an Ethernet, the other two over the ISDN packet access network. Router RX and RY are intercon- nected over an ATM network, using the AAL3/4 service. The Ethernet addresses are ea1 through ea3, the ISDN packet network addresses for the hosts are ia1 and ia2. The ATM addresses are aa1 through aa3 (ISDN and ATM addresses are part of the same numbering system, standardized by ITU-T in ITU-T E. 164. IP and Ether- net each have their own numbering system). 3.4 Modelling Telecom Resource Systems ISDN packet access service Router RY IP-UDT IP-UDT Ethernet AAL3/4U AAL3/ Router RX AAL3/4 aa ATM address ETU Ethernet user ipa IP address AAL3/4U ATMU ATM user ia ISDN address IPU IP user ea Ethernet address IP Internet protocol PHU ISDN packet service user Fig. 3.73 An IP network The model in Fig. 3.73 also shows the boundary of the corresponding physical nodes. Note that the physical nodes must include functionality that is defined by external transport network concepts, and that they are referred to by different addresses in the IP network and in the external transport networks. Relaying of packages (IP-PDUs) is performed by the packet-switching units in routers, based on the routing analysis that is performed by the routing analyser (RA) included in every IP router. Each packet is routed separately and, therefore, the RA is connected directly to the packet switch. To use the route concept as an abstraction of how switching elements and users are interconnected is very impor- tant in IP networks, since one does not want to make routing and relaying depend- ent on the external transport networks that are used for interconnecting hosts and routers. The previous model can (from a routing point of view) be abstracted to the route-oriented model in Fig. 3.74. Compared to Fig. 3.73, I have included two extra IP-users (ipa5 and ipa6) and two additional packet switches, PCZ and PSV (PCZ is the packet switch in router RZ, etc.). The IP network now consists of six IP users and four packet switches. If you compare this model with the c