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Transcript
Chapter 6 Computer Networks OBJECTIVES After reading this chapter, the reader should be able to: Understand the rationale for the existence of networks. Distinguish between the three types of networks: LANs, MANs, and WANs. Understand the OSI model and TCP/IP. List different connecting devices and the OSI layers in which each device operates. Understand client-server models. 6.1 NETWORKS, LARGE AND SMALL Network Computer network – A combination of computers connected through transmission media. LAN (Local Area Network) MAN (Metropolitan Area Network) WAN (Wide Area Network) Internetwork – (e.g. Internet) Networks can be connected using connecting device. Model and Protocol Model – the specification set by a standards organization as a guideline for designing networks. Protocol – a set of rules that controls the interaction of different devices in a network/internetwork. 6.2 OSI MODEL Note: The OSI (Open Systems Interconnection) model is a theoretical model that shows how any two different systems can communicate with each other. OSI Model – is a framework of 7 layers that gives network designers an idea of the functionality of each separate but related layer. Figure 6-1 The OSI model control information is added to the data in the form of headers or trailers. the header or trailer is dropped in each layer. Flow of data in the OSI model Figure 6-2 Functions of the Layers Physical 1. • transmit a bit stream over a physical medium. Data-Link 2. • • Organizes bits into logical units called frames. Node-to-node delivery Network 3. • Source-to-destination delivery of a packet. Transport 4. • Source-to-destination delivery of the entire message. Session 5. • Establishes, maintains, and synchronizes the dialog between communicating systems. Presentation 6. • Deals with the fact that different systems use different coding methods. Application 7. • Enables the user to access the network. OSI Layers 實例: 7 Application Layer 6 Presentation Layer 5 Session Layer 4 Transport Layer 3 Network Layer 快遞公司:決定使用那個機場、經由哪條路徑傳遞 2 Data Link Layer 機場管理局:班次﹑泊位﹑進場/出場時間間隔﹑收發貨物 1 Physical Layer 飛機:運送包裹 產品行銷:策略、合同、邀訪、樣品 秘書:將型錄、報價單、合約翻譯成各國語言的文件格式 一筆生意:寄型錄、寄報價單、寄合約、… 資料打包寄送:將資料拆開、分裝、編號成幾個信封﹐再進行郵寄 6.3 CATEGORIES OF NETWORKS Categories of networks Figure 6-3 LAN (Local Area Network) Allow resource sharing between computers. Computers Peripheral devices Transmission medium (e.g. cable) 3 types of topology Bus topology Star topology Ring topology LANs Figure 6-4 Hub – a device that facilitates connection LAN acts logically like a bus. Star – the dominant topology today. MAN Figure 6-5 MAN (Metropolitan Area Network) Uses services provided by a network service provider. (Tel. Company) Individual users’ computers Organizations’ LANs Many Tel. Company provide a popular MAN service called SMDS (Switched Multimegabit Data Services) Prior to SMDS's arrival in 1995, the only way to connect LANs was through a dedicated private line. WAN Figure 6-6 WAN (Wide Area Network) The connection of individual computers or LANs over a large area (country, world). User using a telephone line to connect to an ISP (Internet Service Provider) is using a WAN. Negotiates fee ISP Tel. company 6.4 CONNECTING DEVICES Figure 6-7 Connecting devices Repeater (L1) Regenerates the signal. Extends the physical length of a network. Figure 6-8 Note: Repeaters operate at the first layer of the OSI model. Bridge/Switch Bridge A traffic controller Divide a long bus into smaller segments so that each segment is independent trafficwise. Regenerate data Switch A sophisticated bridge with multiple interfaces. A station that needs to send a frame sends it directly to the switch. Bridge (L1-L2) Figure 6-9 Note: Bridges operate at the first two layers of the OSI model. Figure 6-10 Switch Router Connect LANs, MANs, and WANs. Routes a packet based on the logical address (network layer) of the packet. Connect two independent networks: LAN-WAN, LAN-MAN, WAN-WAN… V.S. Bridge filters a frame based on the physical address (datalink layer) of the frame. Connect two segments of a LAN or two LANs belonging to the same organization. Routers (L1-L3) in an internet Figure 6-11 Note: Routers operate at the first three layers of the OSI model. Gateway (L1-L7) A protocol converter. Understands the protocols used by each connected network and is able to translate from one to another. Figure 6-12 Connecting devices and the OSI model internetworking IP address Mac address packet frame networking ISP (Internet Service Provider) a company that provides access to the Internet. For a monthly fee, the service provider gives you a software package, username, password and access phone number. Equipped with a modem, you can then log on to the Internet and browse the World Wide Web, and send and receive e-mail. ICP (Internet Content Provider) 網際網路內容提供者。