ost of the other routes are based on the SCCP of CCS, where each logical node of concern has an SCCP address (ISUP is often based on MTP). 5.2 An Example: The GSM Process Control System External networks ISUP, etc. External SMS-C networks Manage- system (OSS) BSSAP(DTAP) BSSAP(BSSMAP) LAPDm (BCF) Reference point based on an SCCP route Reference point based on several routes Reference point based on an MTP route Reference point based on a node platform route Auxiliary reference point Fig. 5.4 GSM process control system (GSM-PCS) logical network The SCCP address is composed by an MTP address (called point code, PC) and an additional SCCP address (called subsystem number, SSN). This addressing solution allows physical nodes to comprise more than one type of logical node, see Fig. 5.5. MAP+TCAP mess. SSN SDU destDPC MTP(a) MTP(b) Fig. 5.5 Addressing logical nodes of the GSM-PCS A PC identifies a particular physical node in the CCS network, while an SSN identifies a particular logical node in the physical node that uses TCAP. MTP mes- sages contain a destination point code (destDPC) and an MTP-user identification called service indicator (SI). If the destDPC is not the PC of the MTP- ode, it routes the message to some other physical node. If the destDPC is the PC of the receiving MTP and the SI is SCCP, the MTP-SDU is delivered to the SCCP (note, 5 Protocol Network Modelling however, that not all physical nodes have SCCP, and that MTP also services nodes other than just SCCP). The SCCP-PDU contains an SSN parameter, which identi- fies the logical node of the process control system. Figure 5.5 shows a received SCCP-message that identifies the EIR node. The SCCP-SDU is delivered to that node. An alternative to this way of addressing logical nodes is to give each logical node that runs on SCCP a global title, GT, which is a unique identity within the network. GT substitutes for the destDPC and the SSN. When GTs are used, SCCP must be able to perform address translation, i.e. translate a GT into a destDPC and a SSN. MTP a