Bandwidth and Throughput

Written by: think12 Published by: Nightscript Published on: 2004-03-11 16:06:43 Topic: Networking

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This is yet another short (but useful) article spawned from a co-worker asking me. By the end of this you’ll (hopefully) know the difference between bandwidth and throughput and how to calculate both!

Note: B == Byte, b == bit (if you didn’t know of course…) Bandwidth: A data transmission rate; the maximum amount of information (bits/second) that can be transmitted along a channel Throughput: The amount of data transferred in one direction over a link divided by the time taken to transfer it, usually expressed in bits or bytes per second. (thanks dictionary.com) So basically, “bandwidth” is the figure your ISP tells you you’re able to get. i.e. “XYZnet ISP now offers DSL at 2Mbps downstream and 1Mbps upstream!” And “throughput” is when you hear “Wtf doodz, Eye can never get 2Mbps!!!111 KTHX” (sorry for the 1337 speak :-) Throughput is what you *ACTUALLY* get. Many things can affect why you don’t get the speed your ISP tells you. broadcast traffic collisions routing protocols etc So anyways, let’s pretend you have an advertised 3Mbps downstream Internet connection and you want to transfer a 700MB ISO. Note: I’m using this type of ideal example for simplicity) We want to find the time it’ll take to transfer, aka “transmission time”. Before we can do that, we must convert everything to a common unit. In our case, we’ll use bits… 3Mbps is 3,000,000 bits per second 700MB ISO is 700,000,000 bytes. Now we multiply by 8 to get it in the ‘bit’ unit. The answer is 5,600,000,000 bits. Now that we have everything in a common unit, we can find the transmission time. That is deduced through a simple formula: Transmission time (in seconds) = Size of file (in bits) / Bandwidth (in bits/second) Plugging in our values, we get 1,867 = 5,600,000,000 / 3,000,000 Converting that into something more readable… It is slightly over 31 minutes to transfer a 700MB ISO over a 3Mbps line. Alright, that was an ideal condition based on the fact you can get the full advertised line speed from your ISP. In reality, that never happens. Even in a 10/100Mbps LAN you won’t get 10/100Mbps :-) Under the same assumptions that we used previously, we’re going to find the throughput (actual line speed) for transferring that huge ISO. Just to recap, we have a 3Mbps and a 700MB file. Finding throughput is just another simple equation. Throughput (in bits) = Size of file (in bits) / Transmission time (in seconds) Assuming we timed how long a file took on our own line to transfer, we’d have a real “Transmission time”. For the sake of this, we’ll just make one up. Keeping in mind, it can’t be less than 1,867 seconds (see above) as that was quickest time that amount of data could be transferred using our 3Mbps line. (see above again). As for our guess, let’s say it took 40 minutes to transfer – which of course is 2,400 seconds. So we enter our values and get… 2,333,333 = 5,600,000,000 / 2,400 In our mock-realistic test (that’s rather ironic wording), we see our line was operating at an average of 2.3Mbps, not the advertised 3Mbps. In conclusion, I hope I was able to explain the differences between the two and give you a better understanding of what I described. This will be (if not already) mirrored on libexec.net I’ve included a little reference sheet below that is always useful. +++++++++++++++++++++++++++++ T1……….....1.536 Mbps T3……….....44.736 Mbps OC-1…..…..51.84 Mbps OC-3…..…..155.52 Mbps OC-12……..622.08 Mbps OC-48……..2.488 Gbps OC-192……9.953 Gbps OC-256……13.27104 Gbps OC-768……39.81312 Gbps +++++++++++++++++++++++++++++

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Comments: SAJChurchey 2004-03-11 16:18:05 When is OSI going to get an OC-768? Is it in the budget? :-D Domuk 2004-03-11 17:00:09 They're currently paying for mine. Whilst you think you posted it 40 mins ago, I actually read it on here an hour ago. Ph33r my OC-768! (I just haven't bothered replying 'til now) jake_null 2004-03-11 21:56:35 You always mention "mirrored on libexec.net" yet when i got to libexec.net it tells me i a whore. :( glitch 2004-03-12 08:48:46 jake_null, I don't think they're calling you a whore, I think it's a request for whores. gabbana 2004-03-13 23:36:31 It's been some time since I saw those figures, but is this taken from Cisco Academy CCNA Semester 1, Chapter 1 ? only thing missing are E1 etc lines, thou those aren't beeing used that much anymore anyway. Nice tut, good for freshing up my old brain cells think12 2004-03-14 15:35:20 glitch: You're right... It *was* a request ;P gabbana: Perhaps those values were at one point, I don't recall. They've been sitting in my home directory for quite some time now. jonny 2004-03-16 16:29:35 think12: 1Mb = (2^20)b = 1 048 576 b not 1000 000b so 3Mb = 3 * 1 048 576b 700MB = 700 * 1 048 576B think12 2004-03-16 18:08:23 No jonny, 1 Mbps == 10^6, not 2^20... Remember, this is (M)ega (b)its (p)er (s)econd Check here and here cyben 2004-03-22 19:44:31 So when your web-host claims that you have 1000 megs of bandwidth per month, they are actually using the term incorrectly? They ought to be using "Throughput" measured by month? think12 2004-03-22 20:32:21 It goes both ways, for instance... If you signed up for a shared hosting type solution, it's generally measured as X GB (GigaByte) of transfer per month. Which means you can generally 'spike' the throughput to whatever you need (though I'm sure most admins will regulate it - it'd be dumb not to :-) *until* X GB is reached. The more expensive dedicated solutions measure in X Mbps - which means you can use whatever they give you in speed, 24x7. If you were given an unmetered 10Mbps link on your dedicated box at XYZ datacenter, you could use that entire 10 Mbps pipe all day everyday. Hopefully that made sense. t12 Anonymous 2007-10-30 22:25:54 Jonny is RIGHT !! 1 Mbps = 2^20 1 Mbps = 1 048 576 bits To verify: Go to www.google.com and in the search bar type the following: 1 Megabit in bits Anonymous 2007-10-30 22:27:24 3rd line in the previous post should read: 1 Mbps = 1 048 576 bits per sec. Anonymous 2008-05-16 17:39:01 The issue is standards. What you really need to know is if you are talking about Mibi or Mega. They are different units. Mibi = 1024 Mega = 1000 When in doubt contact the manufacturer or provider, if it not on their website already. Anonymous 2008-05-16 17:43:29 Sorry, that should have been Mebi = 1024 Anonymous 2009-08-11 06:35:04 I've always used this : Tranfer Time In (Seconds) = File Size (MB) / Throughput (Kbps) X 9600 Throughput (Kbps) = File Size (MB) / Transfer Time (Seconds) X 9600 Works For Me Anonymous 2011-01-26 17:05:41 Thanks so much for exaplaining it so well! and even showing the examples, you explained it much better than all my network teachers in 2 years! ObatAsamUrat 2011-06-16 07:43:18 Very extraordinary article. I merely stumbled on your blog post along with planned to declare i get genuinely liked looking at your notions. However I shall be ever coming back i we imagine you article yet again before long. obat asam urat Anonymously add a comment: (or register here) (registration is really fast and we send you no spam) BB Code is enabled. Captcha Number: