Throughput and bandwidth difference

Question

The throughput of a channel is a measure of amount of data actually moves through the channel  Why is it substantially less than the bandwidth of the channel

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Although there are already few answers to this questions but I think some people still may have doubt in actually visualising the differece b w throughput and bandwidth just like I had    until I read this analogy on quora full credits to that  which proved really helpful  Consider  A highway which has a capacity of moving  say  200 vehicles at a time  but  at a random time someone notices only   say  150 vehicles moving through it    say due to some traffic-jam in between     i e    capacity is 200 but not all the time it is fully utilised  actual traffic is only 150 out of a max of 200   i e  the bandwidth is 200 per unit time but still actual throughput is 150      I thought it might help someone

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Because there are protocol overheads  and because there are other users of the network

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The bandwidth of a link is the theoretical maximum amount of data that could be sent over that channel without regard to practical considerations  For example  you could pump 10 9 bits per second down a Gigabit Ethernet link over a Cat-6e or fiber optic cable  Unfortunately this would be a completely unformatted stream of bits   To make it actually useful there s a start of frame sequence which precedes any actual data bits  a frame check sequence at the end for error detection and an idle period between transmitted frames  All of those occupy what is referred to as  bit times  meaning the amount of time it takes to transmit one bit over the line  This is all necessary overhead  but is subtracted from the total bandwidth of the link   And this is only for the lowest level protocol which is stuffing raw data out onto the wire  Once you start adding in the MAC addresses  an IP header and a TCP or UDP header  then you ve added even more overhead   Check out http   en wikipedia org wiki Ethernet frame  Similar problems exist for other transmission media

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In most cases with  bandwidth  and  throughput  it is OVER complicated  like trying to learn calculus in one day  There is NO need for this  in MOST cases when referencing  Bandwidth  and  Throughput    All you need to know in MOST cases is this    MB  means mega  BYTES   OR 8 bits and 8 bits and 8 bits  etc  is being sent down the line  Mb means mega  bits   OR a single bit and bit and bit  etc  down the line   Example  IF your carrier says this is a  6 Mb line   it means that is the maximum Bandwidth  More succinctly it means that you ONLY are going to benefit 750 kilobytes per sec  throughput   Now why  Because the line is only sending a series of  bits   which uses 8 bits sec to create a byte  Thus  you must divide bits sec by 8 to get to bytes sec  Thus  a 6Mb line can ONLY deliver 750 thousand bytes sec   Another example  I just got a fiber optic line from A T  amp  T  and they LOVE to talk about  bits   So they advertise a whopping  100 mega bits per second   Big deal  Because that is only 12 5  MBytes per second   Remember  EACH  character  on your keyboard or printed on the screen  etc  requires 8 bits  for the other end to  distinguish  what character it is  etc   So even though I have a  Gargantuan  fiber line touted as  100Mb   it is really only 12 5 MBytes  characters  per second  100 divided by 8    Worse  MOST interchange the terms  MB  and  Mb   Worse yet  EVEN The technician that installed the Fiber Optic line and router in my home  did not know what the terms meant  So he thought  and his co-workers  according to him  believed the same  IE  That 100Mb line was a 100MB line  This is very sad   A T  amp  T reps on the phone rarely know the difference either  Even some of their supervisors do not know it either  Even sadder   To summarize   Bandwidth  uses  bits    Throughput  uses  bytes   And   one byte takes up 8 bits  So again  a 100Mb line  bandwidth  can ONLY produce 12 5 MBytes sec  throughput      For whatever it s worth

