Is single/multi TT important by USB3 hubs

usbusb hubusb-3

I'd like to buy a new USB3 hub to replace my old one, which has only 4 slots. By USB2 hubs single TT was a problem, because USB1 devices could slow down the entire hub. Is there something similar by USB3 hubs? Can too many USB2 devices slow down USB3 data transfer by these hubs or was this issue addressed by designing the new standard?

Best Answer

  • In USB 2.0 framework USB1 devices didn't slow down anything. The problem with single TT (Transaction Translator, to translate HS traffic into FS/LS packets locally, in each hub) was that a single TT would slow down USB1 devices on the same hub, not the overall HS hub performance.

    In USB 3.x specifications, electrically the USB 2.0 interface is completely independent from SuperSpeed path, it runs in parallel, on dedicated old-style D+/D- wires. Unlike USB 2.0, USB 3.x doesn't introduce "TT" (Transaction Translator), the USB 3.x super-speed transactions cannot be "translated" into HS USB 2.0 transactions.

    However, internal pipes of the controller and system interfaces (internal fabric) have limited bandwidth, so the system must manage it. The xHCI specifications define the concept of "BI", "bus instance". BIs can be of SS type, or HS type, or LS/FS type. System can assign several BIs to a port, and allocate certain bandwidth for each port, so it is possible that too many USB2 devices may encroach into SS bandwidth. This will depend on xHCI implementation and on software driver stack. It is unclear to what extent modern USB stack is intelligent to manage all this flexibility. This write-up is an example of general confusion in the area of USB bandwidth allocation.