s from complete, exe- cutable specifications that are free of redundancy (called "specification refine- ment"), An implicit support for information management (the model management con- cept). T. Muth, Modeling Telecom Networks and Systems Architecture © Springer-Verlag Berlin Heidelberg 2001 2 Foundations of Sysnet Modelling This chapter gives an overview of the characteristics of sysnet modelling. We will discuss the basic features that characterize sysnet modelling, and how these features support modelling activities. We will concentrate on concepts and model- ling principles, not on notations and symbols (most symbols used in the figures of this chapter are intuitive). Application of SM, including notational details of AML, is left to later chapters. Focus on Telecom Systems and Networks In this book we use the concept telecom system to denote any type of system that provides some type of teleservice between remote users. Teleservices are any kind of communication services for voice, audio, video, fax, data and multimedia. Users of teleservices are teleservice users. Telecom systems are processing systems according to the definition of that con- cept in Sect. 1.4. What makes a telecom system unique is the type of flows between the resource system and users and the type of resources in the resource system. The separation between resource system and process control system, see Fig. 1.8, is useful to apply to providing functional models, so-called logical networks, of tele- com systems, since it represents two major tasks in designing such systems: to design the telecom devices (the resources) and to design the call and connection handling systems (two major parts of the process control system). Logical net- works are valuable if they hold for all, or at least a number of sysnet network solu- tions. We will discuss the main logical networks of telecom systems in this section. Telecom networks (i.e. sysnets) are also concrete systems, i.e. they have M/E/I boundaries on many structural levels. The most obvious of 