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Table 1 lists the basic dimensions of a computer network design space. Real networks can be represented as a point in design space by specifying the values of each dimension. Frequently real networks incorporate more than one alternative value (intercon-
Table 1 The Network Design Space
>1 km (global)
100-1,000 m (building) <100 m (room)
2.4-1 9.2 Kbit/s
1 ³ Mbit/s
Local delay estimate
nections or different capacities, varying physical separation, etc.). This multiplicity and range of values for a single dimension is encouraged by:
Thus it is extremely rare that a network can be described by selecting one parameter from each of the dimensions in Table 1. However, we offer the taxonomy as an orderly way of discussing the various decisions that make up a network's design. During our discussion of the various dimensions, the reader will note that some of the dimensions are interrelated, i.e., the selection of a value for one dimension may dictate, or at least bias, the selection of a value for another dimension. This phenomenon is characteristic of complex system design. It is not sufficient to use one dimension to optimize, because of its impact on other dimensions. Hence the would-be designer must first understand the available alternatives and then seek a harmonious blend of the design parameters in order to optimize against cost-, performance-, and reliability-based objective functions. It is essential that the designer identify constraints, design variables, and objective functions. Moreover, a network changes with time as the objective functions of machines, links, and user costs dynamically change. The next section will present a brief discussion of the design parameters. The following section will examine actual networks; the student is encouraged to analyze them to see the interdependence of the design parameters.
Network Design-Space Parameters
As in most system designs, the intended application is the primary shaper of the eventual structure. The network may only exist to tie remote terminals to a centralized computer. Historically, terminals had relative low data rates limited by available voice-grade phone lines (e.g., 100-300 bit/s), but the advent of interactive graphics, intelligent terminals, better links, and modulator-demodulators (modems) will cause this attribute to be upgraded.
Computer-to-computer communication involves higher data rates and larger blocks of data than terminal-to-computer communications. The type of information transmitted (e.g., data files, facsimile, real time voice, or real time video) significantly impacts
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