ems) to be modelled sepa- rately from distribution characteristics (a relatively dynamic aspect of the architecture of telecom systems). This approach is unique among modelling lan- guages. The top-down approach in developing SM&AML has resulted in well-defined and separated modelling components, as well as a seamless method for modelling complex systems, from high-level network architectures to the finest structures of system components. It was natural to look at the systems to be modelled as abstract systems, in the sense that no preconceived assumptions can be made on how system components will be implemented (i.e. if hardware or software, if ADA or C++, etc.). A major advantage is the reusability of models produced in SM&AML, compared to models of implemented systems. Why Modelling Systems Is Important Modelling is at the heart of all kinds of design activity because that's the way humans solve problems. Most modelling activities are invisible to an observer, since they are done in the minds of humans and result in mental models. Mental models have, however, the disadvantage that they are easily forgotten, they change unconsciously, and they are difficult to communicate. We will therefore regard as "systems modelling" only such activities that result in information models, which means that the model can be stored, communicated and interpreted by other people, and, to some extent, also by computers. Figure 1.2 shows modelling activities and uses of information models in different phases of systems engineering. System modelling, System model very early phase Common Translate System modelling modelling language early phase Products Formal system Design model Information Activity element Other artifact Fig. 1.2 Modelling in systems engineering 1 Introduction to Systems Modelling In very early phases there are normally small, homogenous groups of people involved in developing and analysing a system concept. People in such groups communicate frequently and have developed a common language for expressing their ideas