of fabrics and dyeing solutions as its inputs, and delivers mat- ter in the form of dyed fabrics as its output. A telecom system is also a process- ing system. Its basic goal is to transfer data between points on its boundary with minimum delay. The users of a processing system are humans or machines. Figure 1.3 also indicates that most processing systems of interest include infor- mation systems as subsystems. Information subsystems are needed to control the behaviour of the system. For example, in mobile networks there exist large infor- mation systems, so-called "home location registers" (HLR), that are used by other subsystems of the telecom system to keep track of the movements of mobile phones. We can therefore conclude that information systems is a more general class of system than processing systems. A construction architect may provide a specific information model for each building project, in terms of the model/drawings he has created. The model, if reused for another building project, will result in an identical concrete system. In systems engineering however, we produce system models in some kind of structure of systems abstractions. We say that a model that describes the structure and func- tions of a class of systems in terms of its software and hardware systems (or "com- ponents") describes an implemented system. Concrete systems can be instantiated from implemented systems. Within the set of all concrete systems that can be instantiated from an implemented system, the individual concrete systems 1 Introduction to Systems Modelling will have structural and functional differences only within the constraints that are defined by the implemented system model. An implemented system model can be regarded as a realization of a more abstract system model that does not refer to any particular software and hardware technologies. We will call such a model a sysnet model. A sysnet model can be reused from time to time to create implemented system models that refer to specific software and hardware technologies