USB hub

A USB hub

A USB hub is a device that allows many USB devices to be connected to a single USB port on the host computer or another hub.

A USB hub with the plastic cover removed

USB hubs are often built into equipment, normally keyboards or, more rarely, monitors. Separate USB hubs come in a wide variety of form factors from boxes that look similar to a network hub to small designs intended to be plugged directly into the USB port on a computer (that is, without a connecting cable).

Power

A bus powered hub is a hub that draws all its power from the host computer's USB interface. It does not need a separate power connection. However, many devices require more power than this method can provide, and will not work in this type of hub.

In contrast a self powered hub is one that takes its power from an external power supply unit and can therefore provide full power to every port. Many hubs can operate as either bus powered or self powered hubs.

Speed

To allow high-speed 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 when plugged in to a full-speed hub (or connected to an older full-speed computer port). While high-speed hubs support 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 only a significant benefit when connecting multiple high-bandwidth full-speed devices.[link]

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

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