dations of Sysnet Modelling Introduction Sysnet modelling (SM) is a specification and construction method for large and complex systems. SM is partly a domain-specific method. It comprises a set of con- cepts, architectural principles and generic patterns for modelling process control systems in general, and telecom systems in specific. The modelling language Abstract systems Modelling Language (AML) can be used for model creation. However, this book is however not primarily a description of AML. The impor- tance of systems modelling lies in understanding the concepts, architectural princi- ples and generic patterns (i.e. SM), which also is the primary theme of this book. The language (symbols, terms and syntactical constructs) acts more like a tool in this context. It is conceivable that languages other than AML could serve the same purpose. To support modelling in general, tools are also needed. Presently, how- ever, no SM&AML tool exists. This book, therefore, also addresses tool makers, by providing the requirements for an SM&AML tool. Sysnet models are executable on different levels of abstraction, which implies that SM is more a modelling method for describing the run-time structure and behaviour of such systems, than a method for managing the products and docu- ments used for real systems provisioning. A sysnet model allows architects and designers to explore the structure, functions and behaviour of systems long before any products are implemented. Sysnet models describe "abstract systems", which means that sysnet models are expressed without reference to any particular languages for software and hardware design. Thus, a sysnet model is an abstraction of systems that may be built by soft- ware and hardware components. SM&AML takes into consideration some of the characteristics of systems modelling methods that were discussed in Chap. 1, such The separation between system abstractions (standardized, applied, imple- mented and concrete system models), The separation of incomplete and redundant specification