r an IN function is needed to handle a call is detected through the destination address the user provides, as e.g. a 800-number. The functions for handling centralized services are performed by SCFs. A phys- ical network might have one or several SCFs. The data needed for the IN func- tions may be stored in separate network databases, called SDF. Typical IN services are those services that are accessed by the user by communi- cating with the network through tone-signalling (DTMF signalling) in the voice channel (over the B-channel in ISDN). The system includes special resources (SRFs) that are connected to the switch in physical nodes where the SSFs reside (these resources are modelled by the resource system logical network model). SRFs are primarily advanced announcement machines. The SCF interprets user signals that have been detected by an SRF and decides the response the SRF must give to the user. The SSF and the SRF controller act as agent between the SCF and the SRF for commands and reports between SCF and SRF. SMF is the part of the management system that manages the service logic in SCFs and special IN subscriber data (in SCF or in SDP). The abstract protocol INAP defines how IN nodes communicate. INAP uses TCAP for message handling. Communication between switching nodes follows ISUP as usual. The complexity of the model is frightening, in spite of the fact that I dropped most of the interfaces between the nodes and different kinds of resource control- lers. One useful simplification of the model is to visualize it in an overlay mode. Figure 3.95 shows the traffic system part of the previous model in overlay mode. 3 Logical Network Modelling controller INAP/ INAP/ INAP/ (MTP&SCCP) LAPD SAPIO INAP/ user-user Q.931 messages LAPD SAPIO Fig. 3.95 Visualization in overlay mode This is a reasonable view of how the process control system of the traffic system depends on external transport networks. A further simplification omits these net- works completely, since they do not belong to this logical network model.