TABLE OF CONTENTS A man-made system is a collection of entities that are meant to interact in predictable ways with an environment and with each other via relations between them to achieve a useful function identified and articulated by a customer as a system need statement. Therefore, systems are composed of entities and relations between the entities. The system is intended to satisfy the system need statement, the system's ultimate function, depicted on system diagrams as a rectangular block titled System Need and identified with a functional identifier F. The need is allocated to the system depicted on system models by a rectangular block named "system" (or a particular system) and identified with an architecture identifier A.
A system interacts within an environment as shown in Figure 3.10-1. The environment for every system is everything in the Universe less the entities that are part of the system architecture (E = U - A). One can reduce the scope of the environment to those entities that will have some influence on the system. The line joining the system and environment in Figure 3.10-1 (I2) indicates the relations between the two (external interfaces). The line joining the system on both terminals (I1) indicates the internal relations between system entities (internal interfaces) yet to be defined within the system. The line labeled I3 relates to relations between environmental elements that are of interest to the system.
A system, then, is defined by identifying...
