telecom resource controller (defined by AML) transport service user (defined by AML) terminal system (defined by AML) tree and tabular combined notation (specified by ITU-T) telephony user part (part of CCS) user data termination (defined by AML) user datagram protocol (part of Internet) unified modelling language (specified by OMG) UML-RT UML-real-time user network interface virtual channel (defined in ATM) virtual channel identification (defined in ATM) virtual circuit number (defined in X.25) visitors location register (part of PLMN) virtual path (defined in ATM) virtual path identification (defined in ATM) wide area network WCDMA wideband CDMA 1 Introduction to Systems Modelling Engineering is about designing system solutions that fulfil requirements for what we expect the system to do. Systems modelling is an essential part of the concep- tual tools we use for visualizing possible solutions, be it just in our minds, on a white board, on paper, or with the aid of a computerized tool. We need models to understand the complexity of systems, to validate that the system will satisfy requirements and to trust that the system will work, long before it is implemented. We might also use system models to support the design, the implementation and the testing of components of our systems. In that case, we need formal modelling languages to use the computer to help us translate the system model into the soft- ware and hardware components we need to build real systems. The modelling method described in this book is called sysnet modelling (SM). The term sysnet is used instead of "system" to emphasize that SM is oriented towards modelling systems that have (or may have) a network structure (spatial, physical and functional). This also implies that any network can be regarded as a single system. Sysnet modelling uses a modelling language called the Abstract sys- tems Modelling Language (AML). This chapter discusses some basic concerns that constitute a framework for sys- tems modelling in general, as well as how these conc