tiplying the stable state space of an LSM with the volatile states of the LPM that supports the LSM. The LSM-LPM separation principle is not applicable on relaying and switching layers, since actors of such layers do not interpret the data they relay or terminate, and therefore control no layer states. Thus the behavior of such a layer is described by a single machine, which is an LPM. Figure 1.20 shows the generic operations (send, terminate, relay and receive) that can be associated to layer interfaces and actor-agent interfaces of all connectivity layers. 1.4 Modeling Network Systems in AMLn receive Message terminate relay receive Fig. 1.20 The LPM of a connectivity layer The semantics of these operations are: send: an explicit or implicit request from a user to send some data to another user. receive: an explicit or implicit report to a user that some data has been received from another user. terminate: an explicit or implicit order from an actor to its agent to deliver some data to a defined user. relay: an explicit or implicit order from an agent to an actor to relay some data to a defined user. The reason for making a distinction between relay and terminate is that an agent has no business of knowing if the actor is a relaying layer element or if it is just a terminating layer element. This implies that an actor that, at one point in time, is just terminating all incoming data in layer interfaces to local users, may later be replaced by a relaying or switching element, that can relay data to remote users as well. Note that the realization of these operations depends on the type of connectivity service: when the service is asynchronous, it depends on explicit primitives in interfaces; when the service is synchronous, the primitives are implicit. 1.4.2.5 Agent Layers From these basic separation principles we can derive another, very profitable prin- ciple for modeling: to redefine an OSI RM layer, consisting of layer elements, to a structure of an agent layer that support one or several actors. Figure 1.21 s