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WHAT IS A NETWORK? a group of >= 2 computers/devices linked together. BENEFITS

WHAT IS A NETWORK? a group of >= 2 computers/devices linked together. BENEFITS Information sharing ( 分享資料) - e.g.database, data and files Hardware sharing ( 分享硬件) -e.g. printer Software sharing ( 分享軟件) -a license purchased to install a copy for many users Data Communication ( 數據溝通)

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WHAT IS A NETWORK? a group of >= 2 computers/devices linked together. BENEFITS

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  1. WHAT IS A NETWORK? a group of >= 2 computers/devices linked together.BENEFITS Information sharing(分享資料)-e.g.database, data and files Hardware sharing (分享硬件)-e.g. printer Software sharing (分享軟件)-a license purchased to install a copy for many users Data Communication(數據溝通) Collaborative environment(共同作業) – work together in group project online Cut costs(減成本) 2 TYPES LAN (local area network) (區域網絡)– limited geographical area WAN(wide area network) (廣域網絡)- large geographical area DATA COMMUNICATON-data transmission from one place to another. Requirements: A sending device (a computer) A communication hardware(modem, cable modem)a network transmission hardware ( NIC , Hub, Bridge, Gateway, Router) A communication media (twisted pair, coaxial or fiber optic cable)radio, microwaves, satellites signals) A receiving device(a computer) Communication software (email, icq) USES OF COMMUNICATION TECHNOLOGIES – Voice Mail, Fax, Email, Instant Messaging, Chat rooms, newsgroups,telephony, videoconferencing, collaboration, groupware, Global Position System

  2. Data Communication Internet Digital signals Digital signals Analog signals Analog signals Sending computer Receiving computer modem modem ISP ISP Telephone lines Telephone lines Digital signals(數碼 訊息)(e.g. bits 101010) are used by computers.Analog signals (類比 訊息)(electrical wave) are used by telephone line.A modemconverts digital signals from the sending computer into analog signals to be used by telephone line.(modulate) Data travels through telephone lines to sender’s ISP server. Data are divided into packets(細訊息郵包)which are then routed(路由)through Internet to the ISPserver of the receiver. Data are then reassembled(重新整合).Another modem converts analog signals back into digital signals to be used by the receiving computer. (demodulate)

  3. Communication Media Bandwidth頻寬–width of communication channels. The higher the bandwidth, the more data/information/signals the channel can transmit. Broadband寬頻 media can transmit multiple signals simultaneously.

  4. Network hardware

  5. ARCHITECTURE OF NETWORKS Client/Server(主從式網絡) A high speed , high storage host computer (server) manages network resources.Servers can be printer server, database server, file server, web server, mail server, FTP server or network server. Other computers (clients) rely on the server for resources. Peer-to-Peer(對等式網絡) All computers are equal. Less than 10 computers connected.Each computer has its own resources to be shared by others. Easier to setup and manage. Used by small offices. workstations(Client) workstations(Client) (server) workstations(Client) workstations(Client) workstations(Client) workstations(Client) workstations(Client)

  6. Network Topology(網絡的鋪排) Stara central computer(server) connects to terminals. Messages from one terminal must pass throughthe central computer to get to another terminal. Advantages of Star Network efficient centralized control.(有效中央控制) add/remove terminals from networking without affecting it.Disadvantages of Star Network If the central computer fails, the whole network breaks.Bottleneck /jam(樽頸擠塞)if too many computers. terminals server terminals terminals terminals

  7. Ring(圈狀網絡) All computers are connected in a circle.Has no central computer. One special signal called token is passing around the circle freely.沒有記載訊息叫“代幣”無時無刻圍繞網絡行走 The computer which pick up the free token can send data by writing message on it and send it out to the network again.任何電腦如果想傳送信息給另一電腦, 先從網絡中取下 空“代幣”,寫下信息和對方的地址,然後再傳送出去網絡 If one of other computers find out the token is for it, it will get the token and read the message.Then it pass back the token to the sending computer. It then free the token again. (如果其他電腦發現“代幣”是給他的,便取下它和讀取其內容,再把空“代幣”傳送回去傳送電腦,由他刪除內容) Advantages- signals transmits in one direction and at high speed.訊息高速單方向傳送 Disadvantages -If one computer fails, the whole network break. token A D B C

  8. Bus (線狀網絡) All computers are connected to a single cable(backbone)and share it.所有電腦連接去和分享一條網絡中輸線 Only one computer allowed to occupy the cable at any one time.任何時間,只有一部電腦可佔有中輸線傳送訊息 Thus, if two nodes try to send message at the same time, a collision occurs. Then the nodes must try to resend.如同時一部以上的電腦想傳送訊息,訊息便會互撞擊.電腦們便要重新鬥快傳送 Advantagescomputers can be attached/detached without affecting network電腦隨時插入網絡或可從網絡取走而不影響網絡運作relatively inexpensive比較平宜easy to install and maintain容易安裝和保養 Disadvantages-Speed is low,because many collisions happen速度慢,因有大多訊息互撞,電腦要重新鬥快傳送

  9. INTRANETS(內 聯網) • An internal network that uses Internet technologies and makes company resources accessible to employees and facilitate working in groups. • Use TCP/IP technologies • Has a web server • FIREWALLS(防火牆) • A security system of hardware/softwares that • restricts access to specific authorized users • prevent unauthorized access(非法存取)to data on the Intranet. • Screen all incoming and outgoing messages. • Check domain name/IP address of the message. • Install a proxy server (代理服務器)outside the Intranet that controls which communications pass into the company’s network.

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