USB3 Hub question / test request

m00b · July 22, 2019

Hi everyone,

I just created an account to ask the following question here, but I have seen most of the LTT shows on YT (well, not the newer ones (last half year) and those that look too much like drama).

Anyway, why I ask it here?

This is a thing that might require someone to grab a few usb3 hubs and just test it (and maybe make a YT video about it when you run out of topics XD).

Ok, so here is the question:

Say I have an external USB2 HDD (read/write speed is limited by the USB2 protocol) and a USB2 webcam.

Now I connect both to an USB2 hub and start my video stream while also accessing the HDD.

the HDD access speed will be slowed down by my camera due to the shared USB2 connection between the hub and the PC.

But what if I use an USB3 hub?

Will both devices be able to run at full speed (only limited by the individual USB2 limitation)?

Is that ensured for all USB3 hubs or are there some 'good' hubs that do that and some 'bad' ones that don't.

Thanks for your help.

Senzelian · July 22, 2019

I actually had to read up on that and it seems a little more complicated than you might think.
Here's a little quote from Wikipedia, taken with a grain of salt of course:

Quote

Any USB 2.0 hub that supports a higher standard than USB 1.1 (12 Mbit/s) will translate between the lower standard and the higher standard using what is called a transaction translator (TT). For example, if a USB 1.1 device is connected to a port on a USB 2.0 hub, then the TT would automatically recognize and translate the USB 1.1 signals to USB 2.0 on the uplink. However, the default design is that all lower-standard devices share the same transaction translator and thus create a bottleneck, a configuration known as the single transaction translator. Consequently, multi transaction translators (Multi-TT) were created, which provide more transaction translators such that bottlenecks are avoided.^[6] Note that USB 3.0 hubs do not currently^[7]^{[citation needed]} perform transaction translation to super-speed for USB 2.0 devices.

It might be that this information is now outdated, but it might be worth the time to look into it further, considering it seems to be an interesting topic.

Anyway, what I think will happen and is most likely true, is that the USB 2.0 devices will simply run at their full speed (40MB/s) and therefor won't be bottlenecked if connected to a USB 3.0 hub.

Quality, of course, plays a role in that, but I assume that most USB 3.0 hubs are just fine handling multiple USB 2.0 devices.

I wouldn't buy the cheapest pile of sh*t tho...

mariushm · July 22, 2019

HDD slows down because the camera is in a class of products that's basically "time sensitive, give me priority, as low latency as possible" while the hard drive is in the "mass storage" class of devices which gets less priority. Each device receives "slots" , portions of time, where they can place data... the camera device being higher priority can reserve more "slots" and the hdd must give and let the camera take more space.

the usb hub has to mix the camera and hdd packets in a specific way, with camera getting priority.

Your pc probably has multiple usb 2.0 controllers - ex 2-4 usb 2 ports on the back io shield would be connected to one controller. while the usb 2.0 headers on the other corner of the motherboard will probably be connected to another "controller" so you may solve the problem simply by buying a 1-2$ bracket with 1-4 usb ports that you connect to your usb 2 headers on the motherboard.

A usb 3.0 hub will probably improve the performance of your drive, but it depends on the controller (how smart it is, onboard cache , how it handles 2.0->3.0 translation etc)

m00b · July 22, 2019

Thank you for your replies :)

@mariushm: This example situation was more or less just to describe my question. I understand why I would get the slowdown and I am aware that I could solve it by using different USB controllers. But the idea was to pipe multiple USB2 devices through one USB3 cable.

@Senzelian: Very interesting. So I'd need to look at "multi transaction translator" USB3 hubs. If they exist. Makes sense that having one TT creates an bottleneck. I guess you could imagine it as an USB2 hub that goes in the one TT and that goes in an USB3 hub, or something.

Edit: Looks like you can find some if you do some research: https://www.ioiusb.com/Hub/U3H414E.htm
also this TI chip looks like it would do the job: http://www.ti.com/product/TUSB8044A
so I suspect there should be a few more hubs that use this or a similar chip to archive it.
Thank you again @Senzelian for delivering the right keyword to search for.