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Chapter 40 Computer-network examples 505

Fig. 1a. Variable-structure direct switching network PMS diagram.

delay in getting the problem processed. Load sharing implies highly similar facilities at the nodes of the network.

Data sharing. A program is run at a node that has access to a large, specialized data base, such as a specialized automated library. It is less costly to bring the program to the data than to bring the data to the program.

Program sharing. Data are sent to a C that has a specialized program. This might happen because of the size of the program (hence, fundamentally the same reason as data sharing), but it might also happen because the knowledge (i.e., initialization and error rituals) to run the program is available at one C but not at another.

Specialized facilities. Within the network there need exist only one of various rarely used facilities, such as large random-access memories, or special display devices, or special-purpose array processors.

Fig. 1b. Fixed-network PMS diagram.

Fig. 1c. Store-and-forward network PMS diagram (using C switching).

Message switching. There may be a communication task of such magnitude that sophisticated switching and control are worthwhile.

Reliability. If some components fail, others can be used in their place, thus permitting the total system to degrade gracefully. (At the present state of the art, peripheral computers are needed to isolate the periphery from the unreliability of the network, and vice versa.)

Peak computing power. Large parts of the total system can be devoted for short periods to a single task, if there are important real-time constraints to be met. This depends on being able to fractionate the task into independent subtasks.

Communication multiplexing. Efficient use of communication facilities is obtained by multiplexing a number of low data-rate users, for example, T(typewriter; 150 b/s)'s. This may not be a reason for a network per se but may justify a larger network, provided that there is some reason for having one in the first place.

Better communication. A community of users (e.g., a scientific or engineering community) that could mutually use the same programs and data bases and converse about these directly (i.e., not by writing about them but in the context of mutual use) might become a much more productive community, with less duplication of work, faster communication of results, etc.

Better load distribution through preprocessing. Some tasks require very high-data-rate communication with a computer. By doing preprocessing in a smaller computer, a reduced information rate can be sent to the more general system.

With this general view of networks, let us consider several examples.

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