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Data communications and Networking for beginners

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  CIT 852: DATA COMMUNICATION AND NETWORK    NATIONAL OPEN UNIVERSITY OF NIGERIA   CIT 852 DATA COMMUNICATION AND NETWORK COURSE GUIDE CIT 852 DATA COMMUNICATION AND NETWORK Course Adapter Afolorunso, A. A. National Open University of Nigeria Course Coordinator Afolorunso, A. A. National Open University of Nigeria NATIONAL OPEN UNIVERSITY OF NIGERIA ii CIT 852 DATA COMMUNICATION AND NETWORK National Open University of Nigeria Headquarters 14/16 Ahmadu Bello Way Victoria Island Lagos Abuja Office 5, Dar Es Salaam Street Off Aminu Kano Crescent Wuse II, Abuja Nigeria. e-mail: centralinfonou.edu.ng URL: www.nou.edu.ng Published by National Open University of Nigeria Printed 2008 ISBN: 978-058-378-5 All Rights Reserved iii CIT 852 DATA COMMUNICATION AND NETWORK CONTENTS PAGE Introduction…………………………………………………….. 1 What You Will Learn in this Course …………………….……. 1 Course Aims …………………………………………………… 1 Course Objectives ……………………………………………... 1 Working through this Course ………………………………….. 2 Course Materials ………………………………………………. 2 Study Units …………………………………………………….. 2 Textbooks and References …………………………………….. 3 Assignment File ……………………………………………….. 4 Presentation Schedule …………………………………………. 4 Assessment …………………………………………………….. 4 Tutor-Marked Assignment ……………………………………... 5 Final Examination and .Grading ………………………………... 5 Course Marking Scheme ……………………………………….. 6 Course Overview ……………………………………………….. 6 How to Get the Best from this Course ………………………… 7 Facilitators/Tutors and Tutorials ………………………………. 8 Summary ……………………………………………………….. 9 Introduction CIT 852 -Data Communication and Networks is a three 3 credit unit course of 16 units. The main objective of the course is to deal with fundamental issues in Computer Networks. It starts with the philosophy of data communication covering different modulation and multiplexing techniques. Then, it proceeds to cover MAC layer protocols, several routing techniques protocols, congestion techniques and several network layer protocol. The final module of the course takes up issues related to the transport layer mechanism, such as, addressing, connection, establishment, flow control and multiplexing issues. It also covers the transport layer protocol in details. The module ends with the security issue, which is an important topic today. This Course Guide gives you a brief overview of the course content, course duration, and course materials. What You Will Learn in this course The main purpose of this course is to deal with fundamental issues in Computer Networks. It makes available the steps and tools that will enable you to make proper and accurate decision about data transmission and computer systems connectivity whenever the need arises. This, we intend to achieve through the following: Course Aims iv CIT 852 DATA COMMUNICATION AND NETWORK i. Introduce the concepts data communication and computer networks; ii. Provide in-depth knowledge of Data Link Layer fundamental such as, error detection, correction, and flow techniques; as well as introduce data link layer switching concepts; iii. Discuss the concept of routing and congestion as well as highlight the different routing and congestion control algorithms; and iv. Introduce internetworking concepts and protocols; v. Discuss topics like network security which include symmetric key algorithm, public key algorithm, digital signature, etc. as well as topics like addressing, multiplexing, connection establishment, crash recovery and TCPIUDP Protocols. Course Objectives Certain objectives have been set out to ensure that the course achieves its aims. Apart from the course objectives, every unit of this course has set objectives. In the course of the study, you will need to confirm, at the end of each unit, if you have met the objectives set at the beginning of each unit. By the end of this course you should be able to: i. describe the various components and data communication and computer networking; ii. differentiate between different types of computer networks; iii. compare the different network topologies; iv. describe the mechanism and techniques of encoding; v. describe a wireless LAN and Data Link Layer switching, and operations of bridges; vi. explain the Routing concept; vi. explain the basic principle of internetworking and its importance; vii. describe the whole concept/idea behind network security as well as the various network/data security algorithms Working through this Course In order to have a thorough understanding of the course units, you will need to read and understand the contents, practise the steps by designing and implementing a mini LAN for your department, and be committed to learning and implementing your knowledge. This course is designed to cover approximately eighteen weeks, and it will require your devoted attention. You should do the exercises in the Tutor-Marked Assignments and submit to your tutors. v CIT 852 DATA COMMUNICATION AND NETWORK