nd SCCP are designed to support flexible allocation of logical nodes in the physical network. Figure 5.5 shows that a physical node may include a number of nodes of the GSM-PCS, or just one. Typically, product developers use this flex- ibility to provide physical nodes (i.e. products) that consist of MSC and HLR logi- cal nodes in the same physical node, as well as physical nodes that consist of an HLR logical node only. When the logical nodes exist in different physical nodes, they use the defined message protocol (here MAP over TCAP) for communication. Communication between co-located logical nodes can, however, be implemented by using the communication services of the node platform instead of SCCP. Figure 5.6 shows an example of how co-location affects reference points in logical net- works. Node platform CSP (NPC) RP definition is: Route type is: SCCP Protocol is: MAP(HLR-SMS) on TCAP Route id. is: 11-6 (SSN) RP definition is: RP definition is: RP definition is: Route type is: NPC Route type is: NPC Route type is: SCCP Protocol is: MAP(HLR-VLR), Protocol is: MAP(VLR-MSC), Protocol is: MAP(MSC-SMS) local message handling local message handling on TCAP Route id. is: BN-BN2 Route id. is: BN3-BN4 Route id. is: 11-8 (SSN) Fig. 5.6 The effect of co-location on reference points 5.2 An Example: The GSM Process Control System TCAP is an integrator that can be common for a number of protocols that are based on SCCP. In case of the GSM-PCS, these protocols are all defined by the MAP protocol. In Fig. 5.6, the logical nodes HLR, VLR and MSC are co-located, while the logical node SMS is contained in a physical node of its own. The SCCP network identities of HLR, VLR, MSC and SMS are SSNs 6, 7, 8 and 11, respec- tively. TCAP, HLR, VLR and MSC are also separate applications in the physical node to the left, running on the node platform (here called NPC). The NPC addresses of TCAP, HLR, VLR and MSC are BN1 through BN4. Thus, HLR, VLR and MSC have two addresses each: one for external access, one for internal. The way logical 