, if a 155 Mbps ATM access link is used to connect the switch to the ATM network. Note that rerouting of links is independent between the SDH network and the ATM network. Rerouting in the SDH network can make ATM links transparent to transmission failures, congestion and physical node problems in the SDH network. Rerouting in the ATM network can make VP connections transparent to congestion and physical node problems in the ATM network. The basic switching function in an ATM network is virtual path switching. Fig- ure 3.68 indicates how an ATM router works in that mode. 3.4 Modelling Telecom Resource Systems ATM-DNCP operator router (VP-control) commands Switch controll. vpi7/ vpi1 {...} vpi7 {...} ATM transport link L1 vpi1/ vpi1/ vpi1 {...} vpi2{ vpi2/ vpi1/ vpi1{...} vpi6{...} vpi6/ ATM switch Fig. 3.68 A VP router in an ATM network Paths through an ATM switch are connected by operator commands to the ATM-DNPC. This is a configuration activity, which interconnects ATM routers and terminal systems over VP connections. Figure 3.68 shows an ATM router that has set up three VP connections: vpi1 in link L1 to path vpi7 in link L2, vpi2 in link L1 to path vpi6 in link L3 and vpi1 in link L2 to path vpi1 in link L3. Once paths are set up through the switch, cells that come in over one path on a link are always relayed to the same path on another link, no matter on which virtual circuit within the path they arrive. VP switching simplifies the routing of cells in an ATM router, since when the ATM switch receives a cell, it has only to read the VPI in order to know to which link and path to relay the cell. VP switching is intended for interconnecting ATM access routers and for inter- connecting ATM terminal systems that handle many virtual channels, e.g. the switching nodes in private networks. Figure 3.69 shows an example in an intuitive notation. ATM access ATM Transit router VP router ATM access router Terminal System switch virtual path ATM link Terminal ATM access System router Switch Fig. 3.69 Virtual paths in an 