英语科普文选：Local Area Network(局域网)

　　There are two categories of networks: local area networks (LANs) and wide area networks (WANs). Here I'd like to mainly talk about LAN.

　　A LAN is a system of computers and associated peripherals such as Printers that are physically connected by cable within a limited geographical area-typically in an office building or on a college campus. The topology of the network may be ring, star or bus. LANs use fiber optics or coaxial cable to connect computers, and each computer must have special communications software installed oil its hard disk. Software has been developed that allows computers with different platforms to coexist and exchange data on the same LAN.

　　A File Server

　　Many networks are managed by a computer called a file server.A file server has a large－capacity hard disk and special software that manages access to flies on the network. It controls how data and database are shared among users on the network and how users access master copies of data and application software on the centralized hard disk. It is the file server's job to make sure that users don't accidentally try to update a file at the same time and scramble the data. The file server may also manage the access to an expensive piece of hardware such as a laser printer.

　　When a file server is used on a LAN, large databases are stored on the server and users may store all of their work flies there as well.This operation is analogous to someone's manually collecting all the data each day placing them in a file drawer, and then redistributing these data to the workers as needed.

　　The LAN Operating System--Net Ware

　　Novell's Net ware has become the strongest competitor to IBM's own network operating environment and predates IBM in its support for LANs. In fact,IBM now markets Net ware for both the PC Network and for Token Ring in addition to its own software.The key to Net ware is the file server software and directories on the server are mapped to network drives, thus operating like virtual diskettes from a user's perspective. The server software is a multitasking operating system with a file structure consistent with DOS 2.x and DOS 3.x. The Net ware shell in each workstation intercepts 21H DOS calls and redirects appropriate calls to the server.

　　Perhaps the primary advantage of Net ware is that it runs on a wide variety of networking hardware.Regardless of whether you choose an IBM network, Corvus OMNINET, AT＆T Star LAN, etc., you can obtain Net ware as the operating environment. In fact, if you have one of each the user will not be able to tell the difference(unless there are some distinct performance differentials), and they can be interconnected through Novell's system of bridges and gateways.

　　局域网

　　网络有两类：局域网（LANs）和广域网（WAN）。在这里主要谈一下局域网。

　　局域网就是在一个有限的地域范围内，典型的如在办公大楼或校园中，将计算机及相关外设（如打印机）用电缆进行物理连结而组成的一个系统。网络的拓扑结构可以是环形的、星形的或总线形的。在局域网中，可用光纤或同轴电缆连结多台计算机，且每台计算机的硬盘上必须装有专门的通信软件。允许具有不同平台的计算机共存于同一局域网中，且能进行数据交换的软件早已开发出来了。

　　文件服务器

　　许多网络由一称为文件服务器的计算机来管理。文件服务器具有大容量的硬盘和一些用来管理对网上文件访问的专门软件。它控制着网上用户如何共享数据和数据库，以及用户如何存取中央硬盘上的数据和应用软件的主拷备。确保两个用户不能偶然地在同一时刻对同一文件进行更新或争夺数据也是文件服务器的工作。文件服务器还可以管理对昂贵硬件（如激光打印机）的访问。

　　当文件服务器用于局域网时，大型数据库就被存储在服务器上，而且用户也可以将其所有的工作文件存放在上面。这就像是一个人每天手工收集所有的数据，放到一文件抽屉中，然后等需要时，再将这些数据发到工作人员手中。

　　局域网络操作系统——Net ware

　　Novell公司的Net ware操作系统已经成为IBM公司特有的网络操作环境最强有力的竞争对手，它在对局域网络的支持方面也早于IBM公司。事实上，除了自己的一部分软件外，IBM公司也为所有的PC网和令牌环网购买了Net ware操作系统。Net ware 操作系统的最主要部分是文件服务器软件，服务器上的目录被映射到网络驱动器，这样在用户看来，它运行起来就如同是虚拟磁盘。它的服务器软件是一个多任务的操作系统，文件结构也与

　　DOS 2.x和DOS 3.x的文件结构一致。工作站上的 Net ware外壳程序拦截了DOS的21H中断，并重新走向为对服务器的调用。

　　Net ware操作系统的主要优点是能够运行在多种网络硬件上。不论是选择IBM网络，Corvus公司的OMNINET网络，还是AT＆T公司的星形局域网络等，都可获得Net ware的操作环境。