t layer ISUP agent layer T-ROs SCCP MTP 1.3 T-ROs (SS7 connectivity MTP L3. stratum stratum SCCP) stratum Media Media Media layers layers layers BRA or PRA Media PDH agent layers agent layer layer PDII transmission BRA or PRA transmission BRA or PRA agent layer transmission Media Legend: layers transmission Control points BRA or PRA transmission layer Service points Switching layer LPM SDH transmission ES ES LE/SSP Fig. 2.50 The layer structure of an ISDN. L=layer dimension; N=node dimension We can reduce the complexity of this model by collapsing the N dimension and showing control structures and connectivity structures separately, interconnected over types of actor-agent interfaces, as in Fig. 2.51. This implies that the L dimen- sion dissolves into an Lac dimension for the control structure and four Lco dimen- sions for connectivity structures. Except for their mutual dependence on actor- agent interfaces and on the switching layer, these structures are independent, also as to how many layers they comprise. Since the N dimension is collapsed, this model is valid for every ISDN network. 20 In AMLn, a stratum is a structure or an aggregate of layers. 2 Layer Structures ISDN services ISDN services users requesters (user data) DSS1 L3 Call-handling- agent layer (DSS1) actor layer agent layer (ISUP/TCAP) 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. Alternative ways Switching to provide actor layer agent layer SDH path connectivity control actor for MTP L2 multiplexing layer SDH cross- connect or mul (agent layer tiplexing actor Fig. 2.51 The functional structure of the ISDN network system, separated into its control structure and its connectivity structures. SAC=specified actor interface; SAG=specified agent interface In an AMLn model of the ISDN network system, a layer interface realizes either a control point or a service point, and the structures behind these types of interfaces are separated (corresponding to the "user