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Here is another example to help you imagine what the difference could be depending on the situation   Imagine   You have a 1 MB file  say a photo  that you would like to share with 3 friends  Each of them  however  uses different cloud storage application - Google Drive  Microsoft OneDrive and Dropbox  Therefore they all create a folder and give you access so you can upload the file to their folder   You open the URLs to these three folders in your browser  say in 3 different tabs  and drag-n-drop the file  more or less at the same time  to each tab so to upload it  You are connected to the Internet via a fiber-optic cable which gives you the ability to transfer at speed of 1 Gb per second  i e  this is your bandwidth - the max capacity that you can utilise your connection      Knowing that  theoretically  assuming no data lost  no big protocol overhead  connection to the cloud storage services offers at least the same bandwidth  etc   you can transfer one 1 MB file over the 1 Gbps connection for roughly  1MB   1 Gbps     1 x 10 3 x 8   1x10 9 which gives about 8x10 -6 seconds or say roughly 10 ms  Now  you have 1 file that you want to upload to 3 destinations  your connection bandwidth is big enough so you can transfer the same file to 3 destinations at the same time  we can also assume you have a modern laptop equipped with multi-core CPU so data transfer over the 3 connections to Google Drive  MS OneDrive and Dropbox can be done in parallel   Therefore  instead of waiting for 30ms to transfer the same file to 3 different destinations you only have to wait for 10ms as you have a very good bandwidth   Now let s consider what protocol is being used and what implications that brings  As you use your browser to upload the file  the protocol that is being used is HTTP S which is running on top of the TCP protocol  An important property of the TCP protocol is that it makes sure that a batch of data has reached any destination successfully before sending the next batch of data  That is being done by the TCP sender waiting for acknowledgement  for short an ACK  that the first batch of data has been received before it starts sending the second batch of data  What this means is that if it takes 0 5s to transfer 1 batch of data one direction and then 0 5s to received an ACK then you need to wait for 1 second until 1 batch of data is transferred and successfully confirmed to have been received  again  let s assume no data lost  therefore no need to re-transfer the same batch   Because of this round trip needed by the TCP protocol there appears to be a blocker  The blocker is the delay you experience for one round trip which includes transferring 1 batch of data and its successful acknowledgement  With that in mind we need to see how big is one batch of data  This can vary but it s usually 64KB  So your actual traffic to 1 destination  i e  the throughput  is bound by this delay  i e   latency  and the batch size by the following equation   throughput   batch size   latency   In our example the throughput is 64KB s and as we can split 1 MB in roughly 15 6 batches of 64KB size  you will need about 15 6 seconds to transfer 1 MB of file  That is a major slowdown compared to the bandwidth-only based calculations we made early

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As an analogy consider a water pipe as a channel  Pipe diameter corresponds to bandwidth or capacity  and pipe contents corresponds to throughput or usage  In the following image we can see three pipes  or channels   all of which are under-utilised  hence  usage could be increased without the need for a bigger pipe

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Bandwidth is the maximum amount of data that can travel through a  channel    Throughput is how much data actually does travel through the  channel  successfully   This can be limited by a ton of different things including latency  and what protocol you are using

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Bandwidth - theoretical maximum units of work per unit of time     Throughput - actual units of work per unit of time   As opposed to the time per unit of work  speed latency     This question in network engineering stack exchange contains good responses  https   networkengineering stackexchange com questions 10504 what-is-the-difference-between-data-rate-and-latency

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Imagine it this way  a mail truck can carry 5000 sheets of paper each trip so It s bandwidth is 5000  Does that mean it can carry 5000 letter each trip  Well  theoretically  if each letter didn t need an envelope telling us where it was coming from  going too  and possessing proof of payment  Envelope   Protocol Headers and Footers   But they do  so each letter  1 sheet of paper  requires an envelope    to about 1 sheet of paper  to get it to it s destination  So in the worst case scenario  all envelopes only have one page letters   the truck would carry only 2500 sheets Throughput  Data that we want to send from source gt destination  THE LETTERS  and would have 2500 sheets Overhead  Headers Footer that we need to get the letter from source gt destination but that the recipient won t be reading  THE ENVELOPES   The Throughput  2500 Letters   the Overhead  2500 Envelopes   Bandwidth  5000 sheets of paper  Bigger letters  4 pages  still only require 1 envelope so that would move the ratio of Throughput to Overhead higher  i e  Jumbo Frames  and make it more efficient  so if all the letters were 4 page letters throughput would change to 4000  and overhead would reduce to 1000  together equaling the 5000 Bandwidth of the truck

[networking] Throughput and bandwidth difference?

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