nodes are located in physical nodes affects the reference point specifications, as shown in the lower part of Fig. 5.6. The network service inter- faces to TCAP are realized by protocols over the NPC routes BN1-BN2, BN1 BN3 and BN1-BN4 respectively. The MAP protocols between HLR-VLR and VLR-MSC are implemented by local message-handling mechanisms over the NPC route BN2-BN3, BN3-BN4 respectively. Note that these protocols neither use TCAP nor SCCP and MTP. The MAP protocols used for communication with SMS, however, use TCAP and are implemented over SCCP routes 11-6 and 11-8 (subsystem numbers). 5.2.2 GSM Protocol Networks The logical node structure shown in Fig. 5.4 tells almost nothing about the func- tions that are performed by the modelled system. The structure is based on high- level architectural decisions, like: Where to control physical access and detect the need for handover (in BSC), Where to control MS service access and service provisioning (in MSC), Where to register the location and activity status of MSs (in VLR and HLR), Where to store permanent subscriber data n HLR), Where to store temporary subscriber data (in VLR), Where to control cooperation with external networks (in GMSC for circuit- switched connectivity services, SMS-GMCS for short-message services), Where to control the GSM-PCS (part of the OSS). By partitioning the GSM-PCS logical network into a set of cooperating protocol networks, we can define protocol networks that perform specific network functions as services to other protocol networks. To identify suitable protocol networks requires some experience of the type of network one is dealing with. One way to acquire that knowledge is to browse through the standards of concern. More detailed knowledge is given by protocol specifications within the standards, as e.g. the MAP, BSSMAP and DTAP protocols in case of GSM. Figure 5.7 provides some of the protocol networks of the GSM-PCS logical network that I have identi- fied from the GSM standard (note, however, that standards in general, and