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508 Part 5 The PMS level Section 4 Network computers and computer networks

Fig. 5. Typical computer network PMS diagram.

Typical local network

We summarize in Fig. 5 the direction in which the last three networks are moving by presenting a hypothetical, local network, as it may mature on many large university campuses (and large industrial establishments). The network is conceived as a single computing facility, to serve a clientele with many heterogeneous but partially overlapping computing needs. An essential feature of the environment of the network is that the collection of computing resources it connects are not planned all at once but keep growing and changing in imperfectly controlled ways. This arises from the quasi-independent nature of the subparts of large universities and engineering establishments. In any event, the network is a mixture of functionally independent and functionally specialized C's. One probable feature is the duplexed C.files which handle all the Ms functions for all C's, except the C(library). A library's computer, though strongly coupled to the network, would have its own files and specialized terminals, including hard copy devices oriented to library needs. The C.file increases the requirements for the S.central but provides much more economic Ms, as well as easing the ability to connect new C's into the system, since they immediately have access to an organized Ms.

The reader should note that the four switches (S's) can be either fixed links, variable switches (e.g., Telephone Exchange), or a computer used as a direct switch or as a store-and-forward switch.

The most interesting aspect of this network is that it has a general hierarchical structure and is like other hierarchical organizations. Here, the levels of the organization are based on data rates. For example, there is a very low-level computer which deals with the basic communication to typewriters at ~ 150 b/s. This

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