Business Data Communications and Networking

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Updated with the latest advances in the field, Jerry FitzGerald, Alan Dennis, and Alexandra Durcikova's 12th Edition of Business Data Communications and Networking , continues to provide the fundamental concepts and cutting-edge coverage of applications that students need to succeed in their careers. Authors FitzGerald, Dennis, and Durcikova have developed a foundation and balanced presentation from which new technologies and applications can be easily understood, evaluated, and compared.

He received his Ph.


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Alan R. Chambers Chair of Internet Systems, which honors John Chambers, the founder of Cisco Systems and a groundbreaking developer in networking technology. He has written numerous books on data communication, system design, and networking, and he is the publisher of MIS Quarterly, a scholarly quarterly lodged in the Information Systems department at Indiana University.

Chapter 3: Physical Layer: Architectures, Devices and Circuits Introduction As mentioned in Chapter 1, there are three fundamental hardware components in a data communication network in addition to the network software: the servers or host computers, the client computers, and the network circuits that connect them. This chapter focuses on these three components. However, before we can discuss the various types of clients, servers, and circuits, we need to discuss fundamental network architectures-the way in which the application software is divided among the clients and servers.

Network Architectures There are three fundamental network architectures. In host-based networks, the host computer performs virtually all of the work. In client-based networks, the client computers perform most of the work. In client-server networks, the work is shared between the servers and clients. The client-server architecture is likely to become the dominant network architecture of the future.

Table of Contents

The work done by any application program can be divided into four general functions. The first is data storage. Most application programs require data to be stored and retrieved, whether it is a small file such as a memo produced by a word processor or a large database such as an organization's accounting records. The second function is data access logic, the processing required to access data, which often means database queries in SQL. The third function is the application logic, which also can be simple or complex, depending on the application.

The fourth function is the presentation logic, the presentation of information to the user and the acceptance of the user's commands. These four functions are the basic building blocks of any application. Host-Based Architectures The very first data communications networks were host-based, with the host computer usually a central mainframe computer performing all four functions. The clients usually terminals enabled users to send and receive messages to and from the host computer. The clients merely captured key strokes and sent them to the host for processing, and accepted instructions from the host on what to display see Figure This very simple architecture often works very well.

Application software is developed and stored on one computer and all data are on the same computer. There is one point of control, because all messages flow through the one central host. In theory, there are economies of scale, because all computer resources are centralized.

Business Data Communications and Networking, 3rd Edition

We will discuss costs later. The fundamental problem with host-based networks is that the host must process all messages. As the demands for more and more network applications grow, many host computers become overloaded and cannot quickly process all the users' demands. Prioritizing users' access becomes difficult.

Response time becomes slower, and network managers are required to spend increasingly more money to upgrade the host computer. Unfortunately, upgrades to host computers are "lumpy. This relieved only a little of the bottleneck, however, because the host still performed all of the processing and data storage.

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Hosts became somewhat less overloaded, but the network became more complex: developing applications was more difficult, and there were more points of failure. Client-Based Architectures In the late s, there was an explosion in the use of microcomputers and microcomputer-based local area networks. Today, more than 80 percent of most organizations' total computer processing power resides on microcomputer-based LANs, not in centralized mainframe-based host computers.

As this trend continues, many experts predict that by the end of the century, the host mainframe computer will contain 10 percent or less of an organization's total computing power. Part of this expansion was fueled by a number of low-cost, highly popular applications such as word processors, spreadsheets, and presentation graphics programs. Most mainframe software is not as easy to use as microcomputer software, is far more expensive, and can take years to develop.

In the late s, many large organizations had application development backlogs of two to three years. That is, getting any new mainframe application program written would take years. New York City, for example, had a six-year backlog. In contrast, managers could buy microcomputer packages or develop microcomputer-based applications in a few months. With client-based architectures, the clients are microcomputers on a local area network, and the host computer is a server on the same network.

The application software on the client computers is responsible for the presentation logic, the application logic, and the data access logic; the server simply stores the data see Figure This simple architecture often works very well. However, as the demands for more and more network applications grow, the network circuits can become overloaded. The fundamental problem in client-based networks is that all data on the server must travel to the client for processing.

For example, suppose the user wishes to display a list of all employees with company life insurance. All the data in the database or all the indices must travel from the server where the database is stored over the network circuit to the client, which then examines each record to see if it matches the data requested by the user. This can overload the network circuits, because far more data is transmitted from the server to the client than the client actually needs.

Business Data Communications and Networking

If the files are very large, they may also overwhelm the power of the client computers. Client-Server Architectures Most organizations today are moving to client-server architectures.

Business Data Communications- Infrastructure, Networking and Security, 7th Edition

Client-server architectures attempt to balance the processing between the client and the server by having both do some of the logic. In these networks, the client is responsible for the presentation logic, while the server is responsible for the data access logic and data storage. The application logic may reside on the client or on the server, or it may be split between both. Figure shows the simplest case, with the presentation logic and application logic on the client and the data access logic and data storage on the server.

In this case, the client software accepts user requests and performs the application logic that produces database requests that are transmitted to the server. The server software accepts the database requests, per-forms the data access logic, and transmits the results to the client. The client software accepts the results and presents them to the user. For example, if the user requests a list of all employees with company life insurance, the client would accept the request, format it so that it could be understood by the server, and transmit it to the server.

Upon receiving the request, the server searches the database for all requested records and then transmits only the matching records to the client, which would then present them to the user. The same would be true for database updates; the client accepts the request and sends it to the server. The level of difficulty easy, moderate, difficult and the page number s relevant to the topic are also furnished.

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