Each of the past three centuries has been dominated by a single technology. The 18th Century was the time of the great mechanical systems accompanying the Industrial Revolution. The 19th Century was the age of steam engine. During the 20th Century, the key technology has been information gathering, processing, and distribution. Among other developments, we have seen the installation of worldwide telephone networks, the invention of radio and television, the birth and unprecedented growth of the computer industry, and the launching of communication satellites.

Due to rapid technological progress, these areas are rapidly converging, and the differences between collecting, transporting, storing, and processing information are quickly disappearing. Organizations with hundreds of offices spread over a wide geographical area routinely expect to be able to examine the current status of even their most remote outpost at the push of a button. As our ability to gather process, and distribute information grows, the demand for even more sophisticated information processing grows even faster. This led to the birth of Computer Networks.


1.1 What is a Computer Network?

The merging of computers and communications has had a profound influence on the way computer systems are organized. The concept of the "computer center" as a room with a large computer to which users bring their work for processing is now obsolete. The old model of a single computer serving all of the organization's computational needs has been replaced by one in which a large number of separate but interconnected computers do the job. These systems are called computers networks.

"Computer Network" can be defined as the interconnected collection of autonomous computers. Two computers are said to be interconnected if they are able to exchange information. The connection need not be via a copper wire; fiber optics, microwaves and communication satellites can also be used. By requiring computers to be autonomous, we wish to exclude from our definition systems in which there is a clear master/slave relation. If one computer can forcibly start, stop, or control another one, the computers are not autonomous. A system with one control unit and many slaves is not network; nor is a large computer with remote printers and terminals.

There is a considerable confusion in the literature between a computer network and a distributed system. The key distinction is that in a distributed system, the existence of multiple autonomous computers is transparent (i.e., not visible) to the user. He can type a command to run a program, and it runs. It is upto the operating system to select the best processor, and put the results in the appropriate place.

In the other words, the user of a distributed system is not aware that there are multiple processors; it looks like a virtual uniprocessor. Allocation of jobs to processors and files to disk, movement of files between where they are stored and where they are needed, and all other system functions must be automatic.


1.2 Why a Computer Network?

A question may arise that why we need a computer network? The answer is simple and can be answered in the following few points:

    1) To distribute pieces of computation among computers (nodes).
    2) To have coordination between processes running on different nodes.
    3) To access remote I/O devices.
    4) To have remote data/file access.
    5) For personal communication (like e-mail, chat, audio/video conferencing).
    6) The World Wide Web



1.3 Uses of Computer Networks

There can be large number of uses and utilities of computer networks and some of them can be given as:


    1.3.1 Network for Companies

    Many organizations have a substantial number of computers in operation, often located far     apart. For example, a company with many factories may have a computer at each location     to keep track of inventories, monitor productivity, and do the local payroll. Initially, each of     these computers may have worked in isolation from the others, but at some point,     management may have decided to connect them to be able to extract and correlate     information about the entire company.

    Put in slightly more general form, the issue here is resource sharing, and the goal is to     make all programs, equipment, and especially data available to anyone on the network     without regard to the physical location of the resource and the user.

    A second goal is to provide high reliability by having alternative sources of supply. For     example, all files could be replicated on two or three machines, so if one of them is     unavailable, the other copies could be used.

    Another goal is saving money. Small computers have a much better price/performance     ratio than large ones.

    Another networking goal is scalability, the ability to increase system performance gradually     as the workload grows just by adding more processors.

    Yet another goal of setting up a computer network has little to do with technology at all. A     computer network can provide a powerful communication medium among widely separated     employees.


    1.3.2 Network for People

    The motivations given above for building computer networks are all essentially economic     and technological in nature. If sufficiently large and powerful mainframes were available at     acceptable prices, most companies would simply choose to keep all their data on them and     give employees terminals connected to them. In the 1970s and early 1980s, most     companies operated this way. Computer networks only became popular when networks of     personal computers offered a huge price/performance advantage over mainframes. Starting     in the 1990s, computer networks began to start delivering services to private individuals at     home. These services and the motivation for using them are quite different than the     "corporate efficiency" model. Below are the three of the more exciting ones that are     starting to happen:

        1) Access to remote information.
        2) Person-to-person communication.
        3) Interactive entertainment.


     1.3.3 Social Issues

    The widespread introduction of networking will introduce new social, ethical, political     problems. Let us just briefly mention a few of them. A popular feature of many networks is     newsgroups or bulletin boards where people can exchange messages with like-minded     individuals. As long as the subjects are restricted to technical topics or hobbies like     gardening, not too many problems will arise. The trouble comes when newsgroups are set     up on topics that people actually care about, like politics, religion, or sex. Views posted to     such groups may be deeply offensive to some people. Furthermore, messages need not to     be limited to text. High resolution color photographs and even short video clips can now     easily be transmitted over computer networks. People have sued network operators,     claiming that they are responsible for the contents of what they carry, just as newspaper and     magazines are. Computer networks offer the potential for sending anonymous messages. In     some situations, this capability may be desirable. For example, it provides a way for     students, soldiers, employees and citizens to blow the whistle on illegal behavior on the     part of professors, officers, superiors and politicians without fear of reprisals. On the other     hand, in the United States and most other democracies, the law specifically permits an     accused person the right to confront and challenge his accuser in court. Anonymous     accusations cannot be used as evidence. In short, computer networks, like the printing     press 500 years ago, allow ordinary citizens to distribute their views in different ways and to     different audiences than were previously possible. This new found freedom brings with it     many unsolved social, political, and moral issues.