USB controller bandwidth exceeded with multiple HD webcams


#1

“USB controller bandwidth exceeded” on Pluggable 7 port USB3 hub.

Here’s the scenario:
I plug in two USB 2 webcams into the hub and when I turn them both on at HD resolutions, windows displays an error that this is inadequate bandwidth. Both cameras work fine one and a time or using non-HD resolutions. (I actually have four cameras I’d like to hook up this way, but the problem starts with just two).

I’ve researched this problem, and I’ll share some background on what I found:
It turns out that all USB2 devices are still using a physical USB2 bus even when plugged into a super-speed USB3 hub. Consequently, USB2 devices have the same bandwidth limits on a USB3 hub as on a USB2 hub.

This is different from the the USB1 -> USB2 upgrade, since a “transaction translator” allowed multiple USB1.x devices to be connected to USB2 hubs without subjecting all USB1 devices to shared bandwidth limitations. This was possible because the USB2 spec includes a “transaction translator” for USB1 -> USB2, but with the USB3 spec they failed to include one (instead resorting to physically running legacy USB2 channels inside of USB3 cables/hubs/controllers).
https://en.wikipedia.org/wiki/USB_hub…

Without a transaction translator, USB3 hubs are totally useless for plugging in multiple USB2 peripherals that would require more than 480mbps aggregate bandwidth. In other words, USB2 thumb drives, webcams, sound cards, printers, etc are all subject to a *shared* 480mbps bandwidth even on a USB3 hub that supports 5gbps.

If I’ve stated anything wrong, please do correct me as I’m trying to provide useful information for others experiencing this too.

So back to my problem, the cameras I need are limited to USB2. I haven’t found a solution short of buying a PCI card with four *dedicated* USB channels (meaning completely separate 480mbps USB2 controllers). However this means I’ll have four long USB extension cables between the cameras and computer. And I’ll face more problems if I want to add more cameras. Was I naive in thinking I could run one USB3 cable and plug all cameras into a single USB3 hub? Do you have any other solutions or any idea if/when a USB3 transaction translator hub might become available?


#2

Hi Lou,

Sorry to hear about this issue. I’d be happy to help.

I do not expect there to be much new development on USB 3.0 hubs that will have MTT functionality. Most are just STT and that seems to be the industry standard currently which for your needs is certainly an issue.

I think in your situation it would be best to do as you said, have a system with dedicated USB host controllers to spread the bandwidth out and minimize the use of hubs. You might be able to get away with a few cameras sharing hubs, but if a single camera needs the full bandwidth then that’s obviously going to be an issue.

You may also encounter another issue with USB resources - we wrote a blog post about this issue a few years ago that is of some relevance:

http://plugable.com/2015/09/08/not-en…

Also take a look at the comments as some engineers replied and shed some more light on the situation.

Ultimately there’s no great solution to these kinds of issues at this time that I am aware of that don’t involve a big desktop system with lots of USB controllers and cables.

Best wishes,
Joshua Henry
Plugable Technologies


#3

Thank you for the quick response and link.

I initially thought the 5gbps capacity would favor USB 3, but now that I know the limitations, ethernet cameras may have been a better option in hindsight. Maybe I’ll just end up running a lot more wires to the server.

I don’t know what weight you guys might carry with the USB chip manufacturers but maybe you could let them know there is demand for a fix. Thanks again.


#4