called Service Access Handling (SAC). This mechanism is distributed over MSs and MSCs. The protocol between MS and MSC is DTAP (Direct Transfer Application Part, specified as a part of BSSAP, BSS Application Part). DTAP supports all types of service accesses from MSs, including forced registrations. SAC can be modelled as a separate protocol network, see Fig. 5.9. The SAC protocol network is a complex type of integration mechanism. It acts on behalf of the service handling network (SH) in registration (REG), authentica- tion (AC) and equipment control (EIC) procedures. This is because these functions almost always must be used before the SH accepts any service request. It acts as a distributed service access agent in the SH network. It is also an integrator for all those protocol networks by providing them with a message-handling functionality. SAC uses different means for message transport in MS and MSC: in MS it uses a LAPDm route over a radio access control channel (an SDCCH). In MSC it runs on 5.2 An Example: The GSM Process Control System the discrimination mechanism in BSSAP, which uses an SCCP connection between MSC and BSC. The BSSAP discrimination mechanism separates BSSMAP mes- sages from DTAP messages, see Fig. 5.4 (BSSMAP, BSS Management Application Part, is also part of BSSAP). Fig. 5.9 The service access handling (SAC) protocol network In standardization activities, the protocols that can be defined for protocol net- works are seldom specified as protocols of their own. Normally all protocols defined over a specific type of reference point (see Fig. 5.4) are described in a sin- gle protocol specification. Figure 5.10 provides two examples of this type of aggre- gation. It shows the protocol for the reference points between the logical nodes VLR and HLR, and between BSC and MS in Fig. 5.4. Some types of Some types of Part of BTMS BTMS messages MAP_over_TCAP Part of MAP messages Fig. 5.10 Protocol networks in reference points The reason that the protocols of protocol networks are normally aggregated i