Computer Networks

1. Chapter 6 ComputerNetworks 主讲：李艳 云南农业大学

2. 6.1 NETWORKS, LARGE AND SMALL A computer network is a combination of systems (e.g., a computer) connected through transmission media (e.g., a wire, a cable, or air). A computer network can span a small, medium, or large geographical area. Respectively called: LAN, MAN and WAN. These three types of networks can also be connected using connecting devices to form an internetwork (or an internet). MODEL AND PROTOCOL A model is the specification set by a standards organization as a guideline for designing networks. A protocol, on the other hand, is a set of rules that controls the interaction of different devices in a network or an internetwork.

3. 6.2 OSI MODEL The Open Systems Interconnection model is a theoretical model that shows how any two different systems can communicate with each other. SEVEN LAYERS

4. Flow of data in the OSI model

5. FUNCTIONS OF THE LAYERS Physical Layer The physical layer is responsible for transmitting a bit stream over a physical medium. The mechanical and physical specifications of the physical devices are determined by the physical layer. Data-Link Layer The data-link layer organizes bit into logical units called frames. Note that the data-link layer is responsible only for node-to-node delivery of the frame (from one station to another).

6. Network Layer The network layer is responsible for delivery of a packet (the data unit handled by network layer is called a packet) between the original source and final destination. Note that when a packet moves from a source to a desti-nation, the physical address (added at the data-link layer) changes from station to station, but the logical address remains unchanged from the source to the destination. Transport Layer The transport layer is responsible for source-to-destination (end-to-end) delivery of the entire message. The network layer is responsible for end-to-end delivery of individual packets. The transport layer, in contrast, is responsible for end-to-end delivery of the whole message.

7. Session Layer The session layer is designed to control the dialog between users. It establishes, maintains, and synchronization the dialog between communicating systems. Presentation Layer The presentation layer is concerned with the syntax (format) and semantics (meaning) of the information exchanged between two systems. It compresses and decompresses data for efficiency. It encrypts and decrypts data for security. Application Layer The application layer enables the user, whether human or software, to access the network. It defines common applications that can be implemented to make the job of the user simpler.

8. 6.3 CATEGORIES OF NETWORKS We can divide the networks into three broad categories: local area networks (LANs), metropolitan area networks (MANs), and wide area networks (WANs). LOCAL AREA NETWORK (LAN) A local area network (LAN) is designed to allow resource sharing (hardware, software, and data) between computers. A LAN can be simply defined as a combination of computers and peripheral devices (e, g., printers) connected through a transmission medium (e.g., cable).

10. METROPOLITAN AREA NETWORK (MAN) A metropolitan area network (MAN) uses services provided by a common carrier (network service provider) such as a telephone company. It spans a city or a town and provides services to individual users or organizations.

11. WIDE AREA NETWORK (WAN) A wide area network (WAN) is the connection of individual computers or LANs over a large area (state, country, and world). Note that a person using a telephone line to connect to an Internet service provider (ISP) is using a WAN.

12. 6.4 CONNFECTING DEVICES Connecting devices can be divided into four types based on their functionality as related to the layers in the OSI model: repeaters, bridges, routers, and gateways. Repeaters and bridges typically connect devices in a network. Routers and gateways typically connect networks into internetworks.

13. REPEATERS A repeater is an electronic device that regenerates data. It extends the physical length of a network. As a signal is transmitted, it may lose strength, and a weak signal may be interpreted erroneously by a receiver. A repeater can regenerate the signal and send it to the rest of the network. Repeaters operate at the first layer of the OSI model.

14. BRIDGES A bridge is a traffic controller. It can divide a long bus into smaller segments so that each segment is independent trafficwise. The bridge uses a table to decide if the frame needs to be forwarded to another segment. With a bridge, two or more pairs of stations can communicate at the same time.

15. In addition to its traffic controlling duties, a bridge also functions as a repeater by regenerating the frame. This means that a bridge operates at the physical layer. But because a bridge needs to interpret the address embedded in the frame to make filtering decisions, it also operates at the data-link layer of the OSI model. Bridges operate at the first two layersof the OSI model. In recent years, the need for better performance has led to the design of a new device referred to as a second-layer switch, which is simply a sophisticated bridge with multiple interfaces. The media are not shared; each station is directly connected to the switch.

16. ROUTERS Routers are devices that connect LANs, MANs, and WANs. A router operates at the third layer of the OSI model. Whereas a bridge filters a frame based on the physical (data-link layer) address of the frame, a router routes a packet based on the logical (network layer) address of the packet.

17. GATEWAYS Traditionally, a gateway is a connecting device that acts as a protocol converter. It allows two networks, each with a different set of protocols for all seven OSI layers, to be connected to each other and communicate. A gateway is usually a computer installed with the necessary software. The gateway understands the protocols used by each connected network and is therefore able to translate from one to another. For example, a gateway can connect a network using the Apple Talk protocol to a network using the Novell Netware protocol.

18. 6.5 THE INTERNET AND TCP/IP The Internet was originally a research internetwork designed to connect several different heterogeneous networks. It was sponsored by the Defense Advanced Research Projects Agency (DARPA). Today, however, the Internet is an internetwork that connects millions of computers throughout the world. The Transmission Control Protocol/Internet Protocol (TCP/IP) is a suite or a stack of protocols that officially controls the Internet. TCP/IP was developed before the OSI model. Therefore, the layers in the TCP/IP protocol do not exactly match those in the OSI model.

