ks of the corresponding physical network) that we have a stratified logical network structure. What characterizes such structures from other logical network structures is that each stratum has its own resource system and process control system. Thus, the logical network in Fig. 3.64 shows the resource system in an X.25 stratum. Stratification is manifested by recursive link levels (X.25 links and TR links). 3 Logical Network Modelling route endpoint switch route endpoint identifier switch switch switch X.25 links CON-PDUs Disc PDUs X.25 Disc X.25 Disc address X.25 link SAP LAPBU LAPBU translation LAPB-PDUs TR-SAP TR link TR-CSP Fig. 3.64 The route concept applied on X.25 networks As a consequence of stratification, address translation becomes necessary. In case of the model in Fig. 3.64, address translation works as follows: Address translation must be performed between X.25 link SAPIs (A1, A2, B1, B2) and TR-connections (connection endpoint identifiers x1, x2, y1, y2 in TR- SAPIs X and Y). Routing analysis in the X.25 stratum is performed by the X.25 routing analyser, based on called address. The routing analyser suggest a route endpoint identi- fier (rei1, rei2). Note that these endpoints are only known inside the X.25 stra- Each X.25 route identifies one or several X.25 links (identified by X.25 link SAPIs), which also are known only inside that stratum. The X.25 route manager selects an X.25 link SAPI within the X.25 route, which identifies a particular X.25-UDT. The route manager also selects an idle VCN for that link. Each LAPB unit is semipermanently instantiated on a TR-SAP. The TR link may be based on one or several TR connections, depending only on the expected traffic load over the TR-SAP. If the TR link has multiple connections, a LAPB unit refers a TR connection endpoint identifier (x1, x2, y1, y2) when sending, and gets data over the TR-SAP with reference to TR-CEIs as well. 3.4 Modelling Telecom Resource Systems 3.4.5 Resources in ATM Networks Another well-known CON network is ATM. The architecture