Hp RDX Removable Disk Backup System Manual do Utilizador descarregar pdf (Página 35)

7USBConﬁguration Guide

In this chapter:

•

“USB in HP Storage Works devices”onpage35

•

“USB terminology”onpage35

•

“Setting up a USB network” on page 36

•

“USB cables” on page 36

USB in HP Stor

ageWorks devices

The RDX Remov

able Disk Backup system is a USB 2.0 logo certiﬁed device. It supports the high-speed

USB transfe

r rate with a theoretical maximum transfer of over 50 MB/s. (The a ctual backup rate will be

less. This is deﬁned by the transfer rate of the drive and the compressibility of the data.)

To beneﬁt from the high-speed performance of the USB 2.0 interface, the RDX Removable Disk Baakup

System shou

ld be connected to its host system via a USB 2.0 port. Any USB hubs between the device and

thesystemportshouldalsobeUSB2.0compliant.

To maximize device performance, minimize the use of other USB devices when using the RDX Removable

Disk Backu

pSystem.

USB terminology

The Universal Serial Bus, 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 downstre a m 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 por t is connected

directly to the host. All USB systems have a root hub. This is why a system typically h as more than one

USB port. All the USB ports o n 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 ca meras, typically require timely transmission of data, such as

avideostream.

• 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 impor tant as the accuracy of the data.

The host will give network bandwidth priority to periodic devices and will share any remaining bandwidth

equally amon g any active bulk transfer devices.

USB s upports 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.)

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