hows this principle applied to simple layers of only. two actors. Control layers: Layer element Agent layer Connectivity layers: Agent layer Fig. 1.21 The agent layer 1 Introduction to Network System Modeling As can be expected, this principle is equally applicable on control and connec- tivity layers. What differs are the primitives that are used in actor-agent interfaces, and the functions of actors: Actors in control layers handle state machines and may serve users with control points. Actors in connectivity layers are switching, relaying, or just terminating ele- ments that may serve users with service points. When modeling, we can use this principle to separate control structures from connectivity structures. In Fig. 1.22 we have applied it on the ISDN layer structure (L dimension only). The boundary between control and connectivity structures are defined by actor-agent interfaces only, which allows us to store and handle sepa- rate model fragments for these structures. As long as actor-agent interfaces are not modified, a modification of a connectivity structure, for example, cannot affect a control structure, and vice versa. ISDN services ISDN services users requesters DSS1 L3 Call handling agent layer actor layer agent layer T-ROs Packet common agent layer handling (PH) stratum Media Media Other agent actor resources MTP L3 stratum MTP L2 agent layer X. and V. SDH path Alternative ways Switching control actor to provide actor layer agent layer, connectivity for MTP L2 mux. layer SDH cross- connect or mux actor (agent layer Fig. 1.22 By defining agent layers, a layer structure disintegrates into control structures and con- nectivity structures. This model shows the effect on the ISDN layer structure Agent layers are excellent for simplifying models of specific layers. In Fig. 1.23 we have used it to show the LPM of an ISDN call-handling layer. 1.4 Modeling Network Systems in AMLn TCAP common agent layer T-ROs DSSI L3 DSS1 L3 agent layer T-ROs ISUP agent layer SCCP MTP L3 T-ROs TE TE LE/SSP TE TE Fig. 1.23 