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Computer network is a system connecting two or more computers. A computer network allows user to exchange data quickly, access and share resources including equipments, application software, and information.计算机网络是连接两个或多个计算机的系统，它允许用户快速地交换数据，访问和共享包括设备、应用软件和信息在内的资源。
Data communications systems are the electronic systems that transmit data over communications lines from one location to another. You might use data communications through your microcomputer to send information to a friend using another computer. You might work for an organization whose computer system is spread throughout a building, or even throughout the country or world. That is, all the parts—input and output units, processor, and storage devices—are in different places and linked by communications. Or you might use telecommunications lines—telephone lines—to tap into information located in an outside data bank. You could then transmit it to your microcomputer for your own reworking and analysis.
数据通信系统是通过通信线路将数据从一个地方传送到另外一个地方的电子系统。你可以使用数据通信通过你的微机将信息发送给使用另外一台机器的朋友。你有可能在为一家公司工作，其计算机系统遍布一座大楼，或者甚至是全国乃至世界。也就是说，所有的部分——输入和输出单元、处理器和存储设备——都在不同的地方，是通过通信连接起来的。或者你可能使用远程通信线——电话线——接进位于外部数据单元的信息。然后你可能将信息传送到自己的微机上用于重新工作和分析。
To attach to a network, a special purpose hardware component is used to handle all the transmission. The hardware is called a network adapter card or network interface card (NIC), it is a printed circuit board plugged into a computer's bus, and a cable connects it to a network medium.
为了连接到网络上，需要使用特殊用途的硬件部件来处理所有的传送。这个硬件被称为是网络适配卡或网络接口卡,它是插入到计算机总线上的印刷电路板，由电缆将它连接到网络介质。
Communications networks differ in geographical size. There are three important types: LANs, MANs, and WANs.
通信网络由于其占据的地理范围大小而不同。有三种重要的类型：局域网、城市网和广域网。
Local Area Networks：Networks with computers and peripheral devices in close physical proximit—within the same building, for instance—are called local area networks (LANs). Linked by cable-telephone, coaxial, or fiber optic. LANs often use a bus form organization. In a LAN, people can share different equipments, which lower the cost of equipments. LAN may be linked to other LANs or to larger networks by using a network gateway. With the gateway, one LAN may be connected to the LAN of another LAN of another office group. It may also be connected to others in the wide world, even if their configurations are different. Alternatively, a network bridge would be used to connect networks with the same configurations.
局域网：计算机和外部设备在很近的物理范围内的网络被称为是局域网，例如在一座大楼内，由电缆连接——电话线、同轴电缆或光缆。局域网通常使用总线型的结构。在局域网中人们可以共享不同的设备，这样可以降低设备的费用。局域网可以通过使用网关连接到另外一个局域网或者更大的网。使用网关，一个局域网可以被连接到另一个办公团体的局域网上，它也可被连接到世界范围的其他局域网上，即使它们的配置不同。另外一种方法是用网桥来连接具有相同配置的网络。
There is a newly development for LANs: WLAN. A wireless LAN (WLAN) is a flexible data communication system implemented as an extension to, or as an alternative for, a wired LAN within a building or campus. Using electromagnetic waves, WLANs transmit and receive data over the air, minimizing the need for wired connections. Thus, WLANs combine data connectivity with user mobility, and, through simplified configuration, enable movable LANs.
对于局域网有一个新开发的网络：无线局域网。无线局域网是灵活的数据传输系统，实现了大楼或校园内有线局域网的延伸或替换。无线局域网使用电磁波通过空气传送和接收数据，最低限度地减少了有线连接。这样，无线局域网把数据连接和用户移动性结合起来，通过简化的配置，形成了移动的局域网。
Over the recent several years, WLANs have gained strong popularity in a number of vertical markets, including the health-care, retail, manufacturing, warehousing, and academic arenas. These industries have profited from the productivity gains of using hand-held terminals and notebook computers to transmit real-time information to centralized hosts for processing. Today WLANs are becoming more widely recognized as a general-purpose connectivity alternative for a broad range of business customers.
