right © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. THE EVOLUTION OF NETWORK TECHNOLOGY: DISTRIBUTED COMPUTING AND THE CONVERGENCE OF NETWORKS 4 NETWORK PROCESSORS and large (usually multinational) corporations could afford these machines. Some "enlightened" industry executives have even gone down in history affirming that there could not be any potential for more than two to three computers in the market! Soon the card punchers disappeared and were replaced by alphanumeric terminals. People could sit in front of a computer screen and type in their code using a typewriter-like keyboard. The progress of compiling technology and operating systems facilitated interactive work sessions. Programmers no longer had to wait one day to get results. Once the programs were executed, the programmer could sit down and examine the results or reexamine the code and debug the program. Interactivity between man and machine started increasing. The site topology and IT architectures of these machines were mostly based on an inverted tree structure. The mainframe, also affectionately known as the big iron, was at the top of the hierarchy (the root of the inverted tree). The structure contained a series of layers of controllers of variable per- formance. It had a capacity that would individually cluster several nearby or remote downstream devices. This would eventually create an array of terminals that enabled interactive users to use the mainframe's computing power on a time-shared basis. IBM led the industry and the world by creating the first comprehensive and extremely powerful intercomputer communications architecture called the Systems Network Architecture (SNA). This architecture was quite advanced for its time. SNA enabled mainframes to communicate with each other at different sites. Little by little, tasks that were previously tedious or impossible could be done in a complex but well-tested, documented, and straightforward way. Users could easily perfor