19. TCP/IP and OSI model

20. PHYSICAL AND DATA-LINK LAYERS At the physical and data-link layers, TCP/IP does not define any specific protocol. It supports all of the standard and proprietary protocols. NETWORK LAYER At the network layer (or more accurately, the internetwork layer or the internet layer), TCP/IP supports the Internet Protocol (IP). IP is an unreliable protocol and a best-effort delivery service. The term best-effort means that IP provides no error checking or tracking. The data unit at the IP layer is called an IP datagram, an independent packet that travels from the source to the destination.

21. Addressing TCP/IP requires that every computer connected to the Internet be identified by a unique international address. This address is sometimes referred to as the Internet address, or IP address. Each Internet address consists of 4 bytes (32 bits). To make the 32-bit form shorter and easier to read, Internet addresses are usually written in decimal form with decimal points separating the bytes: dotted-decimal notation.

22. TRANSPORT LAYER At the transport layer, TCP/IP defines two protocols: Transmission Control Protocol (TCP) and User Datagram Protocol (UDP). The TCP provides full transport layer services to applications. TCP is a reliable transport protocol. APPLICATION LAYER Communication on the Internet uses the client-server model. A client, an application program running on a local machine, requests a service from a server, an application program running on a remote machine. Usually, the service program is always running, and the client program runs only when needed.

23. Client-server model

24. File Transfer Protocol (FTP) The standard protocol on the Internet for transferring a file from one machine to another is the File Transfer Protocol (FTP). FTP was designed to respond to traditional problems related to file transfer, one problem is the different coding systems in use; one machine may use ASCII, and the other may use Unicode. Another problem is the different file formats in use. FTP was designed to resolve these problems.

25. Simple Mail Transfer Protocol (SMTP) By far the most popular application on the Internet today is electronic mail (email). The protocol that supports email on the Internet is Simple Mail Transfer Protocol (SMTP). To send and receive email, the user must install both client and server SMTP software on a computer. SMTP is often used with another protocol such as Post Office Protocol (POP).

26. Address SMIP uses a unique addressing system that consists of two parts: a local part and a domain name separated by an @ sign. The local part defines the name of a special file, called the user mailbox, where all the mail received for a user is stored for retrieval by the user agent. The domain name defines the computer (often symbolically) that serves as the SMTP server.

27. Hypertext Transfer Protocol (HTTP) Hypertext Transfer Protocol (HTTP) is a client-server program that is used to access and transfer documents in the World Wide Web. Uniform Resource locator (URL) HTTP uses a special kind of addressing called the Uniform Resource Locator (URL), which is a standard for specifying any kind of information on the Internet. The URL defines four things: method, host computer, port, and path. For example: http://www.w3.org/hypertext/project.html method: http, host: www.w3.org，pathname and name of file: hypertext/project.html。

28. WWW World Wide Web (www), or the Web, is based on the idea of distributed information. Instead of holding all information in one place, each entity (individual or organization) that has information to share stores that information on its own computer and allows Internet users to access it. The WWW is a collection of multimedia documents. Hypertext The WWW uses the concept of hypertext, which is a document containing special text, words, and phrases that can create a link to other documents containing text, images, audio, or video. A document of hypertext available on the Web is called a page. The main page for an organization or an individual is known as a home page.

29. Browser To access a page on the WWW, one needs a browser that usually consists of three parts: a controller, a method, and an interpreter.The controller is the heart of the browser; it coordinates all activities. The method is a client application program that retrieves the document. Although it can be any of the application programs we have discussed, it is usually HTTP. The interpreter displays the document in the screen.

30. SUMMARY • A computer network is a combination of devices connected by transmission media. • The Open Systems Interconnection (OSI) model is a theoretical model that shows how any two different systems can communicate with each other. • The seven layers of the OSI model are the physical layer, data-link layer, Network layer, transport layer, session layer, presentation layer, and application layer. • A local area network (LAN) allows resource sharing (hardware, software, and data) between computers. • A LAN can de configured in a bus, ring, or star topology. • A metropolitan area network (MAN) uses the services provided by a common carrier.

31. SUMMARY (continued) • A wide area network (WAN) is the connection of individual computers or LANs over a large area. WANs are installed and run by common carriers. • A repeater is a connecting device that regenerates data and extends the physical length of a network. • A bridge is a connecting device that filters traffic. • A router is a connecting device that routes packets. • A gateway allows two networks, each with a completely different protocol suite, to communicate. • An internetwork is two or more LANs, MANs, or WANs. • Transmission Control Protocol/Internet Protocol (TCP/IP) is the set of protocols used by the Internet, a worldwide internetwork of computers.

32. SUMMARY (continued) • The Internet Protocol (IP) is TCP/IP’s unreliable protocol at the internet layer. • An IP address identifies each computer connected to the Internet. • User Datagram Protocol (UDP) and Transmission Control Protocol (TCP) are protocols at the transport layer. • File Transfer Protocol (FTP) is a TCP/IP client-server application for coping files from one host to another. • The protocol that supports electronic mail (email) on the Internet is Simple Mail Transfer Protocol (SMTP).

33. SUMMARY (continued) • TELNET is a client-server application that allows a user to log on to a remote machine, giving the user access to the remote system. • Hypertext Transfer Protocol (HTTP) is a client-server program for accessing and transferring documents on the World Wide Web (WWW), a collection of multimedia documents. • The Uniform Resource Locator (URL) is a standard identifier for specifying information on the Internet. • A browser is needed to access a page on the WWW. • A document on the Internet can be classified as static, dynamic, or active.

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