r as product structures (by product management). Both forms are relevant, but for dif- ferent tasks in system development and provisioning processes, and may therefore coexist in a methodology. To support early design phases, only the model manage- ment approach is feasible. To support separation of concerns in different phases of the development and provisioning of telecom networks and systems SM&AML suggests that different system abstractions are described in different types of system models: standardized sysnet models, applied system models, implemented system models and concrete 1 Introduction to Systems Modelling system models. SM&AML is the modelling technology to be used for standardized and applied system models, also called sysnet models. A system modelling language must have a considerable semantic distance from modelling languages for software and hardware systems. To some extent, the lan- guage should be graphical, especial as different types of collaboration diagrams are concerned. Graphical representation should, however, not be overrated. There are many properties of a system that lend themselves better to textual representations. The expressiveness of systems models is an important prerequisite for the lan- guage to be embraced by systems designers. It must, however, also be formal, SO that models can be interpreted and analysed by computers, especially if the models will act as requirements for software and hardware design. Since requirements for expressiveness and formality are relevant in different phases of a development project, the language might offer different representations for that. In SM&AML this distinction between representation forms is called specification refinement. A system modelling language should be as complete as possible, with respect to the intended domain and the engineering task it is supposed to support. SM&AML is reasonably complete in this respect, for processing systems in general and tele- com networks and systems in specific, when the task is systems design. 2 Foun