Course Materials These include: 1. Course Guide 2. Study Units 3. Recommended Texts 4. A file for your assignments and for records to monitor your progress. Study Units There are sixteen study units in this course: Module 1 Introduction to Data Communication and Computer Network Concepts Unit 1 Introduction to Computer Networks Unit 2 Data Transmission Unit 3 Data Encoding and Communication Technique Unit 4 Multiplexing and Switching Module 2 Media Access Control and Data Link Layer Unit 1 Data Link Layer Fundamentals Unit 2 Retransmission Strategies Unit 3 Contention-Based Media Access Protocols Unit 4 Wireless LAN and Datalink Layer Switching Module 3 Network Layer Unit 1 Introduction to Layer Functionality and Design Issues Unit 2 Routing Algorithms Unit 3 Congestion Control in Public Switched Network Unit 4 Internetworking Module 4 Transport Layer and Application Layer Services Unit 1 Transport Services and Mechanism Unit 2 TCPIUDP Unit 3 Network Security I Unit 4 Network Security II Make use of the course materials, do the exercises to enhance your learning. Textbooks and References vi CIT 852 DATA COMMUNICATION AND NETWORK Computer Networks, Andrew S. Tenenbaum, PHI, New Delhi. Data and Computer Communication, William Stalling, PHI, New Delhi. Computer Communications and Networking Technologies, by Michael A. Gallo and William M. Hancock, Thomson Asia, Second Reprint, 2002. Introduction to Data Communications and Networking, by Behrouz Forouzan, Tata McGraw-Hill, 1999. Networks, Tirothy S. Ramteke, Second Edition, Pearson Education, New Delhi. Communications Networks, Leon Garcia, and Widjaja, Tata McGraw- Hill, 1999. Computer Networking, J. F. Kurose & K. W. Ross, A Top Down Approach Featuring the Internet, Pearson Edition, 2003. Computer Networks, Andrew S. Tanenbaum 4th Edition Prentice Hall of India, New Delhi. 2003. Communication Networks, Fundamental Concepts and Key Architectures, Leon and Widjaja, 3rd Edition, Tata McGraw-Hill. Data Network, Dmitri Berteskas and Robert Galleger, Second Edition, Prentice Hall of India, 1997, New Delhi. Network Security Essential -Application and Standard, William Stallings, Pearson Education, New Delhi. Assignments File These are of two types: the self-assessment exercises and the Tutor- Marked Assignments. The self-assessment exercises will enable you monitor your performance by yourself, while the Tutor-Marked Assignment is a supervised assignment. The assignments take a certain percentage of your total score in this course. The Tutor-Marked Assignments will be assessed by your tutor within a specified period. The examination at the end of this course will aim at determining the level of mastery of the subject matter. This course includes twelve Tutor-Marked Assignments and each must be done and submitted accordingly. Your best scores however, will be recorded for you. Be sure to send these assignments to your tutor before the deadline to avoid loss of marks. . vii CIT 852 DATA COMMUNICATION AND NETWORK Presentation Schedule The Presentation Schedule included in your course materials gives you the important dates for the completion of tutor marked assignments and attending tutorials. Remember, you are required to submit all your assignments by the due date. You should guard against lagging behind in your work. Assessment There are two aspects to the assessment of the course. First are the tutor marked assignments; second, is a written examination. In tackling the assignments, you are expected to apply information and knowledge acquired during this course. The assignments must be submitted to your tutor for formal assessment in accordance with the deadlines stated in the Assignment File. The work you submit to your tutor for assessment will count for 30% of your total course mark. At the end of the course, you will need to sit for a final three-hour examination. This will also count for 70% of your total course mark. Tutor Marked Assignment There are sixteen tutor marked assignments in this course. You need to submit all the assignments. The total marks for the best four (4) assignments will be 30% of your total course mark. Assignment questions for the units in this course are contained in the Assignment File. You should be able to complete your assignments from the information and materials contained in your set textbooks, reading and study units. However, you may wish to use other references to broaden your viewpoint and provide a deeper understanding of the subject. When you have completed each assignment, send it together with form to your tutor. Make sure that each assignment reaches your tutor on or before the deadline given. If, however, you cannot complete your work on time, contact your tutor before the assignment is due to discuss the possibility of an extension. Final Examination and Grading The final examination for the course will carry 70% percentage of the total marks available for this course. The examination will cover every viii CIT 852 DATA COMMUNICATION AND NETWORK aspect of the course, so you are advised to revise all