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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.
Data communication systems are the electronic systems that transmit data over communication 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 telecommunication 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.
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.
Communications networks differ in geographical size. There are three important types: LANs, MANs and WANs.
Local Area Networks Networks with computers and peripheral devices in close physical proximity—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 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.
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.
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. [1] 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.
Applications for Wireless LANs [2] 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:
● Doctors and nurses in hospitals are more productive because hand-held or notebook computers with wireless LAN capability deliver patient information instantly.
● Consulting or accounting audit engagement teams or small workgroups increase productivity with quick network setup.
● Network managers in dynamic environments minimize the overhead of moves, adds, and changes with wireless LANs, thereby reducing the cost of LAN ownership.
● Training sites at corporations and students at universities use wireless connectivity to facilitate access to information, information exchanges, and learning.
● Network managers installing networked computers in older buildings find that wireless LANs are a cost-effective network infrastructure solution.
● Retail store owners use wireless networks to simply frequent network reconfiguration.
● Trade show and branch office workers minimize setup requirements by installing preconfigured wireless LANs needing no local MIS support.
● Warehouse workers use wireless LANs to exchange information with central databases and increase their productivity.
● Network managers implement wireless LANs to provide backup for mission-critical applications running on wired networks.
● Senior executives in conference rooms make quicker decisions because they have real-time information at their fingertips.
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.
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.
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.
Words
| alternatively | adv. | 二中择一地，换句话说 |
| attach | v. | 附上，连接 |
| audit | v. | 审计，会计检查，查账，核查 |
| augment | v. | 增大，增加 |
| backbone | n. | 构架，中心，中枢，主干线 |
| cafeteria | n. | 自助食堂 |
| candidate | n. | 选择物，候选人 |
| clumsy | adj. | 笨拙的 |
| engagement | n. | 约定 |
| exchange | v. | 交换，调换 |
| facilitate | v. | 易于，便于，助长 |
| cost-effective | adj. | 划算的 |
| gateway | n. | 网关 |
| halfway | adj. | 中途的，不彻底的 |
| infrastructure | n. | 下部结构，永久性基地，基础 |
| lobby | n. | 门廊，休息室 |
| metropolitan | adj. | 大城市的 |
| mission | n. | 使命，任务，代表团 |
| v. | 派遣 | |
| mobility | n. | 灵活性，移动性，可动性 |
| overhead | adj. | 过顶的，头上的，经常的 |
| peripheral | n. | 外部设备，辅助设备 |
| plug | n. | 插头，插塞 |
| profit | v. | 有利于，获益 |
| proximity | n. | 接近，近似，近程 |
| retail | n. | 零售 |
| warehouse | n. | 仓库 |
Abbreviations
| LAN (Local Area Network) | 局域网 |
| MAN (Metropolitan Area Network) | 城域网 |
| MIS (Management Information System) | 管理信息系统 |
| NIC (Network Interface Card) | 网络接口卡 |
| WAN (Wide Area Network) | 广域网 |
| WLAN (Wireless Local Area Network) | 无线局域网 |
Notes
［1］ 例句：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.
分析：句中of using hand-held terminals and notebook computers to transmit real-time information to centralized hosts for processing为介词短语修饰gains，其中to transmit real-time information to centralized hosts for processing为using hand-held terminals and notebook computers的目的状语。此句虽然较长，但是一个简单句。
译文：这些行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，提高生产率，已获益匪浅。
［2］ 例句：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.
分析：often providing…是分词短语作augment rather than replace的伴随状语。
译文：无线局域网常常扩充而并非代替有线局域网的功能，它通常提供骨干网络和移动用户间最后几米的连接。
Exercises
Ⅰ. Put "true" or "false" in the brackets for the following statements according to the passage.
1. ( ) A computer network is only connected by cable lines, such as telephone lines.
2. ( ) The three types of network mainly differ in their geographical size.
3. ( ) With a network gateway, a LAN can be connected to another LAN that has the same configuration.
4. ( ) A network bridge and gateway are connectors that are used to connect two LANs or more.
5. ( ) WLAN is a replacement for LAN.
6. ( ) NIC is an electric circuit board that is a necessary hardware component of a computer network.
7. ( ) WLANs transmit data through the air and they needn't NICs.
8. ( ) A metropolitan area network connects two or more computers within a city.
9. ( ) Internet is the most often used WAN of today.
10. ( ) With WLANs, it is possible for users to connect with networks at any corner of the world.
Ⅱ. Fill in the blanks according to the passage.
1. A computer network allows user todata quickly,andresources.
2. WLAN stands for.
3. Wireless LANs frequentlyrather thanwired LAN networks.
4.phone systems expand the flexibility of MAN by allowing links to car phones and portable phones.
5. Network managers inenvironmentsthe overhead of moves, adds, and changes with wireless LANs, thereby reducing theof LAN ownership.
6. Wide area networks areandnetworks.
7. Data communications systems are the electronic systems that transmit data overfrom one location to another.
8. Senior executives in conference rooms make quicker decisions because they haveinformation at their fingertips.
9. A wireless LAN is a flexible data communication system implemented as anto, or as anfor, a wired LAN within a building or campus.
