or agent B1&B2 Fig. 2.49 Discrimination in the ISDN circuit-switching layer. B1,B2=B-channels; D=D-channel; OH=overhead bits This model is also an example where the discrimination function in a layer ele- ment may be shared between its actor and agent. The agent discriminates between D and B channels and terminates the D channel, while the actor discriminates between media layer interfaces. It terminates (and multiplexes, if required) B chan- nels. 2.3.4 Control Structures versus Connectivity Structures The distinction we have made between control and connectivity layers reveals that interfaces between layers are not just interfaces that offer connectivity. Since the essence with a control layer is that it controls resources in a network system, some layer interfaces are used for control. A type of resource to control in ISDN are the actors of connectivity layers, but there are many other types as well. Let's therefore combine the ISDN call-handling-layer LPM from Fig. 2.36 and the switching-layer LPM from Fig. 2.41 in the same model (see Fig. 2.50). The model shows how the four types of interfaces (connectivity service point, control point, SAG, and SAC) build up the functional structure of an ISDN (in the L-N plane). In this rather complex model, the different symbols for layer interfaces and actor-agent interfaces help us understand how the network works. Control points are also drawn with dotted lines in order to make the model more intelligible. Added functionality in this model are a number of strata?0 (LAPD and the SS7 con- 2.3 Agents and Actors nectivity strata) that are needed in order to provide the call-handling layer with asynchronous connectivity (since the switching layer is not a suitable datacom net- work). In addition, media layers that use service points of switching layer actors are indicated. Another possible layer that relies on service points is a packet-han- dling layer (not shown in Fig. 2.50, however). Call-handling- TCAP common layer LPM agent layer ACSCP DSS1 L3 T-ROs DSS1 L3 ACSSP agen