指的是在網際網路上提供各種服務內容的廠商。 如Yahoo雅虎、AOL美國線上等,都是屬於ICP的一種。任何人只要 有網路的空間放置網頁,都可以成為ICP。而ICP的收入大都來自廣 告收入。 Web portal A Web site or service that offers a broad array of resources and services, such as e-mail, forums, search engines, and on-line shopping malls. ASP (Application Service Provider) ASP 業者向軟體公司購買應用程式,然後將此程式放在 ASP 業者的 機房內,然後供企業或個人以存取網路的方式連接到機房來使用應用 程式,並依據使用方式與規定向客戶收取費用。 6.5 THE INTERNET AND TCP/IP Internet internetwork (internet) A network of networks Connect individual LANs, MANs, and WANs. Internet – the most famous one TCP/IP and OSI model TCP/IP Transmission Control Protocol/Internet Protocol A suite of protocols that officially control the Internet. Physical and data-link layers: Does not define any specific protocol. Network layer: Supports the Internet Protocol (IP) Transport layer: Defines Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) Application layer: Equivalent to the combined session, presentation, and application layers TCP/IP Network layer Supports the Internet Protocol (IP) Every computer connected to the Internet is identified by a unique international address – IP address. (32bits) Figure 6-14 IP addresses in dotted-decimal notation TCP/IP Transport layer Defines Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) TCP is a reliable transport protocol. It divides a message into a sequence of segments that are numbered sequentially. If one segment is lost, it is sent again. If a segment is received out of order, it is ordered with the help of the sequence numbering mechanism. TCP/IP Application layer Equivalent to the combined session, presentation, and application layers of OSI model. Communication on the Internet uses the client-server model. – an AP running on a local machine. Server – an AP running on a remote machine. Client Client-server model FTP File Transfer Protocol The standard protocol on the Internet for transferring a file from one machine to another. SMTP(Simple Mail Transfer Protocol) A user machine that is turned off will not receive email. POP(Post Office protocol) UA(user Agent)- a user interface to facilitates these transactions Email address Local part – user mailbox Domain name – the computer that serves as the SMTP server TELNET TErminaL NETwork A general client-server program on the Internet allows remote login. Enables the establishment of a connection from a local system to a remote system in such a way that the local terminal appears to be a terminal at the remote system. HTTP HyperText Transfer Protocol the underlying protocol used by the World Wide Web. use a special kind of addressing called URL(Uniform Resource Locator). URL Method – the client-server program used for transferring the documents. Host – the computer where the information is located. Port – the port number of server. Path – the path of the file where the information is located. WWW World Wide Web Hypertext – a document containing special text, words, and phrases that can create a link to other documents. Page – a document of hypertext available on the web. Home page Browser Coordinates all activities. Display the document. A client AP that retrieves the document. Categories of Web documents Document types: Static documents Have fixed contents HTML Dynamic documents Programs residing at the server site. Use CGI(Common Gateway Interface) technology includes Perl and HTML to handle the document creation and interpretation. Active documents The browser requests the transfer of the program. After transfer, it is run at the browser site. Java 台灣網路資訊中心(TWNIC) 國家級財團法人機構,以非營利為目的,是由 交通部電信總局及中華民國電腦學會共同捐助 設立,宗旨是以超然中立及互助共享網路資源 之精神,提供IP address分配、網域名稱 (Domain Name)註冊等服務。 網路蟑螂 1999年底,登記使用business.com的權利,以750 萬美金的天價在拍賣網站電子海灣(eBay.com)賣出。 china.com、ibm.com apple.com whitehouse.com 即時傳訊 (Instant Messenger) 它支援在 Internet 上聊天、發送消息和文件等,另更有語音 聊天、網路攝影機等功能 1. ICQ 全球最早出現的即時傳訊軟體,在1998年6月被AOL以2.87億美元所收購 2. AIM﹙AOL Instant Messenger﹚ 由AOL所開發出來、類似ICQ的即時傳訊軟體 3. QQ 中國騰訊公司產品,佔中國95%使用人口(1億500 萬) 4. MSN即時通 由微軟所開發出來的即時傳訊軟體 5. Yahoo!奇摩即時通 由全球入口網站領導品牌Yahoo!所開發的即時傳訊軟體 6. YamQQ 蕃薯藤 +QQ