10. A network bridge would be used to connect networks with the same.
Ⅲ. Translate the following words and expressions into Chinese.
1. telecommunications lines
2. network interface card
3. geographical size
4. electromagnetic wave
5. gateway
6. hand-held
7. clumsy cabling
8. peripheral devices
9. information exchanges
10. flexibility
The OSI, or Open System Interconnection model, defines a networking framework for implementing protocols in seven layers. Control is passed from one layer to the next, starting at the application layer in one station, proceeding to the bottom layer, over the channel to the next station and back up the hierarchy.
Words
| architecture | n. | 建筑，建筑学，体系机构 |
| aspect | n. | 样子，外表，面貌，（问题等的）方面 |
| authentication | n. | 证明，鉴定 |
| carrier | n. | 运送者，邮递员，［电］载波（信号） |
| compatibility | n. | ［计］兼容性，适合，一致（性），互换性 |
| congestion | n. | 拥塞，充血，交通堵塞 |
| constraint | n. | 约束，强制，局促 |
| convey | v. | 搬运，传达，转让 |
| coordination | n. | 同等，调和，配合，（动作）协调 |
| decode | v. | 解码，译解 |
| encode | v. | 编码，把（电文、情报等）译成电码（或密码） |
| encrypt | v. | ［计］加密，将……译成密码 |
| establish | v. | 建立，设立，安置，使定居，确定 |
| framework | n. | 构架，框架，结构 |
| furnish | v. | 供应，提供 |
| handle | n. | 柄，把手 |
| v. | 触摸，处理，操作 | |
| hierarchy | n. | 层次，层级 |
| host | n. | 主机，主人 |
| identify | v. | 确定 |
| implement | n. | 工具，器具 |
| v. | 贯彻，实现，执行 | |
| independence | n. | 独立，自主 |
| layer | n. | 层，阶层 |
| packet | n. | 小包裹，小捆，信息包 |
| v. | 包装 | |
| presentation | n. | 介绍，陈述，赠送，表达 |
| privacy | n. | 独处而不受干扰，秘密 |
| responsible | adj. | 有责任的，可靠的，可依赖的，负责的 |
| sequence | n. | 次序，顺序，序列 |
| session | n. | 会议，开庭 |
| synchronization | n. | 同一时刻，同步 |
| syntax | n. | ［语］语法，有秩序的排列，句子构造，句法 |
| terminate | v. | 停止，结束，终止 |
| tier | n. | 列，行，排，层，等级 |
| v. | 使造成递升排列，使层叠 |
Phrases
| vice versa | 反之亦然 |
| LLC (Logical Link Control) | 逻辑链路控制 |
| MAC (Media Access Control) | 媒体访问控制 |
| OSI (Open System Interconnection) | 开放系统互连 |
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。
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。我们可能经常使用数据通信通过自己的微机将信息发送给使用另外一台机器的朋友。我们所在的公司，其计算机系统可能遍布一座大楼，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，它们通过通信连接起来。我们也可能使用远程通信线（电话线）接进位于外部数据库的信息，然后将信息传送到自己的微机上进行重新加工和分析。
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输出电路板，由电缆将它连接到网络设备上。
通信网络根据其占据的地理范围不同，而被分为三种重要的类型：局域网、城市网和广域网。
局域网 计算机和外部设备在很近的物理范围内（例如在一座大楼内）的网络被称为是局域网。局域网由电缆电话线、同轴电缆或光缆连接，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，也可以被连接到世界范围的其他局域网，即使它们的配置不同。另外，也可以用网桥来连接具有相同配置的网络。
现在有了一种新的局域网：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。无线局域网把数据连接和用户移动性结合起来，通过简单的配置，形成了移动的局域网。
随着近几年的发展，无线局域网在一些领域已经获得了普及，其中包括健康保健、零售业、制造业、仓储业和学术界。这些行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，提高生产率，已获益匪浅。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。
无线局域网的应用 无线局域网常常扩充而并非代替有线局域网的功能，它通常提供骨干网络和移动用户间最后几米的连接。通过无线局域网的灵活性和功能可以实现许多应用，以下描述了其中的一部分：
● 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，及时传递了病人的信息，提高了效率；
● 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；
● 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要进行的移动、添加和修改工作，从而降低了局域网所有者的费用；
● 公司的培训点以及大学的学生使用无线连接以便于访问信息、进行信息交换以及学习；
● 在旧的建筑物内无线局域网是经济实惠的网络基础结构的解决方案；
● 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）；
● 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；
● 仓储工人使用无线局域网和中心数据库交换信息，提高了生产效率；
● 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；
● 因为手头有实时信息可供使用，在会议室的高级行政官因此可以做出快速的决定。
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，这就使得用户在不同的地方（会议室、门厅、休息室、自助食堂、教室等）穿梭时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。
城域网 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城域网的灵活性。
广域网 广域网是国家和世界范围内的网络。在其他的信道种类中，广域网使用微波中继和卫星通信远距离到达用户。使用最广泛的广域网是Internet，它允许用户和用户及设备在世界范围内进行连接。
开放系统互联模型把实现协议的网络架构分为七层，控制从一层传到下一层，从一个站的应用层开始，依次进行到最底层，通过传输通道到达下一层，再逐层返回。
