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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?

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.

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.

Article C3411 - June 9, 2008 « »

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Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

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17 Comments
Ken B
June 10, 2008 7:23 AM

When you said "oldest metaphor", I thought you were going to talk about a 747 full of magtapes. :-)

Leo
June 10, 2008 9:19 AM

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I think the Bible's just a little older. Been using that for
decades. I wrote about it on my personal blog a while ago,
btw... http://leo.notenboom.org/2008/05/_new_unit_of_me.html

:-)

Leo


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Alan Stein
June 10, 2008 10:01 AM

"Bandwidth" used to refer to radio and other eletromagnetic wave phenomena. An AM radio channel (remember AM radio? 550 kHz to 1650 kHz?)was about 10 Khz wide. An FM radio channel is, I believe, about 10 mHz wide - the FM band is 88.7 mHz to 108 mHz. The greater BANDWIDTH carried higher-fidelity sound (like we need high-fidelity for Rap music?). CW (continuous wave) transmission - remember Morse Code? - needed almost no bandwidth. Since digital information is, like Morse Code, simply "on" or "off", I'm guessing that "bandwidth" is not the correct description of what's required for faster data transmission. I'm certain an electrical engineer can explain what goes through the wires and optical cables, and what permits greater speed. - Thanks - Alan

Greg Bulmash
June 10, 2008 10:48 AM

Gotta go with Alan. While the current usage of bandwidth is to describe speed of transmission or capacity (i.e. "I'll get that done tomorrow, I don't have the spare bandwidth today"), it does date back to the width of a radio band and what it could carry.

I don't see anything wrong with it evolving, though, especially when we're moving back into radio transmission with WiFi and WiMax.

Mike Curtin
June 10, 2008 12:31 PM

An FM station uses 10 or 20 KHz, which gives an audio range that's about as much as most people can hear.

To me, Analog vs Digital transmission is like using a real wood fire vs fake fireplace logs with gas. I get a warm glow from the familiar and I'll miss it, but the new way makes enormously more sense from an environmental and an economic sense. Sigh.

David Ball
June 10, 2008 12:52 PM

Two items come to mind

- First, speed tests are often poor examples of sustained download speeds. Comcast, for example, provides a much higher rate of throughput initially, then slows the download to the "rated" speed. This makes websites typically load quickly, but large files (like operating system patches) can take much longer. My cable modem is rated as 6 mbps, but will burst up to 18 mbps for typical speed tests. When I download large files, it slows back down to 6 mbps.

- Second, many websites throttle how much throughput can be sucked up by a single connection. So, even though you may have the capability to download oodles of content very quickly, the website may restrict how quickly they will send it to you.

Speed is nice, but sometimes not all it seems to be.

Derek Miles
June 10, 2008 10:20 PM

I'm sure you've told us before but here goes....
How do you find out your upload and download figures ?

Great articles and thank you.

Eli Coten
June 11, 2008 5:37 AM

Often the high "burst" readings are inaccuracies in the way the timing is measured, because it is an average it relies on the last, lets, say, 15 seconds of transmission to show your speed. If you have only been downloading for lets say, 5 seconds, a speed calculated on the last 15 seconds is going to be erroneous - and depending on the exact calculation it may come out too high or too low.

As to the bandwidth - its the number of communication channels available at any time. So a bandwidth of 10Mbps means that there is a "wide" enough pipe/cable to transmit signals that contain 10Mbits of data in a second.

Also different technologies work differently - and different technologies are available in different countries. Whilst those speeds are typical for connections in the USA, in the UK the standard DSL connection is now 8Mbps (recently upgraded from 2Mbps). Some providers are rolling out ADSL2+ here which offers upto 24Mbps, though I'm not sure anyone actually gets that.

Cable is available in the UK at speeds between 2Mbps and 10Mbps.

George Sella
June 11, 2008 3:37 PM

This is laughable. So many errors.
T1 is 1.5 Megabytes no megabits. It is writen with a capital M. T1 cost about $360 a month (Speakeasy). Basic DSL cost about $15 a month (ATT)and will bring a speed of 96KBps or 768 kbps.
You can get speed up to 6.mbps (6000kbps or 750KBps) for $35 a month.
T1 is about 15 times faster than basic DSL.
In order to getr speed of 17237 kbps one has to shell out a thousand dollars a month!!

