I'm optimizing network stack for latency. We were using SFP+ with fiber than we came to know that Direct Attach Cables (DAC) have lower latency as there is no SFP involve Link. We are measuring wire to wire latency using hardware timestamps, But we are not able to see any difference between DAC cable and SFP+ with fiber.

I have searched the internet but I'm not able to find typical latency of 10G SFP+. The only link I'm able to find is almost 10years old.

Can anyone confirm what is typical latency of SFP+ or how to calculate it?


Latency of 10GBASE-SR/-LR vs SFP+ DAC is very closely the same - in contrast to 10GBASE-T which adds appr. 1.5 μs.

Majorly, latency is caused by the line encoding overhead. In the case of -R PHYs, that's 64b66b code which requires little processing. SFP+ cages are directly fed with an -R data stream, which DACs then couple onto copper twinax and SFP+ modules use to modulate their lasers. Again, very little processing, same low latency for both.

Hypothetically, DAC could be faster because of the potentially higher velocity factor (VF) and, accordingly, lower propagation delay. The kind of twinax used is at the vendor's discretion, but even if that had a VF of .9 in comparison to fiber's .67, the delay difference for a long DAC of 10 m against fiber would be around 12 ns (10 / c * [1/VFfiber - 1/VFtwinax]).

It makes more sense to tailor the patch cables than to worry about that (12 ns is about 2.5 m worth of fiber) but either is rather moot.

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