USB Hub - Speed

Speed

To allow high-speed (USB 2.0) devices to operate in their fastest mode all hubs between the devices and the computer must be high speed. High-speed devices should fall back to full-speed (USB 1.1) when plugged in to a full-speed hub (or connected to an older full-speed computer port). While high-speed hubs can communicate at all device speeds, low and full-speed traffic is combined and segregated from high-speed traffic through a transaction translator. Each transaction translator segregates lower speed traffic into its own pool, essentially creating a virtual full-speed bus. Some designs use a single transaction translator, while other designs have multiple translators. Having multiple translators is a significant benefit when one connects multiple high-bandwidth full-speed devices.

It is an important consideration that in common language (and often product marketing) USB 2.0 is used as synonymous with high-speed. However, because the USB 2.0 specification, which introduced high-speed, incorporates the USB 1.1 specification such that a USB 2.0 device is not required to operate at high speed, any compliant full-speed or low-speed device may still be labelled as a USB 2.0 device. Thus, not all USB 2.0 hubs operate at high-speed.