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©Brooks/Cole, 2003 Chapter 6 Computer Networks
©Brooks/Cole, 2003 Understand the rationale for the existence of networks. Distinguish between the three types of networks: LANs, MANs, and WANs. After reading this chapter, the reader should be able to: O BJECTIVES List different connecting devices and the OSI layers in which each device operates. Understand client-server models. Understand the OSI model and TCP/IP.
©Brooks/Cole, 2003 NETWORKS, LARGE AND SMALL NETWORKS, 6.1
©Brooks/Cole, 2003 Computer networks A computer network is a combination of systems connected through transmission media. Local area network (LAN) Metropolitan ( 都市的 ) area network (MAN) Wide area network (WAN)
©Brooks/Cole, 2003 Model and protocol A model is the specification set by a standards organization as a guideline for designing networks. A protocol is a set of rules that controls the interaction of different devices in a network or an internetwork.
©Brooks/Cole, 2003 OSI MODEL 6.2
©Brooks/Cole, 2003 The Open Systems Interconnection (OSI) model is a theoretical model that shows how any two different systems can communicate with each other. Note:
©Brooks/Cole, 2003 Figure 6-1 The OSI model
Figure 6-2 Flow of data in the OSI model Header Trailer
©Brooks/Cole, 2003 Seven layers of OSI model Physical layer Data-link layer Network layer Transport layer Session layer Presentation layer Application layer
©Brooks/Cole, 2003 Physical layer The physical layer is responsible for transmitting a bit stream over a physical medium. It encodes and decodes bits into groups of bits. It then transforms a stream of bits into a signal.
©Brooks/Cole, 2003 Data-link layer The data-link layer organizes bits into logical units called frames. The data-link layer is responsible only for node-to-node delivery of the frame. The data-link layer is often responsible for error handling between two adjacent stations.
©Brooks/Cole, 2003 Network layer The network layer is responsible for delivery of a packet between the original source and final destination. Using logical addresses (IP addresses) instead of physical addresses. Example of IP address 18.104.22.168 (4 Bytes)
©Brooks/Cole, 2003 Transport layer The transport layer is responsible for source-to-destination delivery of the entire message. The transport layer is responsible for breaking the entire message into several packets and delivery them to the network layer. The transport layer is responsible for ensuring that the whole message is transmitted. If packets arrive out of order, they must be reorganized.
©Brooks/Cole, 2003 Session layer The session layer is designed to control the dialog between users. The synchronization ( 同步的 ) points divides a long message into smaller ones and ensure that each section is received and acknowledged by the receiver. Most network implementations today do not use a separate session layer, their services are usually included in the application layer.
©Brooks/Cole, 2003 Presentation layer The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems. It deals with the fact that different systems use different coding methods. Compress ( 壓縮 ) and decompress ( 解壓縮 ) data Encrypt ( 加密 ) and decrypt ( 解密 ) data Most implementations do not use a presentation layer today, their services are usually included in other layer.
©Brooks/Cole, 2003 Application layer The application layer enables the user to access the network. It defines common applications that can be implemented to make the job of the user simpler. Will be discussed later…
©Brooks/Cole, 2003 CATEGORIESOFNETWORKSCATEGORIESOFNETWORKS 6.3
Figure 6-3 Categories of networks
©Brooks/Cole, 2003 Figure 6-4 LANs
©Brooks/Cole, 2003 Local area network Bus topology When a station sends a frame to another computer, all computer receiver the frame and check its destination address. Star topology Hub (all, like a bus) v.s. switch (one) Ring topology When a computer needs to send a frame to another computer, it sends it to its neighbor.
©Brooks/Cole, 2003 Figure 6-5 MAN
Figure 6-6 WAN
©Brooks/Cole, 2003 Wide area network WAN is the connection of individual computers or LANs over a large area. A person using a telephone line to connect to an ISP is using a WAN. ISP: internet service provider
©Brooks/Cole, 2003 CONNECTINGDEVICESCONNECTINGDEVICES 6.4
Figure 6-7 Connecting devices
©Brooks/Cole, 2003 Figure 6-8 Repeater A repeater is an electronic device and operate only in the physical layer of the OSI model. A repeater can regenerate the signal and send it to the rest of the network.
