 Still, we must define the connectivity structure in this logical network. We can do that using the route concept, see Fig. 3.96. Network operator X.25/IP e.g. IP address X.25/IP X.25/IP INAP/ INAP/ controller Q.931 INAP/TCAP INAP/ Q.931 global title user-user messages Fig. 3.96 Using routes for simplifying logical network models 3.5 Logical Network Structures The model is heavily annotated. A suitable modelling tool hides the annotations the user doesn't want to see. Previously, we used routes to describe the connectivity structure of resource sys- tem models, but we can also use the symbols for process control models. Note that the model structure is built by three types of routes: SCCP routes, LAPD routes and X.25 or IP routes. Route endpoints define the address or identification to the other party in the route. Identification of a LAPD route is SAPIO in both ends. Endpoints in a SCCP route can be identified with a global title. Logical nodes may have names of this type. The SCCP translates that name to an address that can be used inside the SCCP-CSP for routing (on the MTP level). A less flexi- ble addressing variant in SCCP is that the destination address defines both the MTP address (called destination point code, DPC) and a prefix (called Subsys- tem Number, SSN). The prefix identifies a particular node within the connectiv- ity domain that is identified by a DCP. In this model, both types of addressing are used. Route endpoints in IP or X.25 routes use IP or X.25 addresses (SAPIs) to iden- tify the other party. The model is still rather complex. Since every route is used for a specific proto- col, it seems reasonable to further abstract the relations between nodes with a sym- bol that aggregates both the route and the protocol. This symbol is the reference point symbol, see Fig. 3.97. Each type of reference point must be specified, as shown in the model. The modelling tool is expected to allow the user to select any part of the route specification he would like to visualize in the model. RP3 definition: N