EnglishforITandEE-17.ppt

* 11 For personal computer networks, a key requirement is low cost. In particular, the cost of attachment to the network must be significantly less than the cost of the attached device. Thus, for the ordinary personal computer, an attachment cost in the hundreds of dollars is desirable. . 链接网络的成本必须大大低于链接设备的成本。 * 11 For more expensive, high-performance workstations, higher attachment costs can be tolerated. In any case, this suggests that the data rate of the network may be limited; in general, the higher the data rate, the higher the cost. * 12 Back-end networks storage area networks Back-end networks are used to interconnect large systems such as mainframes, supercomputers, and mass storage devices. The key requirement here is for bulk data transfer among a limited number of devices in a small area. High reliability is generally also a requirement. These are some typical characteristics: * 12 ? High data rate. To satisfy the high-volume demand, data rates of 100 Mbps or more are required. ? High-speed interface. Data transfer operations between a large host system and a mass storage device are typically performed through high-speed parallel I/O interfaces, rather than slower communications interfaces. Thus, the physical link between station and network must be high speed * 12 ? Distributed access. Some sort of distributed medium access control (MAC) technique is needed to enable a number of devices to share the medium with efficient and reliable access. ? Limited distance. Typically, a back-end network will be employed in a computer room or a small number of contiguous rooms. * 12 ? Limited number of devices. The number of expensive mainframes and mass storage devices found in the computer room generally numbers in the tens of devices.6 计算机房中昂贵的主机和大容量存储设备的数量一般以几十台计。 * 13 Back-end networks are commonly found at sites of large companies or research installations with large data-processing budgets. Because of the scale involved, a small difference in product