Introduction to Computer Networks and Data Communications

Introduction to Computer Networks and Data Communications Chapter 1 Learning Objectives After reading this chapter, you should be able to: Define the basic terminology of computer networks Recognize the individual components of the big picture of computer networks Outline the basic network connections Cite the reasons for using a network model and explain how they apply to current network systems List the layers of the OSI model and describe the duties of each layer List the layers of the TCP/IP protocol suite and describe the duties of each layer Compare the OSI and TCP/IP models and list their differences and similarities Chapter Outline 1. Introduction 2. The Language of Computer Networks 3. The Big Picture of Networks 4. Computer Networks Basic Connections a. Terminal/microcomputer to mainframe b. Microcomputer to local area network c. Microcomputer to Internet d. Local area network to local area network e. Personal area network to workstation f. Local area network to metropolitan area network g. Local area network to wide area network h. Wide area network to wide area network i. Sensor to local area network j. Satellite and microwave k. Wireless telephone

5. Convergence 6. Network Architectures a. The Open Systems Interconnection (OSI) model b. The TCP/IP protocol suite c. Logical and physical connections 7. Network Connections In Action 8. The TCP/IP Protocol Suite In Action 9. Summary Lecture Notes Introduction The world of computer networks and data communications is a surprisingly vast and increasingly significant field of study. Once considered primarily the domain of communications engineers and technicians, computer networks now involve business managers, computer programmers, system designers, office managers, home computer users, and everyday citizens. It is virtually impossible for the average person on the street to spend 24 hours without directly or indirectly using some form of computer network. Examples of computer networks include banking, transportation, education, retail sales, and telecommunications. The Language of Computer Networks To better understand the area of computer networks, you should understand the basic broad categories of computer networks and data communications. For example, you should be able to define each of the following terms: Computer network Local area network Metropolitan area network Wide area network Personal area network Data communications Voice network Data network Telecommunications Network management Each of these terms is a topic area under the network umbrella. While there may be broader topic areas, these are the ones that will be discussed in detail in this text.

The Big Picture of Networks Sometimes a big picture can help to give an idea of what is going on in a particular subject area. Examine Figure 1-1. Try to reach a basic understanding of WAN to WAN connections, LAN to LAN connections, and WAN to LAN connections. Even though each of these connections will be covered in more detail later, they are necessary to understanding the big picture of how networks intercommunicate. You should also know the terms server, bridge, router, hub, node, communications network, and communications line. Computer Networks - Basic Connections Understand each of the following connections. Examine the figure from the text or create your own example for each connection. Describe how this connection works in simple terms. Describe one or more applications that use each connection: Terminal/microcomputer to mainframe Microcomputer to local area network Microcomputer to Internet Local area network to local area network Personal area network to workstation Local area network to metropolitan area network Local area network to wide area network Wide area network to wide area network Sensor to local area network Satellite and microwave Wireless telephone Convergence Convergence is happening at all levels: convergence of products (like the Blackberry), convergence of companies (such as SBC and AT&T), and convergence of technologies (such as token ring giving way to Ethernet). Be sure to talk about forms of convergence in this chapter and all subsequent chapters. Network Architecture Models To keep the pieces of a network working together harmoniously and to allow modularity between the pieces, it is necessary to use a network architecture model, or communications model, that places the appropriate network pieces in layers. Each layer in the model defines what services are provided by either hardware or software or both. For example, most organizations that produce some type of product or perform a service have a division of labor. Secretaries do the paperwork; accountants keep the books; laborers perform the manual duties; scientists design products; engineers test the products; managers control operations. Rarely is one person capable of performing all these duties. Large software applications operate the same way. Different procedures perform different tasks, and the whole would not function without the proper

