Sometimes bandwidth is also used to refer to the maximum capacity, or the fastest, that a connection could move data.

Now let's define what you were told: 17237 kbps is 17237 "kilo-bits per second". "Kilo" is 1000, so what you're seeing is 17,237,000 bits per second.

Your math is correct: mbps is "mega-bits per second" so that's17.237 million bits per second.

For that to have some meaning, we need to understand what bits are, and how they're used to carry information.

You probably know that a bit is a single "thing" that can be either 0 or 1. Nothing more, nothing less. Everything in your computer, everything digital, everything you communicate on the network and on the internet is built on the fundamental concept of a bit. Everything. The bit is the very definition of digital.

"The bit is the very definition of digital."

Bits are commonly handled in groups of 8 called bytes. If you look at 8 bits whose possible values are either 0 or 1 each, the collection can have up to 256 possible unique combinations:

00000000
00000001
00000010
.
.
11111110
11111111

Now, when we represent text - such as the text you're reading here - on a computer, the most common way to do so is to use one byte for each character. So if I type, "The quick brown fox jumped over the lazy dog," that took 44 bytes to store all the characters, including the spaces between the words.

Now we break out the metaphor: The Bible.

Let's be clear; it doesn't matter if you believe or not. This has nothing to do with the contents of the Bible, only its size. You've probably seen one, perhaps even own one, and have a good sense for how big it feels, how hefty it might be, and how long it might take to read it cover to cover.

The Bible is a fairly sizeable common frame of reference.

You can download the text of The Bible from project Gutenberg as plain text meaning that it has only the text of The Bible, in its simplest form.

A representative copy in this digital form is about 5,000,000 bytes, or more commonly 5 megabytes.

And here comes just a little math.

Five megabytes at 8 bits per byte is, roughly, 40,000,000 bits.

On your 17,237,000 bits-per- second connection, that means you can download the entire Bible in about two and a half seconds. In the other direction you're running 1,615 kbps, so it would take you about 25 seconds to upload it.

Now let's compare that to some other common bandwidth figures and see how long it would take to transfer The Bible at those rates:

Connection Type	Common Bandwidth	One Bible Time
Common Dial-up	28kbps	23 minutes
Max Dial-up	56kbps	12 minutes
Basic DSL	768kbps	52 seconds
T-1 / DS1	1.5mbps	27 seconds
You (Cable)	17.237mbps	2.3 seconds
Max FIOS	50mbps	0.8 seconds

These are approximations meant to be examples of orders of magnitude. Your mileage will almost certainly vary and will likely be not quite as fast as the numbers above should you actually download a 5 -megabyte file. These numbers assume you have 100% of your connection available to you (which is not always true on shared resources like cable), and that the download is the only thing happening. I'm also completely ignoring any overhead caused by the way the internet and networking work in general. Typically, if you're getting within around 80-90% of these numbers, life is pretty good.

Naturally, we don't all go around downloading Bibles all day. But using that as a common physical object that translates into an easy- to -remember number of bits (40 million), perhaps that'll help give a sense of what bandwidth might mean and how fast your connection might compare to others.

As one final exercise for those so inclined, I'll point out that a data CD holds around 700,000,000 bytes, or 5.6 billion bits. A DVD? 4.7 gigabytes, or 37.6 billion bits. I'll let you do the math for your own connections, but for my T-1, that means with ideal conditions it would take me a little over an hour to download a CD and around 7 hours to download a complete DVD.