s, voice recognition and answering machines, announcement machines, etc. It uses the control points for sending messages (that are stored in the actor) over a telephony connection, and for receiving information on messages that users send over such a connection. In the media layer, such messages may be trans- ported as tone signals ("dual-tone multi-frequency" (DTMF)) or as voice signals that are interpreted by a voice recognition media actor. Control structures may have any number of layers. For example, a possible improvement of the ISDN network systems is to separate the call-handling actor layer into a connection-control actor layer and a service-control actor layer²¹, the former controlling all resources for setting up and handling on-demand synchro- nous connections in the switching stratum, the latter for controlling user's requests for services and other resources, e.g., the use of media layers. Figure 2.52 shows how we can model this separation by defining additional resource control points (rCP). In this solution, the service-control-actor layer will now take care of service analysis, media handling, charging etc., while the connection-control-actor layer will take care of connection handling by performing routing and by controlling switching actors. The effect of this separation is that service-control actors no longer need to con- trol the switching actors. The ISUP protocol can therefore be reduced to service handling only (called ISUP/S in the model). The same goes for DSS1 L3 that we reduce to DSS1 L3/S. On the other hand, the connection-control-actor layer needs connectivity as well. In this example we choose to use TCAP to support the abstract protocol of this layer in the core network (which is a simple but not neces- 21 Also referred to as the "call-connection separation," frequently discussed over the years but first realized in "next generation networks" (NGN), that is now under development. 2 Layer Structures sarily ideal solution), and a protocol called DSS1 L3/C to support the abstract pro