red to in the protocols of the supporting CSP (by global identifi- ers). Thus, the port concept is what allows us to distinguish between local identifiers (used only within a physical node) and global identifiers (used only between physical nodes) in, e.g. CON connections, as shown in Fig. 3.16. User- User- connection endpoint connection endpoint identifier Connection Packet-switched CON-CSP CON-UDT network CON-UDT connection identifier Fig. 3.16 How local and global identifiers for CON connections are used 3 Logical Network Modelling Figure 3.16 says that the CEI is a local identifier used by users of the CON serv- ice to refer a logical connection in their communication with other user-UDTs. The global identifier used in protocols inside the CSP is a connection identifier. Most protocols do not, however, use the OSI terminology. They often regard the connec- tion identifier as representing a virtual circuit or a virtual channel, and call it vir- tual circuit identifier (X.25) or virtual channel identifier (ATM8). The network service interface for connection-less packet-switched connectiv- ity is very simple and resembles the original OSI service provider, see Fig. 3.17. User protocol a SAPI b SAPI CNL-CSP Fig. 3.17 The CSP model for connection-less packet services In this case, there are no CSIs, since the SAP is ready for use without prior ordering. Each SAP has an address that is referred to in sending and receiving oper- ations. IP and the MTP service in CCS are examples of CNL services. Most node platforms also offer some kind of CNL service to applications. The ASPs that are used for CNL services have another set of parameters. In CNL services there are no connections, and consequently also no CEIs. Instead the CNL user (CNLU) must insert the destination-SAPI as part of the event. Figure 3.18 shows some send-events that may occur over a CNL-SAP, and how receivers are referred in those events. sendCnlOrder(a SAPI, d1 SDU) b SAPI sendCnlOrder(c) SAPI, d2 SDU) d1 to SAPI=a d2 to SAPI=c CNL-CSP Fig. 3.18 Sendi