User`s guide

Appendix B USB Configuration Guide
USB Terminology
45 Quantum DAT 320 User’s Guide
The host determines how much of the overall USB network bandwidth a
given device can use at any time. Any given USB network has only one
host.
Hubs act as command repeaters and multipliers. This allows many
devices to be supported by one host. A host command can arrive at the
upstream port of the hub and be transmitted via the downstream ports
to many other device and hubs. (Likewise, device commands arriving at
a downstream port are also repeated upstream.) There is a special hub
called a root hub. The root hub’s upstream port is connected directly to
the host. All USB systems have a root hub. This is why a system typically
has more than one USB port. All the USB ports on the system are
connected to the host system via the root hub.
USB devices are not treated equally by the host.
• Periodic devices, which consist of interrupt and isochronous devices,
are given a higher priority than bulk transfer devices.
• Interrupt devices, such as keyboards and mice, typically send brief
but important input information to the system.
• Isochronous devices, such as web cameras, typically require timely
transmission of data, such as a video stream.
• Bulk transfer, such as printers and CD-ROM, must receive or transmit
100% accurate data. But how fast or often the data is transmitted is
not as important as the accuracy of the data.
The host will give network bandwidth priority to periodic devices and
will share any remaining bandwidth equally among any active bulk
transfer devices.
USB supports many types of devices and three data transfer rates, low
speed, full speed, and high speed. (A keyboard does not need to
transfer data to the system at the same speed as a cable modem.)
The transfer speed capabilities of a device are often referred to by the
USB specification revision in which the transfer rate was defined.
Therefore a USB 2.0 Hub will support high, full and low speed, but a
USB 1.1 hub will only support full and low speed.
The USB specification was developed and is maintained by the USB
organization. The USB organization also conducts a certification
program. The USB organization will issue the USB logo to a device,
which has passed a series of tests that verify the device has correctly
implemented the USB interface, as defined by the USB specification.
There are many types of USB logos depending on which features of the