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Some sites selling cables list the USB 3.0 transfer rate possible at up to 4.8 Gbit/s whereas others say speeds upto 5.0 Gbit/s. Are they rounding up when they list it as up to 5.0 Gbit/s? When is it actually up to 4.8 Gbit/s?

4 Answers 4

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5 Gbit/s raw serial rate, 4 Gbit/s after accounting for the overhead of the 8b/10b line code. I am not sure where the 4.8 Gbit/s figure is coming from. Apparently 3.2 Gbit/s is a reasonable real-world performance figure as per Wikipedia, though I am not sure exactly what assumptions were made for this figure.

See: USB 3.0, Data encoding (Wikipedia)

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4.8 Gbit/s is probably the result of USB 2.0's 480 Mbit/s multiplied with 10. But USB 3 is not just USB 2 times 10.

5 Gbit/s is correct (but in reality you'll need to subtract overhead and end up below 4 Gbit/s).

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625 MByte/s is what is touted, which equals 5 gigabits per second (not rounded), but in reality this is just theoretical; I've never seen this sort of performance.

Following is an article to some real-life tests which are much more in line with my own thoughts on the matter. It is nowhere near ten times faster even though it's supposed to be full duplex.

USB 3.0 Tested: How Fast Is It in the Real World?

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    I don't think these drives are limited by USB itself, so it's not fair to say USB can't perform close to its theoretical speed.
    – Ruslan
    Oct 21, 2015 at 5:36
  • Just being practical, I never said it couldn't, I said I've never seen that performance
    – Kilisi
    Oct 21, 2015 at 5:39
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    5 Gbps is the raw serial rate. The 8b10 code has 20% overhead, so the actual data rate is 4 Gbps. Then you have protocol overheads on top of that. It is full duplex, but I think most transfers are going to be quite asymmetrical. Oct 21, 2015 at 7:46
  • good info, I'd never looked into it that far, but makes sense
    – Kilisi
    Oct 21, 2015 at 7:55
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    Heh, I have been doing a lot of work with high speed serial stuff, mainly 10G ethernet on FPGAs. It's interesting to see how things are rated. USB 3, PCIe, and SATA are all rated at the raw serial rate. All of them use 8b/10b encoding (with a 20% overhead) except for PCIe gen 3 which uses the lower-overhead 128b/130b (1.5% overhead). Gigabit ethernet and 10G ethernet over fiber are both rated at the actual data rate, making the raw serial rates 1.25 Gbit/sec and 10.3125 Gbit/sec (10G ethernet uses 64b/66b with 3% overhead). Oct 23, 2015 at 7:44
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According to Wikipedia, USB 3.0 is 5 Gbit/s.

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