n. In fact, such "page references" are symptoms of an insufficient structuring approach and should never be allowed in a modeling language. 1.4 Modeling Network Systems in AMLn cannot be defined in models based on the OSI RM layer concept. These principles are limited in numbers and easy to understand (see Sects. 1.4.2.1-2.4): 1. The control-connectivity separation separates control layers from connectivity layers. 2. The actor-agent separation separates message handling elements (agents) from acting elements (actors). 3. The actor-resource separation separates resources from actors that use or con- trol them. 4. The LSM-LPM separation separates the stable states of a layer from the volatile states that are associated to message handling. Three additional principles are derived from the above: 1. Substituting an OSI RM layer with an agent layer and supported actors allows a layer structure to be divided into a control structure and one or several struc- tures of agent layers and connectivity layers. We call the latter connectivity structures. 2. Defining the common-agent layer. 3. Defining common actors. These principles are described in Sects. 1.4.2.5-2.7. 1.4.2.1 The Control-Connectivity Separation Using-layer Using layers element OSI layer OSI protocol OSI layer layer element element Providing layers Providing layer element AMLn: Connectivity Layer Transport protocol Layer layer element element Control Layer Control protocol Layer layer element element Fig. 1.14 Layer definitions in the OSI RM and in AMLn With the exception of the OSI RM "application layer" (that does not offer any layer interface at all), all OSI layers offer some type of connectivity service. We know, e.g., through the ISDN and ATM architectures, that most layers in real net- 1 Introduction to Network System Modeling works do not offer connectivity, since they deal with different forms of control (e.g., controlling service access, controlling media resources, controlling managed resources). AMLn therefore defines the difference between contr