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I currently have three peripherals (mouse, keyboard, Wacom tablet) all wired to an individual USB port on my computer. They also each have their own 6ft USB extension cable (passive) because the computer is a little far away. They are each USB 2.0 devices.

To cut down on the cables, I kind of wanted to simplify things. Run a single 6-10ft USB 3.0 extension cable from the PC to a powered 4-port USB 3.0 hub that I have lying around, and plug all three USB 2.0 peripherals into it.

I'm curious about a few things:

  1. I know plugging the 2.0 devices into a 3.0 extension won't take any advantage of 3.0 speeds. But what if they're all connected to a 3.0 hub, connected to a 3.0 cable? Would that cause those 2.0 devices to use the extra wires in a 3.0 cable, theoretically?

  2. Might forcing the event calls from all three devices through a single cable theoretically add latency?

  3. I've been told that powered USB hubs act as repeaters. Is that the case, and if so, could that reduce latency? Or is that not how a repeater works?

Answer one or some or all. I realize this might seem like such an inconsequential thing but I care about the details. Thanks very much for reading.

  • Good questions (and I can't answer most of them), but a repeater can't reduce latency, it can only increase it, because it needs to accept the signal, process it and pass it on. The time to process and pass it on will be additional to the original latency. – davidgo Apr 6 '18 at 4:20
  • Forcing multiple calls at the same time can increase latency. Longer cables will increase latency, but not perceptibly. – davidgo Apr 6 '18 at 4:23
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    Possible duplicate of Does a USB hub affect performance? – CaldeiraG Apr 6 '18 at 20:57
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  1. I know plugging the 2.0 devices into a 3.0 extension won't take any advantage of 3.0 speeds. But what if they're all connected to a 3.0 hub, connected to a 3.0 cable? Would that cause those 2.0 devices to use the extra wires in a 3.0 cable, theoretically?

USB 3.0 includes the USB 2.0 connections for backwards compatibility, but the traffic is kept separate. There is a USB 2.0 controller that handles USB 2.0 traffic passing through the USB 3.0 infrastructure. So you can use USB 3.0 cables and hubs, but USB 2.0 devices get USB 2.0 treatment the entire way. The extra USB 3.0 infrastructure is ignored.

  1. Might forcing the event calls from all three devices through a single cable theoretically add latency?

The key word there is "theoretically". Yes, theoretically, it could add latency to combine the signals through a single path, but you would never know it if it did. Your devices (mouse, keyboard, and tablet), have very low data rates. Even compared to USB 2.0 bandwidth, the combined data from all three is like spitting in the ocean. Statistically, there will be occasions where there will be simultaneous attempts to move data, and some data will be delayed. But the delay will be in the microsecond or shorter order of magnitude. Humans can't perceive delays even in milliseconds.

That said, it wouldn't change anything because it is already happening. Groups of USB ports on your computer are connected to an internal hub (the root hub). So feeding those ports separately results in any collisions happening inside your computer. If you combine the devices on an external hub, you're just moving the location of where those collisions occur.

  1. I've been told that powered USB hubs act as repeaters. Is that the case, and if so, could that reduce latency? Or is that not how a repeater works?

Repeaters are used to extend the distance limits of a connection. A hub can be used as a repeater. But you can only add latency, you can't reduce it. Each time the signal is handled, there is the potential to introduce additional delay (and you can't send it back in time). But again, such delays are many orders of magnitude too short to affect anything you would be aware of.

That said, you could potentially reduce theoretical latency by using a hub (which could be a USB 2.0 hub), for a different reason. The USB 2.0 distance limit is 5 meters when using cables designed for that distance.

Say you have a device with a 5' cable and you add a 10' extension. The wire in the device cable may be sized for a total run of 5', and the wire in the extension cable may be sized for a total run of not much over 10', and the design quality of the cables may be similarly targeted to their length. In that case, each cable has already introduced the allowable losses for the connection, and you are combining the losses of the two cables.

So your setup may already be contributing latency, and it just isn't in a range that you are aware of. For this reason, the USB spec specifically defines the use of extension cables as non-compliant, although the unreliability in actual practice may be at levels acceptable for your purpose, especially with low data rate, human input devices.

Thus, combining the devices at a hub and using a long cable to connect the hub to the computer may reduce the latency of your current setup (not that you would know it), by ensuring that each connection is well within the USB limits.

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  1. I know plugging the 2.0 devices into a 3.0 extension won't take any advantage of 3.0 speeds. But what if they're all connected to a 3.0 hub, connected to a 3.0 cable? Would that cause those 2.0 devices to use the extra wires in a 3.0 cable, theoretically?

No, that's wishful thinking given your premise.
I defer to this answer:

According to the USB3 specification from here, USB2 functionality on USB3 hosts/hubs does not change. Therefore, (putting power issues aside) USB2 devices still operate with a broadcast method, meaning it will share the same old USB speed bandwidth with all other USB2 devices on the same host/hub. USB2 devices will not have USB3 capacity available to it, as the SuperSpeed USB3 capacity is on different wires that are not connected to USB2 devices.


  1. Might forcing the event calls from all three devices through a single cable theoretically add latency?

Yes.

  1. I've been told that powered USB hubs act as repeaters. Is that the case, and if so, could that reduce latency? Or is that not how a repeater works?

Every USB hub is "powered", either with an external power supply or via the USB connection to the host.
There's no such thing as a truly passive (i.e. non-powered) USB hub. "Passive USB hub" is a misnomer.
(An active USB extension cable is actually a single-port hub, or more likely a multi-port hub with only one downstream port used.)
The USB hub is recognized by the host as a distinct USB device.

So inserting a USB hub (which does act as a repeater) will add latency, not reduce it.
A repeater uses a receive, store, and forward method, so there is no way it (or any device) can reduce latency as you ask.

BTW your PC probably has its own internal USB hub, and those three "individual" ports probably coalesce into a single USB host controller.

Also note that passive USB extension cables are apparently "illegal", that is, not covered by the USB specifications. See Why do some USB extension cables have an extended shroud at the female end?

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  1. I know plugging the 2.0 devices into a 3.0 extension won't take any advantage of 3.0 speeds. But what if they're all connected to a 3.0 hub, connected to a 3.0 cable? Would that cause those 2.0 devices to use the extra wires in a 3.0 cable, theoretically?

As others have said, this won’t happen. USB 2.0 (and 1.1) cables have 4 connectors and USB 3.0 cables have 8. The remaining 4 when you plug a USB 2.0 cable into a USB 3.0 connection will be nonexistent to the device.

That said, not all controllers are created equal. And as I explain in this answer, I have noticed faster transfer times when plugging USB 2.0 hard drives into USB 3.0 ports when compared to USB 2.0 ports on similar machine.

Basically, protocol compatibility and speed/efficiency of the use of that protocol are two different things. And my assumption is USB 3.0 controllers are simply better at what the do than some USB 2.0 ports.

I realize your question is about latency, but that is only one slim factor in the midst of this all. If there is a USB 3.0 connection on a device, I would recommend using that with USB 2.0 devices to improve performance.

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