operation of each of its parts. Communications software is no exception. As the size of the application grows, the need for a division of labor becomes increasingly important. Know the OSI model and its seven layers, including the basic functions performed at each layer: physical, data link, network, transport, session, presentation, and application. Even though the OSI model is not the actual model used to support the Internet, understanding it is necessary, as many networks and products often refer to the OSI model for definition. It is equally important to learn the TCP/IP protocol suite (Internet model) and its four layers: network access, network, transport, and application. The Internet model is the model used to support all activities on the Internet. Logical and Physical Connections To avoid future confusion, you must know the difference between a logical connection and a physical connection. Note that the only physical connection in a network is at the physical or interface layer. All other connections between peer layers are logical connections and exist in the software only. Examine Figure 1-14. The TCP/IP Protocol Suite In Action The In Action example in this first chapter follows a packet as it moves down through the layers of the transmitting device and then up through the layers of the receiving device. Note how a data packet grows with additional information at each layer. This is called encapsulation. Note further how the router only examines the data packet up to the network or IP layer. We will return to this encapsulation numerous times in later chapters. Quick Quiz 1. Identify each of the different forms of network connections and list an example of each. Terminal to mainframe motor vehicle licensing branch Microcomputer to mainframe business employee accessing corporate database Microcomputer to local area network student in school lab Microcomputer to Internet home user via Internet service provider Local area network to local area network schools and businesses with multiple LANs Local area network to wide area network school or business connection to Internet or external database service Sensor to local area network manufacturing plant Satellite and microwave numerous examples Wireless telephone

2. Define the seven layers of the OSI model and the four layers of the TCP/IP protocol suite. See pages 16-19 for the OSI layers and pages 20-21 for the TCP/IP layers. 3. Show how a Web browser request message originates at the application layer of one computer and proceeds down through the layers, across the network, and up through the layers of the receiving machine. See Figure 1-19. Discussion Topics 1. Ask each student how he or she has used a network today. 2. Discuss an example of a business that incorporates layers of management, and show how the work progresses through the layers. 3. Are there any network connections that have not been addressed in the chapter? What are their applications? 4. Are there any other examples in business where one particular technology or method has won out over the competition? What were the reasons that led to this particular technology winning? Teaching Tips 1. Be sure to emphasize that each of the terms introduced in the Language of Computer Networks section will be discussed in detail in future chapters. 2. Emphasize that telecommunications is the study of voice systems, but the distinction between data and voice is growing smaller each day. 3. Emphasize the important reasons for using an architectural model: compatibility, encapsulation, separation of work responsibilities, and a good pedagogical tool for learning networks. Further Readings and Resources For each chapter, please visit the instructor s Web site (http://facweb.cs.depaul.edu/cwhite) for the latest references to relevant materials.

Solutions to Review Questions 1. What is the definition of: a. A computer network? b. Data communications? c. Telecommunications? d. A local area network? e. A personal area network? f. A metropolitan area network? g. A wide area network? h. Network management? i. Convergence? Computer network: interconnection of computers and computing equipment using either wires or radio waves over small or large geographic areas Data communications: transfer of digital or analog data using digital or analog signals Telecommunications: study of telephones and the systems that transmit telephone signals Local area network: networks small in geographic size spanning a room, a building, or a campus Personal area network: a network of a few meters, interconnecting wireless devices such as PDAs, laptops, and other devices Metropolitan area network: a network covering a metropolitan area that offers high-speed connections between businesses Wide area network: large network encompassing parts of states, multiple states, countries, and the world Network management: design, installation, and support of a network and its hardware and software Convergence: coming together of two areas of technology or two companies 2. What is the relationship between a sub-network and a node? A sub-network is composed of nodes and communication links. 3. What kinds of applications might use a computer terminal to mainframe computer connection? Service industry, retail industry 4. What kinds of applications might use a microcomputer to mainframe computer connection? Many businesses support microcomputer to mainframe connections, including colleges and universities.

5. What language must a microcomputer use to interface to the Internet? Some form of TCP/IP 6. What kinds of applications might use a sensor to local area network connection? Assembly line 7. Why is a network architecture model useful? It allows for compatibility between products and allows products to interoperate. 8. List the seven layers of the OSI model. Physical, data link, network, transport, session, presentation, and application 9. List the four layers of the TCP/IP protocol suite. Interface, network, transport, application 10. How do the layers of the OSI model compare with the layers of the TCP/IP protocol suite? The network access layer covers OSI physical and data link layers; the TCP/IP application layer covers OSI presentation and application layers; there is no session layer in the TCP/IP; other layers are similar between two models. 11. What are some of the more common applications found in the TCP/IP protocol suite? FTP, Telnet, mail, Web browsing 12. What is the difference between a logical connection and a physical connection? A logical connection exists in software, but a physical connection is a communication link. 13. How does convergence apply to the communications industry? Companies are always converging; technologies are converging.