©Brooks/Cole, 2003 Repeaters operate at the first layer (physical layer) of the OSI model. Note:
©Brooks/Cole, 2003 Figure 6-9 Bridge
©Brooks/Cole, 2003 Bridges In bus topology, 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.
©Brooks/Cole, 2003 Bridges operate at the first two layers (physical layer and data-link layer) of the OSI model. Note:
©Brooks/Cole, 2003 Figure 6-10 Switch—one kind of dynamic bridge
©Brooks/Cole, 2003 Figure 6-11 Routers in an internet
©Brooks/Cole, 2003 Routers operate at the first three layers (physical, data-link, and network layer) of the OSI model. Note:
©Brooks/Cole, 2003 Routers Whereas a bridge filters a frame based on the physical address of the frame, a router routes a packet based on the logical address of the packet. Whereas a bridge may connect two segments of a LAN, a router can connect two independent networks.
©Brooks/Cole, 2003 Gateways A gateway is a connecting device that acts as a protocol converter. A gateway is usually a computer installed with the necessary software. Today the term gateway is used interchangeably with the term router. The distinction between the two terms is disappearing.
©Brooks/Cole, 2003 Figure 6-12 Connecting devices and the OSI model
©Brooks/Cole, 2003 THE INTERNET ANDTCP/IP ANDTCP/IP 6.5
©Brooks/Cole, 2003 Protocols TCP: Transmission control protocol UDP: User datagram protocol IP: Internet protocol FTP: File transfer protocol SMTP: Simple mail transfer protocol POP: Post office protocol HTTP: Hypertext transfer protocol
Figure 6-13 TCP/IP and OSI model
©Brooks/Cole, 2003 Figure 6-14 IP addresses in dotted-decimal notation Network layer
©Brooks/Cole, 2003 Figure 6-15 Client-server model Application layer
©Brooks/Cole, 2003 Client-server model Client: an application program running on a local machine Server: an application program running on a remote machine A client requests a service from a server. The server program is always running, and the client program runs only when needed.
©Brooks/Cole, 2003 Figure 6-16 FTP -- Client-server model FTP -- Client-server model
©Brooks/Cole, 2003 FTP -- Client-server model FTP was designed to resolve two problems: Different coding systems in use One machine may use ASCII, and other may use Unicode Different file formats in use
©Brooks/Cole, 2003 Figure 6-17 SMTP/POP -- Client-server model SMTP -- Client-server model
©Brooks/Cole, 2003 Figure 6-18 Email address email@example.com
©Brooks/Cole, 2003 Figure 6-19 a general client-server model TELNET-- a general client-server model Local login v.s. remote login TELNET is a general client-server program on the Internet that allow remote login
©Brooks/Cole, 2003 Figure 6-20 URL http://www.ice.ntnu.edu.tw/~violet ftp://22.214.171.124HTTP URL: Uniform resource locator A special kind of addressing using by HTTP
©Brooks/Cole, 2003 WWW WWW: World wide web Hypertext: home page Browser Document Types Static documents Dynamic documents Active documents
©Brooks/Cole, 2003 Figure 6-21 Browser
©Brooks/Cole, 2003 Figure 6-22 Categories of Web documents Static documents HTML: Hypertext Markup Language Dynamic documents CGI: Common Gateway Interface (Perl) Active documents Java language
©Brooks/Cole, 2003 Key terms Active document Application layer Bridge Browser Bus topology Client Client-server model Computer network Connecting devices Data-link layer Dotted-decimal notation Dynamic document Email HTML HTTP Internet Internet address Internetwork Internet protocol (IP) IP address LAN Lical login MAN Model Network layer
©Brooks/Cole, 2003 Node Node-to-node delivery OSI Physical address Physical layer Presentation layer Protocol Remote login Repeater Ring topology Router Segment Server Session layer SMTP Source-to-destination delivery Star topology Static document Switch Synchronization point TELNET Topology Trailer TCP TCP/IP Transport layer URL UDP Web WAN WWW
Why to learn OSI reference Model? The answer is too simple that It tells us that how communication takes place between computers on internet but how??
OSI MODEL Maninder Kaur
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©Brooks/Cole, 2003 Chapter 6 Computer Networks. ©Brooks/Cole, 2003 Understand the rationale for the existence of networks. Distinguish between the three.
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