随着近几年的发展，无线局域网在一些市场领域已经获得了广泛的普及，其中包括健康保健、零售业、制造业、仓储业和学术界。这些工业通过手提终端和笔记本电脑将实时信息传送到中央主机进行处理，从获得的生产率中已获益非浅。如今，对于广泛的商业客户来说，无线局域网正成为公认的通用连接的替代品。
Applications for Wireless：LANs Wireless LANs frequently augment rather than replace wired LAN networks—often providing the final few meters of connectivity between a backbone network and the mobile user. The following list describes some of the many applications made possible through the power and flexibility of wireless LANs:
无线局域网的应用：无线局域网通常是增加而不是代替有线局域网的功能——通常是在中枢网络和移动用户间提供最后几米的连接。通过无线局域网的灵活性和功能，以下所列描述了可能实现的许多应用中的一些：
Doctors and nurses in hospitals are more productive because hand-held or notebook computers with wireless LAN capability deliver patient information instantly.
医院的医生和护士利用手提或笔记本电脑与无线局域网连接的性能，及时传递/病人的信息，提高了效率。
Consulting or accounting audit engagement teams or small workgroups increase productivity with quick network setup.
顾问或会计审计事务组或一些小的工作组使用快速搭建的网络提高了工作效率。
Network managers in dynamic environments minimize the overhead of moves, adds, and changes with wireless LANs, thereby reducing the cost of LAN ownership.
在动态环境下的网络管理者使用无线局域网最大限度地减少了经常的移动、添加和修改工作，从而降低了局域网所有者的费用。
Training sites at corporations and students at universities use wireless connectivity to facilitate access to information, information exchanges, and learning.
公司的培训点和大学的学生使用无线连接便于访问信息，进行信息交换以及学习。
Network managers installing networked computers in older buildings find that wireless LANs are a cost-effective network infrastructure solution.
在旧的建筑物内安装网络计算机的网络管理员发现无线局域网是划算的网络基础结构的解决方案。
Retail store owners use wireless networks to simply frequent network reconfiguration.
零售商店的老板使用无线局域网简化经常性的网络重新配置（问题）。
Trade show and branch office workers minimize setup requirements by installing preconfigured wireless LANs needing no local MIS support.
贸易展览部门工作人员通过安装预先配置的无线局域网最大限度地降低了配置需求，而不需要当地信息管理系统的支持。
Warehouse workers use wireless LANs to exchange information with central databases and increase their productivity.
仓储工人使用无线局域网和中心数据库交换信息提高了生产率。
Network managers implement wireless LANs to provide backup for mission-critical applications running on wired networks.
网络管理员使用无线局域网提供运行在有线网络上的关键应用程序的备份。
Senior executives in conference rooms make quicker decisions because they have real-time information at their fingertips.
在会议室的高级行政官因为手头有实时信息可供使用，因此可以做出快速的决定。
The increasingly mobile user also becomes a clear candidate for a wireless LAN. Portable access to wireless networks can be achieved using laptop computers and wireless NICs. This enables the user to travel to various locations–meeting rooms, hallways, lobbies, cafeterias, classrooms, etc.–and still have access to their networked data. Without wireless access, the user would have to carry clumsy cabling and find a network tap to plug into.
日益增长的移动用户也成为无线局域网的坚实的后备力量。使用膝上电脑和无线网络接口卡就可实现移动访问无线局域网，这就使得用户可以在不同的地方穿梭——会议室、 门厅、休息室、自助食堂、教室等地方——仍然可以访问其网络数据。假如没有无线局域网，用户就不得不携带笨重的电缆寻找网络插头。
Metropolitan Area Networks：These networks are used as links between office buildings in a city. Cellular phone systems expand the flexibility of MAN by allowing links to car phones and portable phones.
大城市网：这些网络用于一个城市内的建筑物之间的连接。移动电话系统通过允许将汽车电话和移动电话接入而扩展了大城市网的灵活性。
Wide Area Networks：Wide area networks are countrywide and worldwide networks. Among other kinds of channels, they use microwave relays and satellites to reach users over long distances. One of the most widely used WANs is Internet, which allows users to connect to other users and facilities worldwide.
宽域网：宽域网是国家和世界范围内的网络。在其他的信道种类中，宽域网使用微波中继和卫星通信远距离到达用户。使用最广泛的宽域网是Internet,它允许在世界范围内用户和用户及设备的连接。
