Introduction to Computer Network

　　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 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.

　　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 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.#p#

　　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 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.#p#

　　●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.#p#

　　計算機網絡介紹

　　計算機網絡是連接兩個或多個計算機的系統，它允許用戶快速地交換數據，訪問和共享包括設備、應用軟件和信息在內的資源。

　　數據通信系統是通過通信線路將數據從一個地方傳送到另外一個地方的電子系統。你可以使用數據通信通過你的微機將信息發送給使用另外一臺機器的朋友。你有可能在為一家公司工作，其計算機系統遍布一座大樓，或者甚至是全國乃至世界。也就是說，所有的部分——輸入和輸出單元、處理器和存儲設備——都在不同的地方，是通過信息連接起來的。或者你可能使用遠程通信線——電話線——接進位于外部數據單元的信息。然后你可能將信息傳送到自己的微機上用于重新工作和分析。

　　為了連接到網絡上，需要使用特殊用途的硬件部件來處理所有的傳送。這個硬件被稱為是網絡適配卡或網絡接口卡，它是插入到計算機總路線上的印刷電路板，由電纜將它連接到網絡介質。

　　通信網絡由于其占據的地理范圍大小而不同。有三種重要的類型：局域網、城市網和廣域網。

　　局域網 計算機和外部設備在很近的物理范圍內的網絡被稱為是局域網，例如在一座大樓內，由電纜鏈接——電話線、同軸電纜或光纜。局域網通常使用總線型的結構。在局域網中人們可以共享不同的設備，這樣可以降低設備的費用。局域網可以通過使用網關連接到另外一個局域網或者更大的網。使用網關，一個局域網可以被連接到另一個辦公團體的局域網，它也可被連接到世界范圍的其他局域網上，即使它們的配置不同。另外一種方法是用網橋來連接具有相同配置的網絡。#p#

　　對于相同局域網有一個新開發的網絡：無線局域網。無線局域網是靈活的數據傳輸系統，實現了大樓或校園內有線局域網的延伸或替換。無線局域網使用電磁波通過空氣傳送和接收數據，***限度地減少了有線連接。這樣，無線局域網把數據連接和用戶移動性結合起來，通過簡化的配置，形成了移動的局域網。

　　隨著近幾年的發展，無線局域網在一些市場領域已經獲得了廣泛的普及，其中包括健康保健、零售業、制造業、倉儲業和學術界。這些工業通過手提終端和筆記本電腦將實時信息傳送到中央主機進行處理，從獲得的生產率中已獲益百淺。如今，對于廣泛的商業客戶來說，無線局域網正成為公認的通用連接的替代品。

　　無線局域網的應用 無線局域網通常是增加而不是代替有線局域網的功能——通常是在中樞網絡和移動用戶間提供***幾米的連接。通過無線局域網的靈活性和功能，以下所列描述了可能實現的許多應用當中的一些：

　　●醫院的醫生和護士利用手提或筆記本電腦與無線局域網連接的性能，及時傳遞了病人的信息，提高了效率。

　　●顧問或會計審計事務組或一些小的工作組使用快速搭建的網絡提高了工作效率。

　　●在動態環境下的網絡管理者使用無線局域網***限度地減少了經常的移動、添加和修改工作，從而降低了局域網所有者的費用。

　　●公司的培訓點和大學的學生使用無線連接便于訪問信息，進行信息交換以及學習。

　　●在舊的建筑物內安裝網絡計算機的網絡管理員發現無線局域網是劃算的網絡基礎結構的解決方案。

　　●零售商店的老板使用無線局域網簡化經常性的網絡重新配置(問題)。

　　●貿易展覽部門工作人員通過安裝預先配置的無線局域網***限度地降低了配置需求，而不需要當地信息管理系統的支持。

　　●倉儲工人使用無線局域網和中心數據庫交換信息提高了生產率。

　　●網絡管理員使用無線局域網提供運行在有線網絡上的關鍵應用程序的備份。

　　●在會議室的高級行政官因為播送有實時信息可供使用，因此可以做出快速的決定。

　　日益拉長的移動用戶也成為無線局域網的堅實的后備力量。使用膝上電腦和無線網絡接口卡就可實現移動訪問無線局域網，這就使得用戶可以在不同的地方穿梭——會議室、門廳、休息室、自助食堂、教室等地方——仍然可以訪問其網絡數據。假如沒有無線局域網，用戶就不得不攜帶笨重的電纜尋找網絡插頭。

　　大城市網 這些網絡用于一個地市內的建筑物之間的連接。移動電話系統通過允許將汽車電話和移動電話接入而擴展了大城市網的靈活性。

　　寬域網 寬域網是國家和世界范圍內的網絡。在其他的信道種類中，寬域網使用微波中繼和衛星通信遠距離到達用戶。使用最廣泛的寬域網是Internet，它允許在世界范圍內用戶和用戶及設備的連接。