he access side is done by the transport unit CN-UDT, which distributes user data over the ATDN to the circuit switch and process control data to the LAPD system (which in OSI terminology is a link layer). LAPD adds discrimination between dif- ferent functions within the process control system (SAPI 0 for teleservice control, SAPI 16 for packet access service control, SAPI 63 for OM, etc.). Thus, LAPD is 3 Logical Network Modelling specifically designed for supporting the network in providing different kinds of services within the same network. On the network side, discrimination is performed using special channels in core network links for transporting CCS messages. These channels are semipermanently switched in the 64K CS switch to the CCS node. Note that the same core transport links are used for both CCS messages and user data. The model in Fig. 3.54 shows the routing analyser as a unit separated from the process control unit, the TE-DNCP (which corresponds to a unit in the OSI net- work layer). The rationale for this is not obvious from the ISDN point of view. It is, however, justified from a modelling point of view by the fact that routing analysis must also be done in connection-less networks, which have no network layer, see Sect. 3.4.3. Another reason is the distinction that can be made between teleservice process control and connection process control, sometimes called the "call-con- nection separation", see Sect. 3.5. PSTN networks are also circuit switched. The difference from ISDN networks is how discrimination is performed, and that there is no separate transport system for process control messages on the access side. Thus, PSTN networks are not designed for providing multiple services. The model of a PSTN access and switch- ing node looks quite similar to ISDN. The differences lie primarily on the access side, see Fig. 3.55. Switching PSTN access and switching node nodes Network TE-DNCP {SWC} {SWC} Terminal System Switch Switch Routing controller controller analyser Discrimination Transmission PSTN-