your corrected assignments before the examination. This course endows you with the status of a teacher and that of a learner. This means that you teach yourself and that you learn, as your learning capabilities would allow. It also means that you are in a better position to determine and to ascertain the what, the how, and the when of your language learning. No teacher imposes any method of learning on you. The course units are similarly designed with the introduction following the table of contents, then a set of objectives and then the dialogue and so on. The objectives guide you as you go through the units to ascertain your knowledge of the required terms and expressions. ix CIT 852 DATA COMMUNICATION AND NETWORK Course Marking Scheme This table shows the actual course marking is broken down. Assessment Marks Assignment 1-4 Four assignments, best three marks of the four count at 30% of course marks Final examination 70% of overall course marks Total 100% of course marks Table 1: Course Marking Scheme Course Overview Unit Title of Work Weeks Assignment Activity (End of Unit) Course Guide Week 1 Module 1 1 Introduction to computer Networks Week 1 Assignment 1 2 Data Transmission Week 2 Assignment 2 3 Data Encoding and Communication Week 3 Assignment 3 Technique 4 Multiplexing and Switching Week 4 Assignment 4 Module 2 1 Data Link Layer Fundamentals Week 5 Assignment 5 2 Retransmission Strategies Week 6 Assignment 6 3 Contention-Based Media Access Week 7 Assignment 7 Protocols 4 Wireless LAN and Datalink Layer Week 8 Assignment 8 Switching Module 3 1 Introduction to Layer Functionality Week 9 Assignment 9 and Design Issues 2 Routing Algorithms Week 10 Assignment 10 3 Congestion Control in Public Week 11 Assignment 11 Switched Network 4 Internet work Week 12 Assignment 12 Module 4 1 Transport services and Mechanism Week 13 Assignment 13 2 TCP/UDP Week 14 Assignment 14 3 Network Security I Week 15 Assignment 15 4 Network Security II Week 16 Assignment 16 Revision Week 17 Examination Week 18 Total Week 18 x CIT 852 DATA COMMUNICATION AND NETWORK How to Get the Best from this Course In distance learning the study units replace the university lecturer. This is one of the great advantages of distance learning; you can read and work through specially designed study materials at your own pace, and at a time and place that suit you best. Think of it as reading the lecture instead of listening to a lecturer. In the same way that a lecturer might set you some reading to do, the study units tell you when to read your set books or other material. Just as a lecturer might give you an in-class exercise, your study units provide exercises for you to do at appropriate points. Each of the study units follows a common format. The first item is an introduction to the subject matter of the unit and how a particular unit is integrated with the other units and the course as a whole. Next is a set of learning objectives. These objectives enable you know what you should be able to do by the time you have completed the unit. You should use these objectives to guide your study. When you have finished the units you must go back and check whether you have achieved the objectives. If you make a habit of doing this you will significantly improve your chances of passing the course. Remember that your tutor's job is to assist you. When you need help, don't hesitate to call and ask your tutor to provide it. 1. Read this Course Guide thoroughly. 2. Organize a study schedule. Refer to the 'Course Overview' for more details. Note the time you are expected to spend on each unit and how the assignments relate to the units. Whatever method you chose to use, you should decide on it and write in your own dates for working on each unit. 3. Once you have created your own study schedule, do everything you can to stick to it. The major reason that students fail is that they lag behind in their course work. 4. Turn to Unit 1 and read the introduction and the objectives for the unit. 5. Assemble the study materials. Information about what you need for a unit is given in the 'Overview' at the beginning of each unit. You will almost always need both the study unit you are working on and one of your set of books on your desk at the same time. xi CIT 852 DATA COMMUNICATION AND NETWORK 6. Work through the unit. The content of the unit itself has been arranged to provide a sequence for you to follow. As you work through the unit you will be instructed to read sections from your set books or other articles. Use the unit to guide your reading. 7. Review the objectives for each study unit to confirm that you have achieved them. If you feel unsure about any of the objectives, review the study material or consult your tutor. 8. When you are confident that you have achieved a unit's objectives, you can then start on the next unit. Proceed unit by unit through the course and try to pace your study so that you keep yourself on schedule. 