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(jì)算機(jī)網(wǎng)絡(luò)介紹

　　計(jì)算機(jī)網(wǎng)絡(luò)是連接兩個(gè)或多個(gè)計(jì)算機(jī)的系統(tǒng)，它允許用戶(hù)快速地交換數(shù)據(jù)，訪問(wèn)和共享包括設(shè)備、應(yīng)用軟件和信息在內(nèi)的資源。

　　數(shù)據(jù)通信系統(tǒng)是通過(guò)通信線路將數(shù)據(jù)從一個(gè)地方傳送到另外一個(gè)地方的電子系統(tǒng)。你可以使用數(shù)據(jù)通信通過(guò)你的微機(jī)將信息發(fā)送給使用另外一臺(tái)機(jī)器的朋友。你有可能在為一家公司工作，其計(jì)算機(jī)系統(tǒng)遍布一座大樓，或者甚至是全國(guó)乃至世界。也就是說(shuō)，所有的部分——輸入和輸出單元、處理器和存儲(chǔ)設(shè)備——都在不同的地方，是通過(guò)信息連接起來(lái)的。或者你可能使用遠(yuǎn)程通信線——電話(huà)線——接進(jìn)位于外部數(shù)據(jù)單元的信息。然后你可能將信息傳送到自己的微機(jī)上用于重新工作和分析。

　　為了連接到網(wǎng)絡(luò)上，需要使用特殊用途的硬件部件來(lái)處理所有的傳送。這個(gè)硬件被稱(chēng)為是網(wǎng)絡(luò)適配卡或網(wǎng)絡(luò)接口卡，它是插入到計(jì)算機(jī)總路線上的印刷電路板，由電纜將它連接到網(wǎng)絡(luò)介質(zhì)。

　　通信網(wǎng)絡(luò)由于其占據(jù)的地理范圍大小而不同。有三種重要的類(lèi)型：局域網(wǎng)、城市網(wǎng)和廣域網(wǎng)。

　　局域網(wǎng) 計(jì)算機(jī)和外部設(shè)備在很近的物理范圍內(nèi)的網(wǎng)絡(luò)被稱(chēng)為是局域網(wǎng)，例如在一座大樓內(nèi)，由電纜鏈接——電話(huà)線、同軸電纜或光纜。局域網(wǎng)通常使用總線型的結(jié)構(gòu)。在局域網(wǎng)中人們可以共享不同的設(shè)備，這樣可以降低設(shè)備的費(fèi)用。局域網(wǎng)可以通過(guò)使用網(wǎng)關(guān)連接到另外一個(gè)局域網(wǎng)或者更大的網(wǎng)。使用網(wǎng)關(guān)，一個(gè)局域網(wǎng)可以被連接到另一個(gè)辦公團(tuán)體的局域網(wǎng)，它也可被連接到世界范圍的其他局域網(wǎng)上，即使它們的配置不同。另外一種方法是用網(wǎng)橋來(lái)連接具有相同配置的網(wǎng)絡(luò)。#p#

　　對(duì)于相同局域網(wǎng)有一個(gè)新開(kāi)發(fā)的網(wǎng)絡(luò)：無(wú)線局域網(wǎng)。無(wú)線局域網(wǎng)是靈活的數(shù)據(jù)傳輸系統(tǒng)，實(shí)現(xiàn)了大樓或校園內(nèi)有線局域網(wǎng)的延伸或替換。無(wú)線局域網(wǎng)使用電磁波通過(guò)空氣傳送和接收數(shù)據(jù)，***限度地減少了有線連接。這樣，無(wú)線局域網(wǎng)把數(shù)據(jù)連接和用戶(hù)移動(dòng)性結(jié)合起來(lái)，通過(guò)簡(jiǎn)化的配置，形成了移動(dòng)的局域網(wǎng)。

　　隨著近幾年的發(fā)展，無(wú)線局域網(wǎng)在一些市場(chǎng)領(lǐng)域已經(jīng)獲得了廣泛的普及，其中包括健康保健、零售業(yè)、制造業(yè)、倉(cāng)儲(chǔ)業(yè)和學(xué)術(shù)界。這些工業(yè)通過(guò)手提終端和筆記本電腦將實(shí)時(shí)信息傳送到中央主機(jī)進(jìn)行處理，從獲得的生產(chǎn)率中已獲益百淺。如今，對(duì)于廣泛的商業(yè)客戶(hù)來(lái)說(shuō)，無(wú)線局域網(wǎng)正成為公認(rèn)的通用連接的替代品。

