1. Introduction

Each of the past three centuries has been dominated by a single technology:

18^th - industrial revolution with the great mechanical systems

19^th - age of steam engine

20^th - the key technology has been information gathering, processing and distribution - telephones, radio, television, computer industry, communication satellites.

These areas are rapidly converging.

The ability to process information grows - the demand for even more sophisticated information processing grows even faster.

Progress of computers:

• First two decades - highly centralized systems.
• Later - the merging of computers and communications has had profound influence on the way computer systems are organized - replacement of the old model of highly centralized systems by computer networks.

Computer network is an interconnected collection of autonomous computers.

Two computers are said to be interconnected if they are able to exchange information. The connection can be realized by different media.

Autonomous means no master/slave like. A system with one control unit and many slaves is not a network; nor is a large computer with remote printers and terminals.

Computer networks vs. distributed systems:

• In distributed systems the existence of multiple autonomous computers is transparent to the user - the system looks like a virtual uniprocessor.
• With a network, users must explicitly log onto one machine.
• Distributed system is a software system built on top of a network.

Another definition of a distributed system: interconnected collection of autonomous computers, processes. The computers, processes, or processors are referred to as the nodes of the distributed system.