9. When you have submitted an assignment to your tutor for marking, do not wait for its return before starting on the next unit. Keep to your schedule. When the assignment is returned, pay particular attention to your tutor's comments, both on the tutor- marked assignment form and also written on the assignment. Consult your tutor as soon as possible if you have any questions or problems. 10. After completing the last unit, review the course and prepare yourself for the final examination. Check that you have achieved the unit objectives (listed at the beginning of each unit) and the course objectives (listed in this Course Guide). Facilitators/Tutors and Tutorials There are 15 hours of tutorials provided in support of this course. You will be notified of the dates, times and location of these tutorials, together with the name and phone number of your tutor, as soon as you are allocated a tutorial group. Your tutor will mark and comment on your assignments, keep a close watch on your progress and on any difficulties you might encounter and provide assistance to you during the course. You must mail or submit your tutor-marked assignments to your tutor well before the due date (at east two working days are required). They will be marked by your tutor and returned to you as soon as possible. Do not hesitate to contact your tutor by telephone, or e-mail if you need help. The following might be circumstances in which you would find help necessary. Contact your tutor if: xii CIT 852 DATA COMMUNICATION AND NETWORK you do not understand any part of the study units or the assigned readings, you have difficulty with the self-tests or exercises, you have a question or problem with an assignment, with your tutor's comments on an assignment or with the grading of an assignment. You should try your best to attend the tutorials. This is the only chance to have face to face contact with your tutor and to ask questions which are answered instantly. You can raise any problem encountered in the course of your study. To gain the maximum benefit from course tutorials, prepare a question list before attending them. You will learn a lot from participating in discussions actively. Summary This is course, Data communication and Networks exposes you to basic know ledge and skills about data communication and computer networking you need to acquire as an information technology professional. The content of the course material was planned and written to build upon whatever knows you have acquired at undergraduate or post graduate diploma level and to ensure that you acquire the proper knowledge and skills for the appropriate situations. Real-life situations have been created to enable you identify with and create some of your own. The essence is to get you to acquire the necessary knowledge and competence, and by equipping you with the necessary tools, we hope to have achieved that. I wish you success with the course and hope that you will find it both interesting and useful. xiii CIT 852 DATA COMMUNICATION AND NETWORK Course Code CIT 852 Course Title Data Communication and Network Course Adapter Afolorunso, A. A. National Open University of Nigeria Course Coordinator Afolorunso, A. A. National Open University of Nigeria NATIONAL OPEN UNIVERSITY OF NIGERIA xiv CIT 852 DATA COMMUNICATION AND NETWORK National Open University of Nigeria Headquarters 14/16 Ahmadu Bello Way Victoria Island Lagos Abuja Office 5, Dar Es Salaam Street Off Aminu Kano Crescent Wuse II, Abuja Nigeria. e-mail: centralinfonou.edu.ng URL: www.nou.edu.ng Published by National Open University of Nigeria Printed 2008 ISBN: 978-058-378-5 All Rights Reserved xv CIT 852 DATA COMMUNICATION AND NETWORK CONTENTS PAGE Module 1 Introduction to Data Communication and Computer Network Concepts…………. 1 Unit 1 Introduction to Computer Networks………….. 1 Unit 2 Data Transmission……………………………. 49 Unit 3 Data Encoding and Communication Technique 77 Unit 4 Multiplexing and Switching………………….. 97 Module 2 Media Access Control and Data Link Layer 118 Unit 1 Data Link Layer Fundamentals………………. 118 Unit 2 Retransmission Strategies……………………. 141 Unit 3 Contention-Based Media Access Protocols…. 152 Unit 4 Wireless LAN and Datalink Layer Switching.. 166 Module 3 Network Layer………………………………. 185 Unit 1 Introduction to Layer Functionality and Design Issues………………………………………….. 185 Unit 2 Routing Algorithms…………………………… 207 Unit 3 Congestion Control in Public Switched Network 228 Unit 4 Internetworking……………………………….. 239 Module 4 Transport Layer and Application Layer Services……………………………………….. 262 Unit 1 Transport Services and Mechanism…………… 262 Unit 2 TCP/UDP……………………………………… 279 Unit 3 Network Security I…………………………….. 302 Unit 4 Network Security II……………………………. 344 xvi CIT 852 DATA COMMUNICATION AND NETWORK MODULE 1 INTRODUCTION TO DATA COMMUNICATION AND COMPUTER NETWORK CONCEPTS Unit 1 Introduction to Computer Networks Unit 2 Data Transmission Unit 3 Data Encoding and Communication Technique Unit 4 Multiplexing and Switching UNIT 1 INTRODUCTION TO COMPUTER NETWORKS CONTENT 1.0 Introduction 2.0 Objectives 3.0 Main Content 3.1 What is a Computer Network? 