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Figure 2. Packaging - a set of closely interrelated design activities.

room. In either case, the air conditioning system is left with the problem of carrying the heat away, and the fans associated with that system are added to the fans associated with the computer to create acoustical noise pollution in the room, making it more difficult for people to work. Furthermore, if the computer is used in an unusually harsh environment, a special heat exchanger is required in order to avoid contamination of the components within the computer by the pollutants present in the cooling airflow.

Finally, the mechanical characteristics of a particular package such as weight and size directly affect manufacturing and shipment costs. They determine whether a system can be built and whether it can be shipped in a certain size airplane or carried by a particular distribution channel such as the public postal system. The mechanical vibration sensitivity characteristics determine the type of vehicle (ordinary or special air ride van) in which equipment can be shipped.

It is also necessary to examine the particular design parameter in order to determine whether it is a constraint (such as meeting a particular government standard), a goal (such as minimum cost), or part of a more complex objective function (such as price/performance). Table 1 lists the various kinds of design activities and constraints, goals, or parts of more complex objective functions that they determine. The table also gives the dimensions of various metrics (e.g., cost, weight) available to measure the designs; many of these metrics are used in subsequent comparisons.

Given the basic design activities, one may now examine their interaction with the hierarchy of levels (i.e., the systems) being designed (see Table 2). This is done by looking at each level and examining the interaction of the de sign activities for that level with other design activities (e.g., function requires power, power requires cooling, cooling requires fans, fans create noise, and noise requires noise suppression).

Computer Systems Level. The topmost level in Table 2 is the computer system, which for the larger minicomputers and PDP-l0 computers consists of a set of subsystems (processor, memories, etc.) within cabinets, housed in a room, and interconnected by cables. The functional design activity is the selection and interconnection of the cabinets, with a basic computer cabinet that holds the processor, memory, and interfaces to peripheral units. Disks, magnetic tape units, printers, and terminals occupy free standing cabinets. The functional design is usually carried out by the user and consists of selecting the right components

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