The scenario for establishing access to the CON network is almost the same as for any other teleservice. The CONU requests, e.g. the service "X.25 access" from its TEC. A difference, though, is that the user need not provide a SAPI if the ISDN-CSP includes a packet handler for access to an X.25 network. Still the user cannot use the packet services until a CON connection is estab- lished through the CON network. This is done by the CON-UDT in communica- tion with the CON network over the layer 3 signalling protocol, see Fig. 3.32. A CON-CSP Signalling Network Process CON-TPC protocol, L3 Controller for VC Discrimination Discriminator protocol, L3 Discriminator VC-switch L2-connection CON-UDT CON network Fig. 3.32 The structure of a CON-UDT that provides connection-oriented (CON) data communi- cation over a link layer connection The model shows that there is also a process control system also in logical net- works that provide connection-oriented packet services. The model divides the layer 3 protocol into a signalling protocol component (handled by process control- lers) and a discrimination protocol component (handled by a discriminator). The 3.3 Network Service Interface Scenarios discrimination protocol component tells the receiving party if the data in the mes- sage belongs to the signalling protocol (and therefore has to be terminated in the process controller), or is CONU data that will be relayed through the VC switch. The layer 2 protocol is of no interest in this scenario, and is therefore substituted with a link layer connection (L2 connection). Note that the model in Fig. 3.32 resembles the X.25 architecture (ATM has a slightly different approach). The difference from CSPs that provide CN connections is primarily that control information and user data are both sent over the same (L2)connection, and that the operator network uses special CON switches (called virtual circuit switch, VC switch). The discriminator on the network side provides the VC switch a SAP with subflows, each subflow identified by a 