3.2 Network Goals and Motivations 3.3 Classification of Networks 3.3.1 Broadcast Networks 3.3.2 Point-to-Point or Switched Networks 3.4 work Topology 3.4.1 Bus Topology 3.4.2 Star Topology 3.4.3 Ring Topology 3.4.4 Tree Topology 3.4.5 Mesh Topology 3.4.6 Cellular Topology 3.5 Applications of Network 3.6 Networking Model 3.6.1 OSI Reference Model 3.6.2 TCP/IP Reference Model 3.7 Network Architecture 3.7.1 Client/Server Architecture 3.7.2 Peer-lo-Peer Architecture 3.8 Example Networks 3.8.1 Novell Netware 3.8.2 ARPANET 3.8.3 Internet 3.8.4 ATM Network 3.9 Types of Computer Networks 3.9.1 Metropolitan Area Network (MAN) 3.9.2 Wide Area Network (WAN) 3.9.3 Comparison between LAN, MAN, WAN and GAN 3.10 Advantages of Networks 4.0 Conclusion 1 CIT 852 DATA COMMUNICATION AND NETWORK 5.0 Summary 6.0 Tutor-Marked Assignment 7.0 References/Further Readings 1.0 INTRODUCTION These days, practically every business, no matter how small uses computers to handle various transactions and as business grows, they often need several people to input and process data simultaneously and in order to achieve this, the earlier model of a single computer serving all the organisations computational needs has been replaced by a model in which a number of separate but interconnected computers do the job and this model is known as a Computer Netw9rk. By linking individual computers over a network their productivity has been increased enormously. A most distinguishing characteristic of a general computer network is that data can enter or leave at any point and can be processed at any workstation. For example: A printer can be controlled from any word processor at any computer on the network. This is an introductory unit where, you will be learning about the basic concepts regarding Computer Networks. Here, you will learn about Networks, different types of Networks, their applications, Network topology, Network protocols, OSI Reference Mode, TCP/IP Reference Model. We shall also examine some of the popular computer networks like Novell network, ARPANET, Internet, and ATM networks. Towards t4e end of this unit the concept of Delays in computer networks is also discussed. 2.0 OBJECTIVES After going through this unit, you should be able to: understand the concept of computer networks differentiate between different types of computer networks understand the different application of networks compare the different network topologies signify the importance of network protocols know the importance of using networked system understand the layered organisation and structuring of computer networks using OSI and TCPIIP reference model have a broad idea about some of the popular networks like Novell network, ARPANET, INTERNET, ATM etc., and understand the concept of delays. 3.0 MAIN CONTENT 2 CIT 852 DATA COMMUNICATION AND NETWORK 3.1 What is a computer network? Figure 1: A computer-networked environment A Computer network consists of two or more autonomous computers that are linked (connected) together in order to: Share resources (files, printers, modems, fax machines). Share Application software like MS Office. Allow Electronic communication. Increase productivity (makes it easier to share data amongst users). Figure 1 shows people working in a networked environment. The Computers on a network may be linked through Cables, telephones lines, radio waves, satellites etc. A Computer network includes, the network operating system in the client and server machines, the cables, which connect different computers and all supporting hardware ill between such as bridges, routers and switches. In wireless systems, antennas and towers are also part of the network. Computer networks are generally classified according to their structure and the area they are localised in as: Local Area Network (LAN): The network that spans a relatively small area that is, in the single building or campus is known as LAN. Metropolitan Area Network (MAN): The type of computer network that is, designed for a city or town is known as MAN. 3 CIT 852 DATA COMMUNICATION AND NETWORK Wide Area Network (WAN): A network that covers a large geographical area and covers different cities, states and sometimes even countries, is known as WAN. The additional characteristics that are also used to categorise different types of networks are: Topology: Topology is the graphical arrangement of computer systems in a network. Common topologies include a bus, star, ring, and mesh. Protocol: The protocol defines a common set of rules which are used by computers on the network that communicate between hardware and software entities. One of the most popular protocols for LANs is the Ethernet. Another popular LAN protocol for PCs is the token-ring network. Architecture: Networks can be broadly classified as using either a peer-to-peer or client/server architecture. 