Leo
June 11, 2008 3:56 PM

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T-1 is 1.5 megaBITS, not bytes. Cable users can easily get
bursts of 17,237 kbps without paying thousands. (FIOS users
can do even better.) Yes DSL can go as high as 6mbps, I
quoted basic DSL - the slowest - which is 768kbps.

All the costs depend on your ISP and location.

So, I'm still not sure what all the errors you refer to are.

Leo


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Leo
June 11, 2008 3:57 PM

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Derek: there are various speed testing sites around. My
favorite is http://www.speakeasy.net/speedtest/ but there
are others.

Leo


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Bevin
June 11, 2008 11:17 PM

Nice article to explain this evolution in the meaning of bandwidth, although as others have noted, its use is a little confusing. Like many other technical terms, its meaning has moved beyond its original and precise interpretation. In this case the original meaning was the width of the freqency band which a channel or service can utilise. The natural consequence of the higher frequencies used in our data communications is that a channel can have a wider bandwidth but it is essentially the higher frequency that allows for data to be transmitted faster. Hence the consequence of the faster data rates has become synonymous with the term higher bandwidth. Ah well, it would be a most difficult language to use when explaining new technology if it were not allowed to evolve, however far it strays in meaning.

And another pedanticism I highlight is that when talking about data rates, the convention is to use 'bits' not 'bytes' as the units. Hence the attempt by IT engineers to standardise on the lower case b to mean bits and the upper case B to mean bytes. I guess this is too subtle to survive in general use.

And while we are talking about conventions, the prefixes Kilo and Mega when used in computer jargon, are not the same as the standard international prefixes of the same names. The international units (SI units) mean 1,000 (10^3) and 1,000,000 (10^6) but computerspeak has them meaning 1,024 and 1,048,576. Hence KBytes (or Kbits) has 1,024 bytes (or bits), and similarly for MBytes (or Mbits). These values are the result of the binary unit (bit) having two values, 0 or 1.

Hence a byte (consisting of 8 bits) can have 2^8 = 256 distinct values; ie, the number of states (2) raised to the power of the number of bits (8). This 1,024 =2^10 and 1,048,576 =2^20

I am sure you are aware of these facts Leo and that you chose not to confuse people with the details. I guess I am a purist, and think it important that if people are wanting to learn about using computers, then they are intelligent and motivated enough to be told some of the basic facts.

Please keep your enthusiasm for your site, it is excellent source of information.

Ed Vance
June 12, 2008 9:21 AM

I used to use a Teletype(C) Machine a lot.

Way back then Bandwidth was referred as

BAUD or BAUD RATE.

Switching terms now would be confusing for those who use the term Bandwidth.

This article stayed in my mind and I just had to post a comment.

Thanks Leo for being there for us.

Riff
June 12, 2008 4:25 PM

When I was teaching basic IT to telemarketers the best analogy I could come up with was the water system.

The bigger the pipe you have coming into your house the quicker you'd fill your kettle etc.

The original question was how does it compare so big numbers = more water = good :)

Good article

Chris
June 14, 2008 3:51 PM

I don't know if one of the comments covered this, but most downloading programs show KB/s (kilo-bytes per second) and most speed tests will show kbps (kilo-bits per second)...

A byte is 8 bits, so to show your download speed, simply divide your speedtest's kbps by 8 to get KB/s...

17237 kbps = 2 MB/s Down
1615 kbps Up = 202 KB/s Upload

so kbps to mbps, divide by a meg (1000-units)
kbps to to MB/s, divide by a meg (1000-units) then divide again by a byte (8-bits)

Walt Webb
January 29, 2009 9:24 PM

Thanks Leo your explanation was helpful in trying to get the meaning of bandwidth. I had just exceeded it on my blog a day or so ago. Must have been the two videos I had on my post about fighting in hockey.
Walt AKA All Sports on the Web
All Sports on the Web

Chris Ray
September 1, 2010 11:41 AM

Keep it simple.
Nice explanation which I have tried with various clients. However, one mention of kbps, binary,... eyes begin to glaze. I explain now in the following manner. "Imagine you have two one litre bottles (unit value 1), one empty, one full. I wish to transfer the contents of the full container (file) too the other. The easiest way to attain this is by utilising a funnel. If I pour the contents of the full bottle (file) into the funnel which is inserted into the empty bottle the rate of transfer will be defined by the width of the outlet aperture of the funnel. Bandwith. If I increase the size of the aperture my banwidth/speed of transfer will increase correspondingly".

Or you could just say data rate.


Regards
Chrisray

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