Quantum DAT 160 User Manual page 78

Quantum DAT 160 User's Guide
host to a particular device is broadcast to all devices on the network. A
device waits for a command addressed to itself and then responds as
requested. 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
Appendix C USB Configuration Guide
USB Terminology
64