3.2 Network Goals and Motivations Before designing a computer network we should see that the designed network fulfils the basic goals. We have seen that a computer network should satisfy a broad range of purposes and should meet various requirements. One of the main goals of a computer network is to enable its users to share resources, to provide low cost facilities and easy addition of new processing services. The computer network thus, creates a global environment for its users and computers. Some of the basic goals that a Computer network should satisfy are: Cost reduction by sharing hardware and software resources. Provide high reliability by having multiple sources of supply. Provide an efficient means of transport for large volumes of data among various locations (High throughput). Provide inter-process communication among users and processors. Reduction jn del’y driving data transport. Increase productivity by making it easier to share data amongst users. 4 CIT 852 DATA COMMUNICATION AND NETWORK Repairs, upgrades, expansions, and changes to the network should be performed with minimal impact on the majority of network users. Standards and protocols should be supported to allow many types of equipment from different vendors to share the network (Interoperatability). Provide centralised/distributed management and allocation of network resources like host processors, transmission facilities etc. 3.3 Classification of Networks Depending on the transmission technology i.e., whether the network contains switching elements or not, we have two types of networks: Broadcast networks. Point-to-point or Switched networks. 3.3.1 Broadcast Networks Broadcast networks have a single communication channel that is shared by all the machines on the network. In this type of network, short messages sent by any machine are received by all the machines on the network. The packet contains an address field, which specifies for whom the packet is intended. All the machines, upon receiving a packet check for the address field, if the packet is intended for itself, it processes it and if not the packet is just ignored. Using Broadcast networks, we can generally address a packet to all destinations (machines) by using a special code in the address field. Such packets are received and processed by all machines on the network. This mode of operation is known as “Broadcasting”. Some Broadcast networks also support transmission to a subset of machines and this is known as “Multicasting”. One possible way to achieve Multicasting is to reserve one bit to indicate multicasting and the remaining (n-l) address bits contain group number. Each machine can subscribe to any or all of the groups. Broadcast networks are easily configured for geographically localised networks. Broadcast networks may be Static or dynamic, depending on how the channel is allocated. In Static allocation, time is divided into discrete intervals and using round robin method, each machine is allowed to broadcast only when its 5 CIT 852 DATA COMMUNICATION AND NETWORK time slot comes up. This method is inefficient because the channel capacity is wasted when a machine has nothing to broadcast during its allocated slot. Dynamic allocation may be centralised or decentralised. In centralised allocation method, there is a single entity, for example, a bus arbitration unit which determines who goes next and this is achieved by using some internal algorithm. In Decentralised channel allocation method, there is no central entity, here, each machine decides for itself whether or not to transmit. The different types of Broadcast networks are: 1) Packet Radio Networks. 2) Satellite Networks. 3) Local Area Networks. Packet Radio broadcasting differs from satellite network broadcasting in several ways. In particular stations have limited range introducing the need for radio repeaters, which in turn affects the routing, and acknowledges schemes. Also the propagation delay is much less than for satellite broadcasting. LAN (Local Area Network) Local Area Network is a computer network that spans over a relatively small area. Most LANs are confined to a single building or group of buildings within a campus. However, one LAN can be connected to other LANs over any distance via telephone lines and radio waves. A system of LANs connected in this way is called a wide-area network (WAN). Most LANs connect workstations and personal computers. Each node (individual computer) in a LAN has its own CPU with which it executes programs, but it is also able to access data and devices anywhere on the LAN. This means that many users can share data as well as expensive devices, such as laser printers, fax machines etc. Users can also use the LAN to communicate with each other, by sending e-mail or engaging in chat sessions. There are many different types of LANs, Ethernets being the most common for PCs. The following characteristics differentiate one LAN from another: 6 CIT 852 DATA COMMUNICATION AND NETWORK Topology The geometric arrangement of devices on the network. For example, devices can be arranged in a ring or in a straight line. Protocols The rules and encoding specifications for sending data. The protocols also determine whether the network uses peer-to-peer or client/server architecture. Media Devices can be connected by twisted-pair wire, coaxial cables, or fiber optic cables. Some networks communicate via radio waves hence, do not use any connecting media. LANs are capable of transmitting data at very fast rates, much faster than data can be transmitted over a telephone line; but the distances are limited, and there is also a limit on the number of computers that can be attached to a single LAN. The typical characteristics of a LAN are: Confined to small areas i.e., it connects several devices over a distance of5to 10km. High speed. Most inexpensive equipment. Low error rates. Data and hardware sharing between users owned by the user. Operates at speeds ranging from 10Mbps to 100Mbps. Nowadays 1000 Mbps are available. 