HP AJ765A HP StorageWorks RDX Removable Disk Backup System User Guide (484933- - Page 35

7 USB Configuration Guide In this chapter: • "USB in HP StorageWorks devices" on page 35 • "USB terminology" on page 35 • "Setting up a USB network" on page 36 • "USB cables" on page 36 USB in HP StorageWorks devices The RDX Removable Disk Backup system 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 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 should be connected to its host system via a USB 2.0 port. Any USB hubs between the device and the system port should also be USB 2.0 compliant. To maximize device performance, minimize the use of other USB devices when using the RDX Removable Disk Backup System. 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 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.) RDX Removable Disk Backup System 35