Fig. 3.94 shows, in a topological view (visualiza- tion mode is flat mode), the logical network in ISDN that handles teleservices that are based on circuit-switched connections (B-channels). SAF Service access functions Management System SCF Service control functions SDF Service data functions Network SMF Service management functions X.25 or IP operator SRF Special resources SSF Service switching functions STF Service transit functions SWC Switch controller INAP/ controller INAP/ ISUP, Q.931 LAPD SAPIO CCS (MTP&SCCP) user-user messages LAPD SAPIO Fig. 3.94 The ISDN process control system, partitioned according to the IN concept Only SAPIO of LAPD is needed for service access in this case. The model shows only one or a few nodes of each type. In relation to previous models of ISDN, a number of IN nodes have been included. The reason for including this extra complexity in the model is partly because it reflects the actual complexity of logical networks we must handle, partly to demonstrate the need for a simplifying 3.5 Logical Network Structures notation for logical networks. For the reader who is new to the IN concept, a short description follows (the acronyms SAF, STF and SMF are not official IN names; I invented them here for pedagogical reasons). All logical nodes, except the TPC and the SMF, are parts of the ISDN network process controller. Only SAFs, STFs and SSF exist in physical nodes that can switch calls. SAF is the distributed network process controller (DNPC) in a physical node that handles accesses (often called local exchange). STF is the DNPC in a physical node that does not handle accesses (often called transit exchange). SAFs and STFs handle those teleservice aspects that are distributed to every local and transit exchange. They do not handle centralized teleservice aspects (i.e. IN-functions). A few switching nodes in a network have functions for accessing IN functions. These are the SSFs. When a SAF sees that a call must be handled by IN func- tions, the call is routed to the nearest SSF. Whethe