Glossary
A hub consists of two portions: the Hub Controller and Hub Repeater. The repeater is a protocol-controlled
switch between the upstream port and downstream ports. It also has hardware support for reset and
suspend/resume signaling. The controller provides the interface registers to allow communication to/from
the host. Hub specific status and control commands permit the host to configure a hub and to monitor and
control its ports.
Device
A logical or physical entity that performs a function. The actual entity described depends on the context of
the reference. At the lowest level, device may refer to a single hardware component, as in a memory device.
At a higher level, it may refer to a collection of hardware components that perform a particular function, such
as a Universal Serial Bus interface device. At an even higher level, device may refer to the function
performed by an entity attached to the Universal Serial Bus; for example, a data/FAX modem device.
Devices may be physical, electrical, addressable, and logical.
Downstream
The direction of data flow from the host or away from the host. A downstream port is the port on a hub
electrically farthest from the host that generates downstream data traffic from the hub. Downstream ports
receive upstream data traffic.
Upstream
The direction of data flow towards the host. An upstream port is the port on a device electrically closest to
the host that generates upstream data traffic from the hub. Upstream ports receive downstream data traffic.
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