HP StorageWorks DAT Tape Drives User Guide - USB (April 2005)

HP StorageWorks DAT Tape Drives User Guide 49
A USB Configuration Guide
USB in HP StorageWorks DAT devices
The HP StorageWorks DAT tape drive is a USB 2.0 logo certified device. It supports the high-speed
USB transfer rate with a theoretical maximum transfer of over 50 MB/s. (The actual backup rate will
be less. This is defined by the transfer rate of the tape drive and the compressibility of the data.)
To benefit from the high-speed performance of the USB 2.0 interface, the USB StorageWorks DAT
tape drive should be connected to its host system via a USB 2.0 port. Any USB hubs between the
tape drive and the system port should also be USB 2.0 compliant.
To maximize tape drive performance, minimize the use of other USB devices when using the USB
tape drive.
USB terminology
The Universal Serial Bus, better known as USB, is a communication interface where an intelligent
host controls all the network activity. A command from the 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 of secondary importance verses 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.)