es not only simple separa- tion of a model into parts, but also creation of a model to show suitable levels of abstractions of the resource system. For example, "model of {X, Y}" is a high- level abstraction of the resource system. We say that the structure of resource model fragments (and thereby the structure of the process control system), shows an abstraction hierarchy. The process control systems we design are increasingly complex, since we try to control more and more complex resource systems. For a processing system to be reliable, the process control system must have a complexity that matches the com- plexity of the resource system it controls. Oversimplification of a resource system model is therefore dangerous to system reliability. At the same time, we do not want to design process control systems that are more complex than necessary. The only way to achieve this is to have a thorough understanding of the resource sys- tem, and to use a systematic approach to constructing the structure of the process control system. The resource model approach, as depicted in Fig. 2.13, is therefore at the heart of a methodology for designing process control systems. Modelling of process control systems is discussed in detail in Chap. 6; how to design resource models is discussed in Sect. 7.3. 2.3.2 The Dualism Between Interface and Unit A sysnet model consists of several different types of model fragments that show collaboration structures. A physical network shows a structures of physical nodes, other fragment types show structures of logical nodes or service objects. There will 2.3 Architectural Principles also exist many model fragments of the same type. In this discussion we will disre- gard the type of components and interfaces that are described by a particular type of model fragment and just call them unit and interface. A collaboration is commonly regarded as a structure of units. In SM, however, this is a derived view. A more basic view is to look upon a collaboration as a struc- ture of related interfaces. F