Computer Networks

I. INTRODUCTION  

Computer Networks, the widespread sharing of information among groups of computers and their users, a central part of the information age. The popular adoption of the personal computer (PC) and the local area network (LAN) during the 1980s has led to the capacity to access information on a distant database; download an application from overseas; send a message to a friend in a different country; and share files with a colleague—all from a personal computer.

The networks that allow all this to be done so easily are sophisticated and complex entities. They rely for their effectiveness on many cooperating components. The design and deployment of the worldwide computer network can be viewed as one of the great technological wonders of recent decades.

II. MODEMS AND COMPUTER BUREAUX

As recently as the 1970s, computers were expensive, fragile machines that had to be looked after by specialists and kept in a controlled environment. They could be used either by plugging in a terminal directly or by using a phone line and modem to gain access from a distance. Because of their high cost, they tended to be centralized resources to which a user had to arrange their own access. During this time, organizations that offered access time on a mainframe computer—computer bureaux—flourished. Computer networks during this period were not commercially available. Even so, one of the most significant developments to shape the modern world of technology was initiated at this time: experimentation by the US Defence Department in distributing computer resources to provide resilience against failure. This work is now known as the Internet.

III. LOCAL AREA NETWORKS

One of the most dramatic events in computer networking has been the introduction and rapid growth of the local area network (LAN) as a way to standardize the system of linking computers used in office systems. As the name suggests, this is a means of connecting a number of computing elements together. At the simplest level, a LAN provides no more than a shared medium (such as a coaxial cable to which all computers and printers are connected) along with a set of rules that govern the access to that medium. The most widely used LAN, Ethernet, uses a mechanism called Call Sense Multiple Access—Collision Detect (CSMS-CD). This means that each connected device can only use the cable when it has established that no other device is using it. If there is contention, the device looking for a connection backs off and tries again later. The Ethernet transfers data at 10M bits/sec—which is fast enough to make the distance between devices insignificant. They appear to be connected directly to their destination.

Ethernet and CSMA-CD are examples of LANs. There are many different layouts (such as bus, star, ring) and a number of different access protocols. Despite this variety, all LANs share the feature that they are limited in range (typically they cover one building) and are fast enough to make the connecting network invisible to the devices that use it.

In addition to providing shared access, modern LANs can also give users a wide range of sophisticated facilities. Management software packages are available to control the way in which devices are configured on the LAN, how users are administered, and how network resources are controlled. A widely adopted structure on local networks is to have a number of servers that are available to a (usually much greater) number of clients. The former, usually powerful computers, provide services such as print control, file sharing, and mail to the latter, which are usually personal computers.

IV. ROUTERS AND BRIDGES

The facilities on most LANs are very powerful. Most organizations do not wish to have small isolated islands of computing facilities. They usually want to extend facilities over a wider area so that groups can work without having to be located. Routers and bridges are specialized devices that allow two or more LANs to be connected. The bridge is the more basic device and can only connect LANs of the same type. The router is a more intelligent component that can interconnect many different types of computer network.

Many large companies have corporate data networks that are founded on a collection of LANs and routers. From the user's point of view, this arrangement provides them with a physically diverse network that looks like one coherent resource.

V. WIDE AREA NETWORKS  

At some point, it becomes impractical to extend a LAN any further. Physical limitation sometimes drives this, but more often than not there are more convenient or cheaper ways to extend a computer network. Two major components in most real computer networks are the public telephone and data network. These provide long distance links that extend a LAN into a wide area network. Nearly all of the national network operators (such as DBP in Germany, British Telecom in the United Kingdom) offer services for the inteconnection of computer networks. These services range from simple, low speed data links that work over the public telephone network through to sophisticated high-speed data services that are ideally suited to the interconnection of LANs. These high-speed data services are usually referred to as broadband connections. It is anticipated that they will provide the necessary links between LANs that make what is called the information superhighway a reality.

VI. DISTRIBUTED COMPUTING

 It would be easy to assume that computers will all be able to work together once they have broadband connection. But how do you get computers made by different manufacturers in different countries to work together across the world? Until recently, most computers were built with their own interfaces and were structured their own unique way. A computer could talk to one of its own kind but would have difficulty communicating with a "foreigner". There were only a privileged few with the time, knowledge, and equipment to extract what they wanted from a variety of computing resources.

By the 1990s, the level of commonality across different computers reached the stage where they could interwork effectively. This allows virtually anyone to use a remote machine to good effect. The main contributors to this are:

A) Client Server  

Instead of building computer systems as monolithic systems, there is now general agreement that they should be constructed as client/server systems. The client (a PC user) requests a service (such as printing) and the server (a LAN-connected processor) provides it. This consensus view on the structure of a computer system means that there is a separation of functions previously bundled together. The implementation details that flow from a simple concept go a long way to enabling all computers to be treated uniformly.

B) Object Technology  

Another way to build computer systems works from the premise that they should be built from well-defined parts—objects which are encapsulated, defined, and implemented so that they can be independent agents. The adoption of objects as a means of building computer systems has helped to allow interchangeability of parts.

C) Open Systems  

This term covers the general aim of building computer systems so that they can readily be interconnected, and hence distributed. In practice, open systems is all about unbundling all the complexities of a computer system and using similar structure across different systems. And this entails a mixture of standards (which tell the manufacturers what they should be doing) and consortia (groups of like-minded people who help them to do it). The overall effect is that they can talk to each other.

The ultimate aim of all of the work in distributed systems is to allow anyone to buy computers from a number of different manufacturers, to site them wherever is convenient, to use broadband connections to link them, and to operate them as one cooperating machine that takes full advantage of the fast links.

VII. SECURITY AND MANAGEMENT

Having fast computer networks built of machines that can talk to each other is not the end of the story. The spectres of the "information superhighwayman" and the "information superroadworks" have yet to be dealt with.

A) Security  

With ever increasing amounts of important information being entrusted to ever more distributed computers, computer security becomes ever more important. In a highly distributed system it would be all too easy for an informed superhighwayman to access confidential information without being seen. The Data Encryption System (DES) standard for protecting computer data, introduced in the late 1970s, has more recently been supplemented by "public key" systems that allow users to scramble and unscramble their messages easily without a third party intruding.

B) Management  
It is a full-time job to keep a LAN operating as it should. Keeping a computer network that is distributed across the world running smoothly takes the challenge of network management a step further. The essential concepts for managing distributed and diverse networks have received a lot of attention lately. There are now enough tools and standards for this important aspect of computer networks to allow global networks to be supervised effectively.