USB 2.0 connectivity exceeding the theoretical limit of 35 MB/s - how come?

[m654321]

The other day, I backed-up data on my Dell Latitude D630 (an LL3.8 setup), transferring files of various sizes from the laptop's internal drive to an external drive (both SSDs: 1TB Samsung 850 Evo). While doing this, I was surprised to see transfer speeds apparently exceeding the theoretical limit of 35MB/s for USB 2.0 connectivity, according to the onscreen File Operation Progress notification - see link at https://imgur.com/a/mn8QvLs

Several times, peak transfer rates hit around 60 to 70 MB/s, though most of the time hovered between 25 to 35 MB/s. I was unable to find any references on the web regarding overshoot in USB 2.0 connectivity speeds.

[bitsnpcs]

Hello m654321,

this says maximum 60MB/s - https://en.wikipedia.org/wiki/USB#USB_2.0

There is info about USB 2 in this pdf - https://www.pjrc.com/teensy/beta/usb20.pdf

[TheDead]

Heya!

This maybe old school thinking but it could be because of "what" is transfered?
When transfering plain text files over a "modem" internet connexion for example, data can be compressed in real time by the hardware and show as transfering 2-3 times the native bandwith.
Already "compressed" files like pictures (jpg, gif, png) or videos though only show "normal" speeds.

I have not looked at transfered speed for years and taken the "relaxed" approach since have the slowest-high-speed internet available (hence cheapest ) and also have a little network box (NAS) at home doing all the work on my local network.
When I transfert I just click and go do something else. Same with loading screens... but this is a lot less since I'm using Linux now and not so much the "other" OS wink, wink.

_Edit : typos

[m654321]

Hello m654321,
this says maximum 60MB/s - https://en.wikipedia.org/wiki/USB#USB_2.0

Many thanks bitsnpcs - this wiki link was really interesting - not sure where exactly where I got the 35MB/s limit for USB 2.0 connectivity from!  : 

So, according to this link, 60MB/s does appear to be possible for USB 2.0 (designated High Speed, 2000), which explains what I observed.  Interestingly too, is the info it gives for USB 3.0 (designated Superspeed, 2008), where it says it's realistic to get 400MB/s or more.

Based on personal observation, there would appear to be two other factors affecting USB connectivity: CPU speed (e.g. i3 vs. i7) & Drive characteristics (e.g. SSD vs. HDD, HDD rpm) - these were my finds:

1. CPU - when transferring data from an internal SSD to and external 10TB HDD (7200 rpm), USB 3.0 connectivity rates averaged around 80 MB/s for an i3-3110M machine compared to 110 MB/s for an i7-4710HQ one.

2. Drive Characteristics - as expected there  seem to be huge differences in USB 3.0 connectivity rates between an SSD-to-SSD system, compared to an SSD-to-HDD one. For example with a i7-4710HQ machine, I would get rates of 300-400 MB/s (peaking at 400-500 MB/s), when copying from an internal SSD to an external one. This would drop to around 110 MB/s (peaking at 120-130 MB/s), when copying from an internal SSD to an external 7200 rpm HDD, as mentioned above.

Again, there would of course be differences between copying to an 5400 rpm HDD vs. a 7200 one, but I haven't done any observations to quantify this.

Many thanks again bitsnpcs & please excuse the delay in replying to you - I've just been tied up with other things recently ... 

[m654321]

This maybe old school thinking but it could be because of "what" is transfered?

Many thanks for your reply - again please excuse the delay in my reply.
Yes, the types of file being transferred does influence transfer rates. I read somewhere that transferring a collection of mixed files (e.g. different file types & sizes) was slower than if the files were more uniform, and have noticed this myself from time to time.

You mention internet connectivity - this thread was only looking at internal-to-external drive connectivity via USB.

[bitsnpcs]

Glad it helped [member=458]m654321[/member]  your personal observations are interesting, I agree with them.