 come in two 1.4 Modeling Network Systems in AMLn flavours: specified agent interface (SAG) in control layers, and specified actor interface (SAC) in connectivity layers (see Fig. 1.15). The difference between SAGs and SACs is how interactions between agents and actors are defined (see Appendix B). If you look at implementors' network system models, you will find that actors are commonly separated from agents. Standards do not apply the actor-agent sepa- ration, however. It should be specified already in the standard view of a network system, which is why it must be part of AMLn. Figure 1.15 shows that this principle is applicable on any type of layer. There is no real differences when it comes to what the agents do and how message exchange works. This is not SO where actors are concerned, however: When an actor of a connectivity layer receives some data from its agent, it can only choose between two activities: either it decides to relay the data to another agent, or it decides to terminate it in a user layer element. In no case, however, does the actor interpret the data. When an actor of a control layer receives some data from its agent, it interprets the data and chooses an action that is relevant to the received information. This may or may not result in a communication with a using layer element. A consequence of this approach is that while actors of connectivity layers have no states (at least not lasting states), actors of connectivity layers have. As a result of received information, a control-layer actor may affect resources in the layer, which changes the state of the layer for a shorter or longer period. It is important to model the distinction between these two types of actors, which is why we use slightly different symbols for actor-agent interfaces. We also use different names (SAC and SAG) that associate certain (generic) AMLn primitives to those interfaces, as was mentioned before. 1.4.2.3 The Actor-Resource Separation The OSI RM has no notion of resources that a layer needs for fulfilling its ta