The First computers were large, expensive, and dif?cult to manage Originally, these large mainframe computers were not networked as you are familiar with today. Jobs were entered into the system by reading commands from card decks. The computer executed one job at a time and generated a printout when the job was complete. Terminals, which came later, provided the user with a new mechanism to interact with the centralized computer. These terminals, however, were merely input/output devices that had no independent processing power. All processing still took place on the central main-frame, (see Figure) hence the name centralized computing. Networks, therefore, served little purpose other than to deliver commands to and get results from the powerful centralized processing device. To this day, large mainframe systems are still being operated around the world, most often by governments and large corporations. An example of centralized computing to which everyone can relate is using an ATM machine. ATMs function as terminals. All processing is done on the mainframe computer to which the ATMs are connected. In summary, the centralized computing model involves the following:
All processing takes place in the central mainframe computer.
Terminals are connected to the central computer and function only as input/output devices.
This early computing model worked well in large organizations that could justify the need for these expensive computing devices. One of the drawbacks, however, was that the mainframes were not Flexible in their placement (some were the size of a large room) and did not scale down to meet the needs of smaller organizations. New ways of sharing information were necessary to allow computing power to be shared ef?ciently on smaller networks.
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