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I've been searching for a solid answer for this, but haven't found anything conslusive.

Basically, I am getting a powered 7-port USB 3.0 hub, but I am plugging devices that are important or are high-data devices—such as a graphics tablet for professional work (important), and a 1080p webcam. I am wondering if the 7-port USB hub will be overpowered, in terms of loss of transfer speed, due to the number of devices or the number of demanding devices, and if data transfer speed of any of the devices on the hub will be affected because of this.

So, with hard numbers/facts, how much can a powered 7-port USB 3.0 hub handle without causing any loss of speed whatsoever for the devices (not counting external hard drives)? And, arround how much bandwidth would 7 devices (graphics tablet and 1080p webcam out of those devices, as well as mouse, keyboard, printer, etc.) if theoretically used all at the same time?

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  • What aspect are you finding inconclusive? Are you referring to power or bandwidth (a powered hub should deliver full power to every port)? Are you contemplating some relationship between power and transfer speed? What do you mean by "important"? What specific devices besides a graphics tablet and a webcam? What do you mean by not counting external hard drives (are they not in the mix, or in the mix but you are ignoring them for some reason)? What do you mean by theoretically used at the same time? (cont'd) – fixer1234 Jan 30 '17 at 1:17
  • Some devices could be continuously streaming data and some (graphics tablets) could be intermittent. You could have competition for bandwidth at moments well below saturation and never notice it. Different devices would manifest starving bandwidth in different ways, some more noticeable than others. Which device is starved at any moment could be different. A hypothetical question like this can't be answered with "hard numbers" because there are infinite hard numbers. – fixer1234 Jan 30 '17 at 1:17
  • It is more to do with bandwidth. I am trying to understand how the 5 GB/s of the USB 3.0 port is shared on a powered USB 3.0 hub, and if this can affect some of the devices on it (maybe for some technical reason I am not aware of). – samseva Jan 30 '17 at 1:24
  • By devices, the most important is a graphics tablet (it is for work), with also on the hub a mouse, keyboard, printer, USB earphones. So would the graphics bandwidth be affected in any way due to being on a hub? With a 1080p webcam, which is also for my work (tablet not used at the same time), would the other USB devices on the hub (or that it is on a hub itself) affect the bandwidth of the webcam in any way? – samseva Jan 30 '17 at 1:26
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    I can't imagine why a graphics tablet should use much more bandwidth than a mouse. They've been running fine on USB 2.0 for quite a while. It certainly won't be continuous data. For that matter 1080p webcams seem to do ok on USB 2.0 also. – Jamie Hanrahan Jan 30 '17 at 2:11
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Keyboard, mice, audio, and USB2 pen drives have no effect on operations of USB 3.0 channel - these USB2 devices go through ABSOLUTELY SEPARATE hardware, a USB 2.0 hub that is built inside the USB 3.0 hub, leaving the USB 3.0 bandwidth untouched. Please see the USB 3.1 specifications, Section 3.1, figure 3-1, page 3-2. The text says, "The USB 3.1 system architecture (Figure 3-1) is comprised of two simultaneously active buses: a USB 2.0 bus and an Enhanced SuperSpeed bus."

A 1080P HD webcam (30fps) takes about 60 Mbps of effective bit rate, or maybe 2-3% of the available USB 3.0 bandwidth. It does not seem something to worry about.

Therefore, the rest of USB 3.0 bandwidth on the 7-port hub will be available for your "graphics tablet" for your professional work, with all hard data/facts you provided.

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  • Hi, Ali. Thank you very much for your input. It clarifies many things regarding how USB and USB hubs function. Kind regards. – samseva Jan 30 '17 at 18:56
  • I'm not sure where you got those numbers from. 1920x1080 at 30fps and 2 bytes per pixel is about 120 MB/s, which is 20% of USB 3.0's 640 MB/s bandwidth, not counting overhead. – whoKnows Jan 21 at 14:16
  • @whoKnows, I got my numbers from experience. Please be aware that HD webcams don't transmit any raw data these days, the videostream is already compressed/encoded (H.264 or else). More, raw bandwidth of USB 3.0 is 500 MB/s, see here superuser.com/a/1138568/620011 – Ale..chenski Jan 23 at 4:37
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Depends of how much power can this hub deliver. Underpowered devices usually will not just loose speed, but they'll stop to work entirely (or be cause of magic smoke escaping).

If it's a powered hub with maximum current of 900mA × 7 = 6.3A (maximum current according to USB 3.0 specification; you can look for it on PSU of the hub) it can pretty much handle every port at maximum current (unless it's wired badly and have bad circuit). I would say, getting a quality hub it's what you're looking at (you can ask what on Hardware Recommendations Stack Exchange).

The best you can do is to get USB voltage & current meter and check real voltage/amperage of each device (it's extremely cheap) and add it together to see on what kind of amperage you're looking at, then add some current for the hub itself and loses in transit.

If the hub other than 5V PSU you need to check for wattage as well. Then, multiply voltage and amperage of PSU to get wattage and divide by 5V to get 5V possible amperage. Subtract some for loses of DC to DC converter efficiency inside the hub (typically, 5-30%)

To check the bandwidth use Device Manager in Windows: Open properties of (Name) Host Controller, in Properties check Bandwidth-consuming devices list on Advanced tab. By opening Properties on USB Root Hub in Power tab you can see also see required power.

On Mac you could at least check if they are running in USB 2.0 or 3.0 mode. If 2.0, the maximum bandwidth of the device is about 10.5x lower, so theoretically you could run 10 USB 2.0 devices on this hub with maximum speed. Also, benchmarking HDDs you could estimate what maximum bandwidth they'll use.

As to how to calculate bandwidth it's pretty much adding bandwidth together and some overhead. See also: Why are USB devices slower than 480 MBit/s.

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  • Thanks for the response. I'll do the calculations for the power as well. But, rather, I was more worried about loss of data transfer speed due to many devices on a hub, as some of the devices that I use can't slow down at all (for work). – samseva Jan 30 '17 at 1:08
  • @samseva Added how to check. – Hex Jan 30 '17 at 1:28
  • Thanks, but I'm on a Mac! :) – samseva Jan 30 '17 at 1:29
  • @samseva Then i don't know how to do precisely, but i added paragraph on basic checks. – Hex Jan 30 '17 at 1:45
  • I found out how. For Mac users interested: Alt+Apple logo->System Information...->USB. From there you can see the maximum usage of each device (if plugged to the computer or a USB hub). – samseva Jan 30 '17 at 18:52

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