than the one for which it was initially defined is often ambiguous, sometimes misunderstood, and even misleading. In the software system domain, the industry has managed to achieve consensus on a reasonably small common set of concepts just over a couple of dec- 1.4 Modeling Network Systems in AMLn ades. It is therefore astonishing that in the network system domain, which has been developing over 100 years now, there still exists no consensus on concepts, to the extent that they can just be chosen for a modeling language. One of the main stand- ardization bodies for network systems (ITU-T) tried for many years to find a com- mon terminology by publishing a steadily increasing set of the thousands of terms that were used in standards. However, ITU-T gave up some years ago when it found that almost every term had two or more contradicting and/or overlapping definitions. This fact has been a major challenge when developing AMLn. Modeling Network Systems in AMLn In this overview we describe briefly how AMLn deals with modeling the structure of network systems, how an AMLn model may be viewed, the AMLn concepts and notations, and how behavior of a model is specified. As an added-value language to be used for modeling systems that are often extremely complex, AMLn must deal with the subject of how to structure a net- work system model very seriously. This issue is not a matter of defining a structure of documents (as we have commented on previously), but to define a structure that is in line with a sound architecture of network systems. 1.4.1 Structures in Network System Models 1.4.1.1 Layer Structure and Node Structure The term "network system" indicates that such a system is something more than just a number of related functional elements or objects. It is also a system that has a structure of interconnected elements that we call logical nodes. These two aspects of a network system, which we call layer structure and node structure, are com- pletely separated from a specification point of view. A layer structure (a re