　　無(wú)線局域網(wǎng)的應(yīng)用 無(wú)線局域網(wǎng)通常是增加而不是代替有線局域網(wǎng)的功能——通常是在中樞網(wǎng)絡(luò)和移動(dòng)用戶(hù)間提供***幾米的連接。通過(guò)無(wú)線局域網(wǎng)的靈活性和功能，以下所列描述了可能實(shí)現(xiàn)的許多應(yīng)用當(dāng)中的一些：

　　●醫(yī)院的醫(yī)生和護(hù)士利用手提或筆記本電腦與無(wú)線局域網(wǎng)連接的性能，及時(shí)傳遞了病人的信息，提高了效率。

　　●顧問(wèn)或會(huì)計(jì)審計(jì)事務(wù)組或一些小的工作組使用快速搭建的網(wǎng)絡(luò)提高了工作效率。

　　●在動(dòng)態(tài)環(huán)境下的網(wǎng)絡(luò)管理者使用無(wú)線局域網(wǎng)***限度地減少了經(jīng)常的移動(dòng)、添加和修改工作，從而降低了局域網(wǎng)所有者的費(fèi)用。

　　●公司的培訓(xùn)點(diǎn)和大學(xué)的學(xué)生使用無(wú)線連接便于訪問(wèn)信息，進(jìn)行信息交換以及學(xué)習(xí)。

　　●在舊的建筑物內(nèi)安裝網(wǎng)絡(luò)計(jì)算機(jī)的網(wǎng)絡(luò)管理員發(fā)現(xiàn)無(wú)線局域網(wǎng)是劃算的網(wǎng)絡(luò)基礎(chǔ)結(jié)構(gòu)的解決方案。

　　●零售商店的老板使用無(wú)線局域網(wǎng)簡(jiǎn)化經(jīng)常性的網(wǎng)絡(luò)重新配置(問(wèn)題)。

　　●貿(mào)易展覽部門(mén)工作人員通過(guò)安裝預(yù)先配置的無(wú)線局域網(wǎng)***限度地降低了配置需求，而不需要當(dāng)?shù)匦畔⒐芾硐到y(tǒng)的支持。

　　●倉(cāng)儲(chǔ)工人使用無(wú)線局域網(wǎng)和中心數(shù)據(jù)庫(kù)交換信息提高了生產(chǎn)率。

　　●網(wǎng)絡(luò)管理員使用無(wú)線局域網(wǎng)提供運(yùn)行在有線網(wǎng)絡(luò)上的關(guān)鍵應(yīng)用程序的備份。

　　●在會(huì)議室的高級(jí)行政官因?yàn)椴ニ陀袑?shí)時(shí)信息可供使用，因此可以做出快速的決定。

　　日益拉長(zhǎng)的移動(dòng)用戶(hù)也成為無(wú)線局域網(wǎng)的堅(jiān)實(shí)的后備力量。使用膝上電腦和無(wú)線網(wǎng)絡(luò)接口卡就可實(shí)現(xiàn)移動(dòng)訪問(wèn)無(wú)線局域網(wǎng)，這就使得用戶(hù)可以在不同的地方穿梭——會(huì)議室、門(mén)廳、休息室、自助食堂、教室等地方——仍然可以訪問(wèn)其網(wǎng)絡(luò)數(shù)據(jù)。假如沒(méi)有無(wú)線局域網(wǎng)，用戶(hù)就不得不攜帶笨重的電纜尋找網(wǎng)絡(luò)插頭。

　　大城市網(wǎng) 這些網(wǎng)絡(luò)用于一個(gè)地市內(nèi)的建筑物之間的連接。移動(dòng)電話(huà)系統(tǒng)通過(guò)允許將汽車(chē)電話(huà)和移動(dòng)電話(huà)接入而擴(kuò)展了大城市網(wǎng)的靈活性。

　　寬域網(wǎng) 寬域網(wǎng)是國(guó)家和世界范圍內(nèi)的網(wǎng)絡(luò)。在其他的信道種類(lèi)中，寬域網(wǎng)使用微波中繼和衛(wèi)星通信遠(yuǎn)距離到達(dá)用戶(hù)。使用最廣泛的寬域網(wǎng)是Internet，它允許在世界范圍內(nèi)用戶(hù)和用戶(hù)及設(shè)備的連接。