3.3.2 Point-to-Point or Switched Networks Point-to-point or switched, networks are those in which there are many connections between individual pairs of machines. In these networks, when a packet travels from source to destination it may have to first visit one or more intermediate machines. Routing algorithms play an important role in Point-to-point or Switched networks because often multiple routes of different lengths are available. 7 CIT 852 DATA COMMUNICATION AND NETWORK An example of switched network is the international dial-up telephone system. The different types of Point- to-point or Switched networks are: Circuit Switched Networks. Packet Switched Networks. In Switched network, the temporary connection is established from one point to another for either the duration of the session (circuit switching) or for the transmission of one or more packets of data (packet switching). Circuit Switched Networks Circuit Switched networks use a networking technology that provides a temporary, but dedicated connection between two stations no matter how many switching devices are used in the data transfer route. Circuit switching was originally developed for the analog based telephone system in order to guarantee steady and consistent service for two people engaged in a phone conversation. Analog circuit switching has given way to digital circuit switching, and the digital counterpart still maintains the connection until broken (one side hangs up). This means bandwidth is continuously reserved and “silence is transmitted” just the same as digital audio in voice conversation. Packet Switched Networks Packet switched Networks use a networking technology that breaks up a message into smaller packets for transmission and switches them to their required destination. Unlike circuit switching, which requires a constant point-to-point circuit to be established, each packet in a packet- switched network contains a destination address. Thus, all packets in a single message do not have to travel the same path. They can be dynamically routed over the network as lines become available or unavailable. The destination computer reassembles the packets back into their proper sequence. Packet switching efficiently handles messages of different lengths and priorities. By accounting for packets sent, a public network can charge customers for only the data they transmit. Packet switching has been widely used for data, but not for real-time voice and video. However, this is beginning to change. IP and ATM technologies are expected to enable packet switching to be used for everything. 8 CIT 852 DATA COMMUNICATION AND NETWORK The first international standard for wide area packet switching networks was X.25, which was defined when all circuits were digited and susceptible to noise. Subsequent technologies, such as frame relay and SMDS were designed for today’s almost-error-free digital lines. ATM uses a cell-switching technology that provides the bandwidth sharing efficiency of packet switching with the guaranteed bandwidth of circuit switching. Higher-level protocols, such as TCPIIP, IPX/SPX and NetBIOS, are also packet based and are designed to ride over packet-switched topologies. Public packet switching networks may provide value added services, such as protocol conversion and electronic mail. SELF ASSESSMENT EXERCISE 1 1) Explain the difference between Client/Server and Peer-to-peer architecture? 2) List the important aspects that should be kept in mind while designing a network? 3) Write briefly about the areas where networks are used? 4) Differentiate between Broadcast and point-to-point networks.. 3.4 Network Topology Topology refers to the shape of a network, or the network’s layout. How different nodes in a network are connected to each other and how they communicate with each other is determined by the network’s topology. Topologies are either physical or logical. Some of the most common network topologies are: Bus topology Star topology Ring topology Tree topology Mesh topology Cellular topology. The parameters that are to be considered while selecting a physical topology are: 9 CIT 852 DATA COMMUNICATION AND NETWORK Ease of installation. Ease of reconfiguration. Ease of troubleshooting. 