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What is the max Full duplex throughput of Gigabit Ethernet ?

Posted on 2011-09-15
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Last Modified: 2012-06-27
Hi!

This question comes from a friendly argument.

Theoritically, Gigabit Ethernet can Transmit/Receive about 100 MB/Sec.
Now most people understand that Full-Duplex means that you can Transmit/Receive at the same time (in otherwords transmitting and receiving data are independable).


Now this friend says that : "this is very resonable.  BUT in reality Gigabit Ethernet boards do not Transmit/Receive simultanouosly".

So, Who is right ?
Is the theoretical throughput of Transmit + Receive at the same time 100 MB or 200 MB ?
Is it true that in practice "Gigabit Ethernet boards do not Transmit/Receive simultanouosly" ?
I had spent yesterday a lot of time searching for the answer, but did not find :-(


Now here are 2 practical questions :
1) If I connect 2 Windows PCs directly, and try to transmit "tons of data in both directions", what is maximal expected throuput (Using Fullduplex gigabit Ethernet) ?
2) If I connect 2 Windows PCs through a "good gigabit Ethernet switch", and try to transmit "tons of data in both directions", what is maximal expected throuput (Using Fullduplex gigabit Ethernet) ?

Is there some where a C/C++ code which gives the answer for the two above questions ?


zmau
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Question by:zmau
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netjgrnaut earned 2000 total points
ID: 36542281
Gigabit Ethernet provides a 1000Mbps (mega bits, not bytes) channel.  This equates to 125MBps (1000 bits / 8 = 125 bytes).

Full duplex is as you describe it (simultaneous transmission).  Here's a reference from Wikipedia:  

1000BASE-T uses all four cable pairs for simultaneous transmission in both directions through the use of adaptive equalization and a 5-level pulse amplitude modulation (PAM-5) technique.
(link to full article)

Your practical questions are a bit too vague to answer, as effective throughput will depend on what "tons of data" means, as well as what network protocol you're using for the transfer - not to mention all the OS-level variables.  

For detailed answers and methods for testing your effective throughput (aka "goodput"), spend some time with this Wikipedia article.

I hope that answers your basic question, and provides a good jumping off point to better understand network throughput.

If you have questions about developing network test applications in C/C++, I suggest you post that as a separate question in a more programming-oriented zone.

Good luck!
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by:netjgrnaut
netjgrnaut earned 2000 total points
ID: 36542313
Sorry - I left out some math...

As you've probably figured out from the original post, full duplex gigabit Ethernet would have a theoretical throughput of 250 mega bytes per second.  That's for the channel - not any given computer on the link.

Such "double the speed for full duplex" numbers are often misleading marketing speak.  The link is still 125MBps in one direction, so a given host can only SEND 125MBps max.  The fact that it can simultaneously RECEIVE 125MBps means the channel size (bandwidth) is 250MBps.

This means that "tons of data in both directions" can theoretically move at 125MBps in *each* direction *at the same time*.  So (very simply) if you had 250MB of data to send in *each* direction, it works out like this...

half-duplex link:
(computer 1 x 250MB = 2 seconds) + (computer 2 x 250MB = 2 seconds) = 4 seconds

full-duplex link:
(computer 1 x 250MB = 2 seconds) & (computer 2 x 250MB = 2 seconds) = 2 seconds

Hope that helps!
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by:Ernie Beek
ID: 36542315
By default Gigabit Ethernet is full-duplex but it supports half-duplex for hubs (repeaters).

In a departure from both 10BASE-T and 100BASE-TX, 1000BASE-T uses all four cable pairs for simultaneous transmission in both directions through the use of adaptive equalization and a 5-level pulse amplitude modulation (PAM-5) technique.

Source: http://en.wikipedia.org/wiki/Gigabit_Ethernet

The theoretical speed would be 125MB/s, though normally the speed depends on a lot of factors, not only the line speed. Bearing that in mind the maximal throughput in Q1 and Q2 would be: 125MB/s in either direction (theoretically speaking of course ;)
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by:Ernie Beek
ID: 36542321
Ok, didn't realize I was typing that slow :-~
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by:netjgrnaut
ID: 36542333
It must be my full duplex fingers. ;-)
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by:Ernie Beek
ID: 36542339
Ah now I see, I had autonegotiation turned off :)
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by:zmau
ID: 36542351
You are greate guys.
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by:Ernie Beek
ID: 36542356
Just trying to do the best we can (be it full or half duplex ;).
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by:Sushant Gulati
ID: 36542420
That's why we guys have always are link up ;) Loved to read it...

~SG~
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by:netjgrnaut
ID: 36542432
You can show some love by awarding points and marking this question as answered.  :-D
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by:zmau
ID: 36544577
I am sorry it took me so long to mark this question as answered.
I am not always near my computer...
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