, network systems), i.e., on the knowledge of what this type of systems do, what their functions are, and how to combine them into an architecture that is open for its evolution over a long time. In other words, an added-value lan- guage is based on, and preserves the intellectual capital of a system domain. Since mapping an added-value model on a common-value model means that the seman- tics and architecture of the added-value system gets completely (and deliberately) lost, creating and maintaining added-value models is extremely important to any system-developing enterprise. The added-value modeling language that is presented in this book is called AMLn (Abstract Modeling Language, network view). To my knowledge, it is the first and, SO far, the only attempt to create a modeling language for network sys- tems. The background to AMLn is the enormous complexity and rapid evolution of today's network systems, caused by the introduction of new technologies in elec- tronics, software, and transmission media over the last 25 years. As a result, new network systems appear in an ever increasing pace. One would expect that, over these years, well-proven and efficient architectural principles have been identified and commonly applied. On the contrary, only a single attempt in that direction has been made: the OSI RM (Open Systems Interconnection Reference Model), pub- lished in 1984. This generic model exhibits such a set of general architectural prin- ciples. In spite of the fact that the OSI RM was only a first attempt in the right direction and was constrained to experiences from a small class of pre-standard datacom networks (e.g., IBM's SNA, DEC's DNA, DARPA's Arpanet), the indus- try took it to its heart with 100% consensus, and with great enthusiasm. In retro- spect, we can conclude that this was more an effect of a fear of losing control of the evolution than of a serious evaluation of the quality and applicability of the OSI Unfortunately, the industry still has not pursued such an evaluation. Even though the id