3.4.1 Bus Topology Figure 2: Bus topology In Bus topology, all devices are connected to a central cable, called the bus or backbone. The bus topology connects workstations using a single cable. Each workstation is connected to the next workstation in a point- to-point fashion. All workstations connect to the same cable. Figure 2 shows computers connected using Bus Topology. In this type of topology, if one workstation goes faulty all workstations may be affected as all workstations share the same cable for the sending and receiving of information. The cabling cost of bus systems is the least of all the different topologies. Each end of the cable is terminated using a special terminator. The common implementation of this topology is Ethernet. Here, message transmitted by one workstation is heard by all the other workstations. Advantages of Bus Topology Installation is easy and cheap when compared to other topologies Connections are simple and this topology is easy to use. Less cabling is required. Disadvantages of Bus Topology Used only in comparatively small networks. 10 CIT 852 DATA COMMUNICATION AND NETWORK As all computers share the same bus, the performance of the network deteriorates when we increase the number of computers beyond a certain limit. Fault identification is difficult. A single fault in the cable stops all transmission. 3.4.2 Star Topology Figure 3: Star topology Start topology uses a central hub through which, all components are connected. In a Star topology, the central hub is the host computer, and at the end of each connection is a terminal as shown in Figure 3. Nodes communicate across the network by passing data through the hub. A star network uses a significant amount of cable as each terminal is wired back to the central hub, even if two terminals are side by side but several hundred meters away from the host. The central hub makes all routing decisions, and all other workstations can be simple. An advantage of the star topology is, that failure, in one of the terminals does not affect any other terminal; however, failure of the central hub affects all terminals. This type of topology is frequently used to connect terminals to a large time-sharing host computer. Advantages of Star Topology Installation and configuration of network is easy. Less expensive when compared to mesh topology. Faults in the network can be easily traced. Expansion and modification of star network is easy. Single computer failure does not affect the network. 11 CIT 852 DATA COMMUNICATION AND NETWORK Supports multiple cable types like shielded twisted pair cable, unshielded twisted pair cable, ordinary telephone cable etc. Disadvantages of Star Topology Failure in the central hub brings the entire network to a halt. More cabling is required in comparison to tree or bus topology because each node is connected to the central hub. 3.4.3 Ring Topology Figure 4: Ring Topology In Ring Topology all devices are connected to one another in the shape of a closed loop, so that each device is connected directly to two other devices, one on either side of it, i.e., the ring topology connects workstations in a closed loop, which is depicted in Figure 4 Each terminal is connected to two other terminals (the next and the previous), with the last terminal being connected to the first. Data is transmitted around the ring in one direction only; each station passing on the data to the next station till it reaches its destination. Information travels around the ring from one workstation to the next. Each packet of data sent on the ring is prefixed by the address of the station to which it is being sent. When a packet of data arrives, the workstation checks to see if the packet address is the same as its own, if it is, it grabs the data in the packet. If the packet does not belong to it, it sends the packet to the next workstation in the ring. Faulty workstations can be isolated from the ring. When the workstation is powered on, it connects itself to the ring. When power is off, it disconnects itself from the ring and allows the information to bypass the workstation. The common implementation of this topology is token ring. A break in the ring causes the entire network to fail. Individual workstations can be isolated from the ring. 12 CIT 852 DATA COMMUNICATION AND NETWORK Advantages of Ring Topology Easy to install and modify the network. Fault isolation is simplified. Unlike Bus topology, there is no signal loss in Ring topology because the tokens are data packets that are re-generated at each node. Disadvantages of Ring Topology Adding or removing computers disrupts the entire network. A break in the ring can stop the transmission in the entire network. Finding fault is difficult. Expensive when compared to other topologies. 3.4.4 Tree Topology Figure 5: Tree Topology Tree topology is a LAN topology in which only one route exists between any two nodes on the network. The pattern of connection resembles a tree in which all branches spring from one root. Figure 5 shows computers connected using Tree Topology. Tree topology is a hybrid topology, it is similar to the star topology but the nodes are connected to the secondary hub, which in turn is co