to- col network identifier for static discrimination between data that belong to the "protocol networks" DTAP and BSSMAP, respectively. The protocols that are specified in GSM for the GSM-PCS are annotated in Fig. 5.4. Four protocol specifications exist: BTSM, BSSAP, MAP over TCAP and ISUP. If one does not specify the protocols of protocol networks separately, the next level of specification is obviously protocols over types of reference points. With the exception of ISUP, the GSM protocol specifications are, however, not uniquely related to reference points. BTSM describes communication over two ref- 5.2 An Example: The GSM Process Control System erence points, as does BSSAP. Figure 5.4 shows that there are nine different refer- ence points between MAP logical nodes. MAP is described in a single protocol specification. The MAP protocol is an abstract protocol, since MAP logical nodes communicate by remote operation calls, supported by TCAP. TCAP creates the messages that carry MAP operation calls and responses. Each reference point defines its own set of operation calls. possible method to discriminate between reference points was to use a reference point identifier (RPid) in TCAP messages, as modelled in Fig. 5.12. AP(MSC-SMS) operation request in reference point MSC-SMS SMSid MSCid paramet. caller called Other MSCid Other userData destPC origPC userData Fig. 5.12 Possible method for separating MAP reference points Figure 5.12 shows how logical nodes that run on TCAP could address each other. TCAP also supports logical nodes other than those defined by MAP, such as INAP, which is used between service switching points (SSF) and service control points (SCF) in the IN concepts. All TCAP-logical nodes can have a unique identi- fier, consisting of the type of logical node and an individual suffix (there may exist many MSCs, HLR, SMSs, etc. in a GSM-PCS). A reference point is identified by such an identifier in each endpoint (e.g. MSCid). TCAP translates between the RPid and the SCCP-address, which is either a global