echanism for BSSAP is connection-oriented and connection-less message transport, provided by SCCP. The transport mechanism for BTSM is connection-oriented message transport, provided by LAPD. However, the effectiveness of integrating all kinds of network functions in a low-level document as a protocol specification can be questioned. By not making the RPid part of TCAP messages, TCAP users must know the SCCP addresses of all their partners. If TCAP had supported a PNid, it would have further facilitated the mapping of independent network functions on protocol solutions. 5.2.3 Summary The integration mechanism of logical nodes is the key not only to the internal structure of service elements in logical nodes, but also to how service elements can communicate (i.e., by operation calls, messages, etc.). Each logical node must implement some type of integration mechanism. This mechanism is generally mod- elled as a separate protocol element, called integrator. Seen over a particular logical network, all logical nodes may either implement the same type of integrator, or use different types of integration in different parts of the network. For example, if we look at the GSM-PCS logical network model in Fig. 5.4, different integration mechanisms are used in the core network and in the access network. Most refer- ence points in the core network rely on TCAP as an integration mechanism. Refer- ence points in the access network rely on different mechanisms, one of which is discrimination between BSSMAP and DTAP over the MSC-BSC reference point. Thus, the MSC logical node must implement several different integrators, while, 5.3 Discrimination e.g. the VLR, SMS-GMCS, HLR, EIR and AUC logical nodes can rely on TCAP only (disregarding reference points to the management system). In the following sections, we intend to look more systematically at how to implement the integration functions of logical networks. Comparing different net- work concepts reveals that there are many different solutions to integration mecha- nisms. This d