©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  (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 address

©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 ftp:// HTTP  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   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