c modelling languages is such that, even in such a large and established domain as telecom systems, commonly agreed upon languages for systems modelling do not exist. An early effort to provide a system of concepts for the domain of data communication networks was the OSI Refer- ence Model (OSI RM). OSI stands for Open Systems Interconnection, and the ref- erence model was developed and first published by the International Telecommunication Union (ITU, former CCITT) in the early 1980s. The model is described in the ITU-T recommendation X.200. In spite of good intentions, the model failed as a guide for modelling, most likely because of lack of involvement from the experts in the telecommunication domain. Since then the Telecommunica- tion sector of the International Telecommunication Union (ITU-T) has standard- ized a number of languages for supporting the development of telecom systems: 1 Introduction to Systems Modelling The Message Sequence Charts language (MSC). MSC is a graphical language described by ITU-T in recommendation Z.120. A Message Sequence Chart shows chronological sequences of messages sent between system components. The Abstract Syntax Notation One (ASN.1). ASN.1 is a generic notation described by ITU-T in recommendation X.208. It is used for describing mes- sages, data types and values. It is frequently used in various data and telecom protocols and applications. The Tree and Tabular Combined Notation (TTCN). TTCN is described by ITU- T in recommendation X.292. TTCN is a notation for the specification of tests for communication systems, and has been developed within the framework of standardized conformance testing. The Specification and Description Language (SDL). SDL is described by ITU-T in recommendation Z.100. The development of SDL started 20 years ago. The language is promoted as a language "for specification of complex, event-driven, real-time and interactive applications". This means that SDL adds a state-ori- ented behaviour description technique to MSC and ASN.1. MSC, ASN.1 and SDL a