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DAT 160 USB
USB terminology USB configuration Guide
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.
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Periodic devices, which consist of interrupt and isochronous devices, are
given a higher priority than bulk transfer devices.
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Interrupt devices, such as keyboards and mice, typically send brief but
important input information to the system.
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Isochronous devices, such as web cameras, typically require timely trans-
mission of data, such as a video stream.
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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 organi-
zation. 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.