计算机网络基础介绍（英文）AD：Introduction to Computer NetworkComputer 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 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 devicesare in different places and linked by communications. Or you might use telecommunications linestelephone linesto 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 computers 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 proximitywithin the same building, for instanceare 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.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.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 Wireless LANs frequently augment rather than replace wired LAN networksoften 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. -