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There are a lot of factors involved in downloading a file to your computer. When adding it up, however, you need to know the difference between a bit and a byte.

Hi Leo, I get a pretty good download speed of around 14mb/s. Yet when I go to download for example, a 14 MB file, why does it take so much longer than just a second? Yours confusedly.

In this excerpt from Answercast #37, I look at download speeds and methods for determining how fast a file should download.

Download speeds

There's definitely a confusion here, absolutely.

  • The speed that your internet connection is measured in is in megabits per second. So 14 Mbs (usually lower case "b") is a 14-megabit connection.

  • The file that you're downloading is a 14-MB file ("megabyte", normally that's with an uppercase "B").

The difference, of course, is a factor of eight. There are 8 bits to a byte and thus, one would expect a 14 megabyte file to take eight times longer than downloading 14 megabits.

So, it's not gonna be a second. At best (and I do mean "at best"), it's going to be something like eight seconds.

Download variables

Now, it's not even going to be eight seconds; and that's basically because nothing is perfect. A 14-megabit connection may, in fact at times, actually download 14 megabits per second – but there's more to your data than just the data:

  • There's lots of overhead in the protocol;

  • There's error checking information that is added to the data;

  • There are headers;

  • There's block information;

  • There's destination information that says, "This packet of data is intended to go to this particular computer."

A good rule of thumb that I always use is if I'm getting for a download something on the order of 80% of my rated speed, I'm pretty darned happy.

Sometimes it can go faster; sometimes it will be much slower, especially if there's other things going on online at the time.

Rough estimate...

In a case like what you're experiencing, downloading 14 megabytes on a 14 megabit connection, I would expect it to take somewhere between (I don't know...) 12 and 15 seconds.

  • In other words, definitely not one second.

  • Unfortunately, not eight seconds.

  • But something on the order of magnitude of that eight-second time: 10, 12, 15 seconds to get that kind of data coming down.

Equipment limitations

Once you get into the higher speeds that you're talking about, 14 megabits per second, then you're also starting to look at just how quickly all of the equipment involved can handle the data.

  • Sometimes, your router won't be fast enough (although that's not very common).

  • Sometimes, the upstream equipment is actually too busy to give you your full 14 megabits per second.

You've indicated that you're getting a pretty good download speed of 14 megabits per second. I'm assuming you're using things like speedtest.net to verify that; but that's really the difference. It's a very common confusion.

  • Remember that files are measured in the number of bytes they contain;

  • But your download speed is measure in the number of bits that are transferred.

Article C5613 - July 21, 2012 « »

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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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4 Comments
Hoc Nguyen Triet
July 22, 2012 1:22 AM

Thanks a lot, Mr. Leo! Have a nice day :)

Gordon
July 22, 2012 7:33 PM

Ah yes, the old bit/byte question. I remember when my family got its first computer, we had a 56 kbps modem. But I was always upset that I could only download around 3-5 kB per second. Back then, I knew so little about searching the internet for an answer. I just had no idea that a bit and a byte were different. When I finally found out that my 56k modem could really only download at 7 kB/s, it was like a light going on. I still think, however, that it is misleading for internet companies to use bits as a way to make their connection speed sound bigger than it is to the many people who don't know there's a difference.

Actually not everything that can be transmitted across a connection comes in bytes. Almost everything these days does, but that wasn't always the case. Everything, however, breaks down to bits at one point or another, so measuring speed in bits per second is actually the most accurate.
Leo
22-Jul-2012
Jason
July 24, 2012 9:29 AM

Also remember most providers only tell you the speed to where they connect to the internet... So inside their network you'll most likely get 14 Mbps, but they can't control anything outside their own network... Whether it be internet congestion, or the server on the far end, the provider can not control that... You may have a 14Mbps connection, but the other end could be at 1.5Mbps... The best you'll get is the 1.5Mbps since that is the slowest connection...

Johnny
July 28, 2012 8:46 PM

What ever happened to the 56kbs routers & download speeds that Fios introduced back in 2008-2010? I'm still not getting nowhere near that on a "good" day, other times, way better. what gives?

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