Suggested Solutions to Exercises 1. Create a list of all the things you do in an average day that use data communications and computer networks. Answers will vary but could include riding mass transit, driving on the highway, 24-hour banking withdrawal, working on homework on computer, and checking your e-mail. 2. If you could design your own home, what kinds of computer network or data communications labor-saving devices would you incorporate? Answers will vary but could include alarms, remote control of appliances, remote monitoring of windows and doors, indoor and outdoor video surveillance, motion sensors, and distributed video and music systems. 3. Two companies consider pooling resources to perform a joint venture. The CEO of the first company meets with his legal team, and the legal team consults a number of middle managers in the proposed product area. Meanwhile, the CEO of the first company sends an e-mail to the CEO of the second company to offer a couple suggestions concerning the joint venture. Does this scenario follow the OSI model? Explain. No, this does not follow the OSI model. Upper layers do not communicate directly. All physical communications proceed only through the lowest layer. 4. Using a laptop computer with a wireless connection into the company s local area network, you download a Web page from the Internet. List all the different network connections involved in this operation. Possible answers might include: Laptop to receiving antenna wireless connection Receiving antenna to local area network microcomputer to LAN connection Corporate network connections LAN to LAN connection Corporate connection to Internet LAN to WAN connection 5. You are working from home using a microcomputer, a DSL modem, and a telephone connection to the Internet. Your company is connected to the Internet and has both local area networks and a mainframe computer. List all the different network connections involved in this operation. Possible answers might include: Microcomputer to Internet connection Local area network to wide area network

6. You are sitting at the local coffee shop, enjoying your favorite latte. You pull out your laptop and, using the wireless network available at the coffee shop, access your e-mail. List all the different network connections involved in this operation. Microcomputer to local area network connection Local area network to wide area network or Microcomputer to Internet 7. With your new cell phone, you have just taken a snapshot of your best friend. You decide to send this snapshot to the e-mail account of a mutual friend across the country. List all the different network connections involved in this operation. Wireless telephone connection Wide area network to wide area network perhaps 8. You are driving in a new city and have just gotten lost. Using your car s built-in navigational system, you submit a request for driving directions from a nearby intersection to your destination. List all the different network connections involved in this operation. Satellite connection Wide area network to wide area network, perhaps 9. The layers of the TCP/IP protocol suite and OSI are different. Which layers are missing from the TCP/IP suite? Are they really missing? TCP/IP doesn t have session layer, but if it wants to create a session, it can do so at the transport or application layer. TCP/IP doesn t have a presentation layer, but these features are often done elsewhere, such as in the application layer. 10. If the data link layer provides error checking and the transport layer provides error checking, isn t this redundant? Explain. Not exactly. The data link layer performs the error checking when the data arrives at the next computer on the transmission path. The transport layer performs the error checking only when the data arrives at the final destination machine. 11. Similarly, the data link layer provides flow control, and the transport layer provides flow control. Are these different forms of flow control? Explain. Once again, the flow control provided by the data link layer is between two adjacent devices at the data link level, while flow control provided by the transport layer is only between the endpoints of the connection (at the transport level).

12. You are watching a television show and somebody is suing somebody else. The lawyers for both parties meet and try to work out a settlement. Is there a logical or physical connection between the lawyers? What about between the two parties? Physical between the lawyers, but logical between the two parties 13. You want to download a file from a remote site using the File Transfer Protocol (FTP). To perform the file transfer, your computer issues a Get File command. Show the progression of messages as the Get File command moves from your computer, through routers, into the remote computer, and back. Answer should be very similar to Figure 1-19. Replace Get Web Page in the application layer with Get File and at all other similar points. 14. What characteristics distinguish a personal area network from other types of networks? Very short distance and wireless, essentially used for eliminating wires between electronic and peripheral devices 15. Isn t a metropolitan area network just a big local area network? There are many similarities, but a metropolitan area network is almost always fiber optic, very high speed, and very resilient to failures. 16. List the OSI layer that performs each of the following functions: a. Data compression b. Multiplexing c. Routing d. Definition of a signal s electrical characteristics e. E-mail f. Error detection g. End-to-end flow control OSI answers are: a. Presentation b. Physical, transport c. Network d. Physical e. Application f. Data link, transport g. Transport

17. For each of the functions in the previous exercise, list the TCP/IP protocol suite layer that performs that function. TCP/IP answers are: a. Application b. Network access, transport c. Network d. Network access e. Application f. Network access, transport g. Transport 18. You are sending and receiving instant messages (IM) with a friend. Is this IM session a logical connection or a physical connection? Explain your answer. This is a logical connection. The physical connection is a wire or airwave that is used to transmit the IM message as a signal.