cross-connecting systems. However, when we classify systems after the type of switched routes they offer, PSTN, ISDN, PDH, and SDH all belong to the same class of systems (i.e., circuit switching). We will study the structure of route abstractions in one of these systems here: ISDN. The ISDN switching layer operates synchronously, i.e., it realizes synchro- nous connections. These connections are used by other layers in ISDN for creating asynchronous routes (such as LAPD routes) and media routes between ISDN users (e.g. voice connections). Figure 3.25 shows an ISDN in three views. 3.4 Route Type Examples Derived network: Voice Voice source/ Service Service source/ V-NI4 (Voice "protocol", voice route) sink requester acceptor V-NI3(A-law,CS-route) V-NI3(u-law,CS-route) V-NII V-NI2 V-NII (SH,LAPD,BRA) (SH,SS7,SDH) Layers and logical networks: Voice "protocol" Voice Voice source/ Voice route source/ (Service Service requester acceptor A-law u-law T-ROs T-ROs A quasi logical-network structure: Voice Voice Media network source/ source/ Service (Service requester acceptor A-law A-law u-law u-law hosts Fig. 3.25 An ISDN modelled in three views. CS=circuit-switching; ES=an ISDN end-system; LE=an ISDN local exchange; Med.=media encoding function; CH=call-handling layer The top portion shows a network model based on derived nodes, while the mid- dle section shows the same model expanded in the layer dimension with possible logical networks indicated. The model at the bottom indicates a possible logical- network structure, extracted from the model in the middle. We included only two terminating nodes of the switching network, both of the BRA type. In reality, the 3 Node Structures network would support PRA-type terminals as well, and have thousands of termi- nating switching nodes. Some comments on these structures are needed: In the derived-network model we apply the AMLn definition of a network interface, comprising protocols and the routes on which the protocols run. All network interfaces, except those that define how u