Network Latency Question: Gigabit Fiber vs Gigabit Copper

Posted on 2006-11-13
Last Modified: 2013-11-15

So I'm in the process of setting up a iSCSI EqualLogic PS300 SAN.  The unit uses 6 - Gigabit Ethernet ports (3 on each storage processor card) and comes with the ability to use Gigabit Fiber (1000BaseSX) via SFP or standard 1000BaseT ethernet jacks [SFP slot next to a gigabit jack].

What I'd like to know is:

If you use 1000BaseSX (via SFP modules), will it actually decrease your network latency compared to copper?

My network back bone is a Cisco 4507R with a series of 6 port blades that support either SFP modules (1000BaseSX or LX) or 1000BaseT.

My interest is that I'll be setting up a HPC (High Performance Cluster) and rather than using 10Gig, Infiniband, or Myrnet I'd like to get the best bang for the buck I've already spent.

I'd like a complete and through answer to this question...
Question by:gerhardub
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Expert Comment

ID: 17933391
Your switch, or more accurately your switch modules, are the greatest single influence on latency.  There was a recent article in Network World about the untold world of switching, and it may actually have been a module for the 4500 that was the lowest latency out there.
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Expert Comment

ID: 17937467
pjtemplin - got a link, title, or date on that article?
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Expert Comment

ID: 17937764
There is no difference in speed between copper and fiber. Gigabit is gigabit is gigabit. As long as your copper cables meet CAT5e standards, they support Gigagbit. I would use CAT6 just to be safe.
Fiber does give you an electrical buffer and is resistent to Electro Magnetic Interference. For these characteristics it might be a better choice for a cluster.
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Expert Comment

ID: 17938170
Sorry, the mag is already in the circular file, but it was within the past two weeks that it showed up in my mailbox.
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Expert Comment

ID: 17938262
Network World - 11.6.06
"LAN switching: The burning questions"
 - Which switches are fastest?
Page 12
'Newman says he clocked a Cisco Catalyst 4948 at around 3 microsec at 10G rates, "which is the lowest I've measured", he adds.'

Author Comment

ID: 17938797
Ok guys,

To restate my question:

Does fiber gigabit have lower latency than copper gigabit?

Specifically, does multimode fiber plugged into a Cisco 4507 or 6503 using 1000BaseSX-SFP have lower network latency than 1000BaseT?

The "gigbit is gigabit" argument doesn't take into account that possibility that light is faster than then movement of electrons down a copper wire...

When you compare 10G, Myrnet, and Infiniband to COPPER gigabit, you tend to see 5 times higher latency with copper (25ms comapred to 3-6ms).

What I want to know is if anyone actually KNOWS (hard numbers) if there is a difference in latency between fiber gigabit using a SFP and normal 1000BaseT copper?


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Assisted Solution

pjtemplin earned 250 total points
ID: 17939462
Go ask the manufacturer.  Are you really going to trust our answer anyway?

Speed of light through glass vs. speed of electrons over copper is going to be absolutely insignificant for any sort of single building application.  186,000 miles per second (speed of light in a vacuum) x .7 (efficiency of typical fiber optics) is still 130,200 miles per second, or 130 miles per millisecond, or 686 feet per microsecond.

As I said before, the fundamental factor in your latency is going to be how the switch or switch module handles transferring the packet from buffer to wire/glass.  If it's an inefficient process (or you choose store and forward switching, etc.), you'll have latency.

One of the factors in higher TGE latency over copper is most likely the line encoding.  Copper Ethernet in higher speeds is usually 4B/5B or 8B/10B encoding; 8B/10B means eight bits have to be received, encoded as a 10-bit value, and then a voltage level and possibly a polarization value are calculated before the bits can be transmited on the wire.  This keeps the signal rate to 1/4th or 1/8th of data rate, resulting in a lower frequency of pulses and therefore less susceptibility to interference.  Fiber can simply blink at line rate (i.e. in a true serial fashion), so the transmitter doesn't have to wait for 8 bits to arrive prior to encoding, and the receiver doesn't have decode the voltage value.

I apologize for remembering the wrong Cisco switch as having the lowest latency value.
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Accepted Solution

lrmoore earned 250 total points
ID: 17939860
Yep, speed of electricity vs speed of light is roughly the same for such short distance anyway.
Other published articles:

"Fiber provides a future path for upgrading bandwidth. But at gigabit speeds on office networks, copper delivers the data slightly faster.
     That’s right. Copper just barely edges out fiber. "

"Signals do not travel faster in fiber than copper"

Here's a good discussion of fiber vs copper and where fiber might be appropriate. It does not, however, address any difference in latency between fiber or copper


Author Comment

ID: 17940287

The answer is that the coversion process of electrons to photons and then from photons to electronics slowss things down slightly... and in one of the artices indicates that at full capacity [due to the switch] the difference was 5 times that of copper, with respect to latency.

On a 750Gb backplane like the Cisco units I'd probably not see the 100% utilization that would cause the worst case of 5 times the latency...

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