parate access transport system for the service access protocol (as the SAPI 0 in ISDN). The messages of the PSTN service access protocol are sent in the same channel as the one used for the teleservice protocols. Thus, the ATN for PSTN is much sim- pler than the ATN for ISDN. 3.3.2 Packet Switched Services The logical node structure in a Terminal System that relies on packet switching looks quite different. Figure 3.31 shows the logical node structure of an ISDN Ter- minal System that provides a packet-switched, connection-oriented data communi- cation service, based on 64-kbps CN connections over the ATN. Terminal System A CON-CSP CON-SAP (64 kbps Teleservice Access CS Network PS-data) Protocol Process Control Packet data System protocol, L3 CON-UDT Network (PS data) Access Node Packet data (x.25,IP) {64 kbps} protocol, L2 SAPI 0 Fig. 3.31 The logical node structure of an ISDN Terminal System that provides packet-switched, connection-oriented data services over ISDN 64-kbps connections A short explanation of the model in Fig. 3.31 follows: Since the services provided by the CON-UDT are not defined as part of ISDN teleservices, we call users of CON data services CONU (instead of TEU). 3 Logical Network Modelling An ISDN-CSP does not provide CON services, only access to CON networks (X.25 or IP networks). The function that handles CON services in the terminal system is an UDT that terminates a CON data protocol and handles the CON connections for users. This UDT is called CON-UDT. The CON-UDT is, in this case, established over an ATN 64-kbps CN connection, as any other teleservice. The CON packet data protocol comprises two protocol levels, normally referred to as layer 2 (packet link level) and layer 3 (packet network level) protocols. The layer 2 protocol may be terminated in a packet handler (PH) inside the ISDN- CSP (for concentration purposes), but the layer 3 protocol is terminated in an external packet-handling network. Since the CON-UDT provides a connection-oriented service, it interfaces the CONU with a 