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5.2 Local and Wide Area Networks

A network that provides communication among computers over a relatively short distance, such as one floor of a building, is called a Local Area Network(LAN). If the computers are located further away within a building, or across town, across the country, or across the world, then a Wide Area Network (WAN) is used. Fundamentally, a LAN and a WAN perform similar actions from a user's perspective. However, the types of hardware used, the time involved in transferring data, and other issues such as security may distinguish a LAN from a WAN. The Internet is an example of a WAN. This section describes some of the basic principles of networks, focusing on LANs, while WANs are discussed in a later section.

Reading Sequence:

  • Parsons/Oja, Chapter 5-Section A. Learning Goal: Knowledge of the basic network building blocks.

  • Parsons/Oja, Chapter 5-Section B. Learning Goal: Knowledge of wired networks.

  • Parsons/Oja, Chapter 5-Section C. Learning Goal: Knowledge of wireless networks.

  • Parsons/Oja, Chapter 5-Section D. Learning Goal: Knowledge of the system and application software used to support LANs.

        

Assessments:

  • Multiple-Choice Quiz 18

5.3 Communication Strategies

This module covers the various types of communications architectures available.

Reading:

  • 5.3.1 Client-Server FrameworkLearning Goal: Understanding thin and thick clients.

  • 5.3.2 Peer-to-Peer ConnectivityLearning Goal: Understanding how each machine is both a client and a server, and what impact this has on computing.

          

Assessments:

  • Multiple-Choice Quiz 19

5.3.1 Client-Server Framework

Many network applications are organized as client-server systems. The client and the server are both software programs. The client runs on the user's computer and interacts with both the user and the server. The server can accept requests from any number of clients. It performs some service for them and returns the results. The World Wide Web is an example of a client-server system. The client is the Web browser program, such as Firefox or Internet Explorer. For a machine to host a Web site, it must run a Web server program that clients can talk to. One widely used Web server is Apache. Email is another example of a client-server application. The mail-reading program, such as Microsoft Outlook, is the client. The server is a program that accepts and stores incoming mail for users, and delivers the mail to them when their client requests it. Servers can also forward mail to other servers if necessary.

Yet another client-server application is Usenet news, which is a kind of distributed bulletin board system. There are thousands of newsgroups covering all kinds of topics, like computers, sports, politics, science, and entertainment. News servers (with big disks!) store messages, called "postings," and exchange them with other servers. They also accept new postings from clients. When users want to read a newsgroup, their client sends a request to the server to download the most recent messages. There are many client programs specialized for reading news, but today many people use their Web browser, since Firefox and Internet Explorer both contain newsreaders. Modern Web browsers can communicate with many kinds of servers, including Web servers, news servers, and mail servers.

Two terms you may run into are thick clients and thin clients. The distinction between thick and thin clients is based on how much work is done on the client side: a thin client does relatively little work, typically providing little more than a user interface. A thick client, on the other hand, carries out a substantial portion of the overall work of the system. For example, in a typical Web surfing use, the browser acts as a thin client, while the work of searching a database and organizing the results is carried out by servers. However, a system might download a Java applet to the browser, and that applet might act as a thick client. How much work is assigned to the client in the design of the system depends on the logic of the system itself and the power of the client device. For example, a personal computer can support clients that do a great deal of work, while a handheld device or a television set-top box is more likely to be suited to support thin clients only.