In summary, the preceding discussions provide the basis with which to establish the guiding principles that govern the architecture of a system s operating environment. Principle 11.1 A system s OPERATING ENVIRONMENT consists of two classes of domains: HIGHER ORDER SYSTEMS and a PHYSICAL ENVIRONMENT. Principle 11.2 A system s PHYSICAL ENVIRONMENT domain consists of three classes of system elements: NATURAL, HUMAN-MADE, and INDUCED. NATURAL and HUMAN-MADE ENVIRONMENTs systems interact; the INDUCED ENVIRONMENT represents the timedependent result of that interaction.

Our discussion in this section provides an orientation of the OPERATING ENVIRONMENT, its levels of abstraction, and classes of environments. Based on identi cation of these OPERATING ENVIRONMENT elements, we introduced Figure 11.1 to depict relationships among PHYSICAL ENVIRONMENT levels of abstraction and its system elements. Next we introduced the concept of the OPERATING ENVIRONMENT architecture as a framework for linking the OPERATING ENVIRONMENT system elements. The architectural framework of interactions provided a basis for us to de ne several system element interaction principles.

1. Answer each of the What You Should Learn from This questions identi ed in the Introduction. 2. Refer to the list of systems identi ed in 2. Based on a selection from the preceding chapter s General Exercises or a new system selection, apply your knowledge derived from this chapter s topical discussions. Identify the following: (a) HIGHER ORDER SYSTEMS domain and its system elements (b) PHYSICAL ENVIRONMENT domain, its system elements, and levels of abstraction.

1. Contact a system development program in your organization. Research how they analyzed their SYSTEM OF INTEREST (SOI), its OPERATING ENVIRONMENT, and their respective system elements. How was this analysis re ected in the SOI architecture

Kossiakoff, Alexander, and Sweet, William N. 2003. Systems Engineering Principles and Practice. New York: Wiley-InterScience. MIL-HDBK-1908B. 1999. DoD De nitions of Human Factors Terms. Washington, DC: Department of Defense (DoD).

One of the crucial factors of system success is determined by what happens at its internal and external interfaces. You can engineer the most elegant algorithms, equations, and decision logic, but if the system does not perform at its interfaces, the elegance is of no value. System interface characterizations range from cooperative interoperability with external friendly systems to layers of protection to minimize vulnerability to external threats (environment, hostile adversary actions, etc.) and structural integrity to ensure survivability. This chapter introduces the context of system interfaces, their purpose, objectives, attributes, and how they are implemented. Our discussions explore the various types of interfaces and factors that delineate success from failure. This information provides the basis for the next chapter, which addresses interface design and control.

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. What is an interface What is the purpose of an interface What are the types of interfaces What is a point-to-point interface What is a logical interface What is a physical interface How do logical and physical interfaces interrelate Identify seven types of physical interfaces What are the steps of the interface de nition methodology What is a generalized interface solution What is a specialized interface solution How does a generalized solution relate to a specialized solution How do generalized and specialized solutions relate to logical and physical interfaces What are some methods of limiting access to interfaces What constitutes an interface failure What are some examples of interface failures

System Analysis, Design, and Development, by Charles S. Wasson Copyright 2006 by John Wiley & Sons, Inc.

Human-Machine Interface The actions, reactions, and interactions between humans and other system components. This also applies to a multi-station, multi-person con guration or system. Term also de nes the properties of the hardware, software or equipment which constitute conditions for interactions. (Source: MIL-HDBK-1908, p. 21) Interchangeability The ability to interchange, without restriction, like equipments or portions thereof in manufacture, maintenance, or operation. Like products are two or more items that possess such functional and physical characteristics as to be equivalent in performance and durability, and are capable of being exchanged one for the other without alteration of the items themselves or of adjoining items, except for adjustment, and without selection for t and performance. (Source: MIL-HDBK-470A, Appendix G, Glossary, p. G-7) Interface The functional/logical relationships and physical characteristics required to exist at a SYSTEM or entity boundary with its OPERATING ENVIRONMENT that enable the entity to provide a mission capability. (Source: Adapted from DSMC Glossary of Terms) Interface Control The process of: (1) identifying all functional and physical characteristics relevant to the interfacing of two or more items provided by one or more organizations; and (2) ensuring that proposed changes to these characteristics are evaluated and approved prior to implementation. (Source: Former MIL-STD-480B, para. 3.1.43) Interface Device An item which provides mechanical and electrical connections and any signal conditioning required between the automatic test equipment (ATE) and the unit under test (UUT); also known as an interface test adapter or interface adapter unit. (Source: MILHDBK-470A, Appendix G, Glossary, p. G7) Interface Ownership The assignment of accountability to an individual, team, or organization regarding the de nition, speci cation, development, control, operation, and support of an interface. Interoperability The ability of two or more systems or components to exchange information and to use the information that has been exchanged. (Source: IEEE 610.12-1990) Peer level Interactions SYSTEM OF INTEREST (SOI) interactions namely MISSION SYSTEM and SUPPORT SYSTEM with external systems in the OPERATING ENVIRONMENT. For analytical purposes, we aggregate these systems into a single entity abstraction referred to as the PHYSICAL ENVIRONMENT SYSTEMS domain. Point-to-Point An interface con guration that characterizes the physical connectivity between two points, typically accomplished via dedicated, direct line. For example, a light switch connection to a room light. Based on this introduction, let s begin our discussion by exploring what an interface is.