e present context. A relaying function handles no message protocol. It only takes bits of an inlet and relays them to an outlet. A relaying function may take bit streams from several inlets and relay them to a common outlet (which means it performs a multiplexing function). This still does not change anything in the individual streams, however, i.e., the multiplexing function does not handle any message protocol. This implies that agents (i.e., elements that can communicate by messages) must be connected to inlets and outlets of relaying functions in order to create an OSI-layer element. Consequently, a relaying function is an actor, and its inlets and outlets must there- fore be defined as actor-agent interfaces. Using OSI layers to identify where in a layer structure relaying and terminating elements exist has always been a definition problem. OSI layers normally make these functions parts of a "network layer," i.e., a layer that performs some type of connection control and is defined by one or several protocols. This principle does not work for connection-oriented networks, such as circuit-switching and virtual- circuit-switching networks, since layers that control switching are definitely sepa- rated from layers that perform switching in these networks. Therefore, in order to create a generally valid principle for modeling in AMLn, we define any layer ele- ment that contains a relaying/terminating actor and connects to agents that handle 2 Layer Structures the protocols of this element as a "connectivity layer element", no matter how the relaying/terminating process is controlled, i.e., control of relaying/terminating is always modeled as a separate actor or layer. If we exclude functions such as transcoding and regeneration, all relaying ele- ments are common actors. Such an actor offers one or a few types of specified actor interfaces (SACs) to agents, and connects an often large number of identical or different types of agents. Note that a common actor defines the primitives of the actor-agent interface, c