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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 USB specification a device
supports. A USB 2.0 logo-ed device correctly supports high, full, and low
speed data transfer.
Setting up a USB network
USB is very user friendly. The interface automatically assigns addresses to
devices; you simply need to connect a USB device to a USB port on a system
using a USB cable. If you have many USB devices or need to place a device
away from your system, you may place up to a maximum of five USB hubs
between the device and host system and connect them with additional
cables.
1018932 DAT USB Tape Drives
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Chapter 8–USB Configuration Guide