tool. Such a tool can have an enormous impact on the development process as a whole, since it has the potential for eliminating many sources for mis- understanding and errors in the translation process, not to mention the potential for time and costs savings. Even if automatic translation to UML models is not applied, an AMLn model is an important tool for facilitating communication between all designers in the added-value process, as well as with sub-contractors and network designers. 1.3 Contributions to AMLn Contributions to AMLn AMLn has been developed over the past eight years, based on the author's experi- ences of over 20 years of systems development in the telecommunication industry. AMLn is used for modeling structural relations in network systems. Other parts of AML (AMLs and AMLp) can be used for specifying behavioral aspects of an AMLn model. These language components have been described in details in Muth (2001). A brief presentation of AMLs and AMLp is given in Appendix D in the present book. The development of AMLn has been influenced by improvements of the archi- tecture of network systems over the last 20 years. We can summarize this evolution in seven major steps: 1. OSI RM (Open Systems Interconnection Reference Model) introduced a layered functional structure for network systems in the early 1980s. As a result the con- cepts "layer," "layer element," "layer interface," and "protocol" became widely used. OSI RM also introduced an operation-oriented technique for communica- tion and behavior description (the Remote Operation Service Element concept (ROSE), supported by Abstract Syntax Notation No. 1 (ASN.1), used for typing data and other types of entities). 2. ISDN (Integrated Services Digital Network) introduced the separation of streams of user data from streams of control data in the mid-1980s. As a result layers were classified as either belonging to the "user plane" or the "control plane." 3. TCAP (Transaction Application Part) was the first layer outside the OSI RM context that used opera