Quantum DAT 160 用户指南

Appendix C USB Configuration Guide

USB Terminology

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