definition of the word troughput

Posted on 2001-07-12
Last Modified: 2013-11-29
Hello can you give me the definition of the word troughput? Is there a difference between troughput and data rate or transmission rate? Which word do we use when we want to speak about the rate of useful data bytes only and ignore the encapsulation bytes of the protocol?

Please be clear because I'm confused.

Thank you!
Question by:pascal_lalonde
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Expert Comment

ID: 6278675
Webster defines it as "Output or production, as of a computer program, over a period of time."  I guess I'd define it as the total output achieved as a result of multiple input and processing devices.  How about a cold one first?  :-)  I would look at data rate and transmission rate as they would pertain to a single pipe.  Say a 10 meg input.  Take a switch for example, if you have a single 10 meg input, the most you'll get out is 10 meg, even if the output is rated at 100 meg.  Now aggregate 5 - 10 meg inputs, assuming that your output is capable of 100 meg, you get a theoretical output of 50 meg.  The Through-put is more the total output.  Does that make sense?  

Accepted Solution

dcgames earned 100 total points
ID: 6278774
In general, when talking about performance of a site, you have three values to consider:

a) The transaction time (how fast a request can be sent and a reply received).

b) The SPEED of the pipe (bits per second)

b) Throughput (How much DATA can be served over the pipe.

The first one is NOT affected very much by the speed of your connection beyond a certain initial threshold. For example, if you PING your favorite site, you will get roughly the same milliseconds whether you are on a 56kbps modem, a 128kbs ISDN line, or a 768kbps DSL link.

HOWEVER, if you start downloading a lot of data, you will notice a very significant difference because the speed of the pipe and the throughput define how many bytes can be received a the same time without saturating the pipe.

In general modems and connections are quoted in kilobits per second. This is the raw bits that can travel the pipe in a second.

768kbps means you can get 768,000 bits over the pipe in one second. HOWEVER, actual THROUGHPUT depends on much more than just the speed.

a) transport protocol overhead means to get one byte you actually have to transmit about 9 or 10 bits, not 8. That is assuming a good quality link. If lots of transmision errors occur, then the ratio can easily go to 16 bits for every 1 byte of data.

b) In addition, you have higher-level protocols which can consume 10-40% of your pipe. SPecially if you are doing lots of things at the same time over that pipe.

FTP (file transfer), HTTP (hypertext), SMTP & POP3 (mail), all of these are protocols running on top of TCP running on top of IP, running on top of ETHERNET. All these protocols contribute a bit to this overhead.

c) Client Speed (CPU, BUS, Memory) all affect how fast you can get the data. DOesn't do you any good to install giga-bit ethernet if your computer is has to slow it down because you are playing an MP3 in the background that consumes 50% of your CPU capacity.

d) routing (delays in the route between the two points).

This doesn even consider that some connections are faster in one direction than the other (like DSL and 56kbps modems).

Take for example a 56kbps modem. You can't get 56kbps from it becuase:

a) The copper isn't usually as good a quality as it could, resulting in degrated signal quality

b) You only get 56kbps if you are NOT transmiting anything in the other direction.

c) The 56kbps standard is for stuff going from the ISP to you (downloading), but the up-stream is still 33kbps at best.

So: How do you know what your REAL, PRACTICAL throughput is?

ok, to check for an "upper" bound (i.e. under best conditions, what can I get), it is simple. Download a large file (40MB for example) and see how long it takes.

With a large file you minimize the overhead and concentrate on maximium throughput for the one connection. You should obviously not be checking e-mail, browsing the internet or otherwise disturbing your test with any other actions. Do this at 4am in the morning to make sure you don't get impacted by internet and ISP loads.

You will find that the maximum throuput cannot exceed your quoted speed divided by 10 (768kbps connection has max about 76k bytes per second). But most likely it will be much lower. If it's less than 70% you've been duped and arent getting what you were sold.



Author Comment

ID: 6280775
Excellent! You were surely a teacher in another life.

Expert Comment

ID: 6280907

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