Chapter 40 Computer-network examples 509

C switch concentrates several typewriters into a time-multiplexed 2,400-b/s link. Several of the 2,400-b/s links can in turn be concentrated prior to transmitting via a 50-kb/s link. Thus the general organizing principle, like that of most large organizations, is to handle problems at the lowest (cheapest) possible level. Another organization principle of the hierarchy is that only relevant information be passed between the levels. For example, encoding would be used so that only some fraction of the bits flowing at the periphery would enter the highest-level computers. At each of the levels we assume that specialized, time-shared computers are employed to handle the very simpler tasks of editing, simple calculations, etc.

At the network periphery there are a number of terminal computers, i.e., C(terminal; CRT, card, lines, analog, plot, keyboard). Although they are computers, they behave as terminals. The DEC 338 (Chap. 25) is typical of this terminal class. Part of the periphery connects to other networks and part connects to specialized processes, e.g., a process control, or experimental apparatus on a dedicated basis. The peripheral computers are able to do local tasks independently of the larger, more unreliable computers.

ComLogNet was developed for the U.S. Air Force in the early 1960s for the purpose of sending messages (or information) among T's [Segal and Guerber, 1961]. It is built to transmit both at low