ATM network 3 Logical Network Modelling The ATM network is configured SO that all ATM access nodes are fully intercon- nected with several VP connections. VP connections between terminal Systems that must be permanently interconnected are established by interconnecting paths in ATM access switches. Terminal systems that are interconnected over VP con- nections can use their virtual channels at will, since the ATM network is not sensi- tive to when or how the virtual channels within a path are used. However, this is not the case for users of virtual channels on demand. In that case, virtual channels must be established when they are needed and released when they are no longer, similar to any other connection-oriented services. Requesting virtual channels is done from terminal systems and requires that switches in access routers can also switch virtual channels. Figure 3.70 shows a scenario for connect- ing two virtual channels between link L1 and L2. The sequence diagram shows how the VC connection L1/vpi2/vci124-L2/vpi6/vci13 is set up through the switch. The request for setting up a VC connection comes from the access side in this scenario. A short description of the setup procedure follows. Note that release is not covered by this scenario. Neither are the correct names of the Q.2931 and B-ISUP messages used: 1. To a set up a virtual channel is a two-stage process (more or less similar to how mobile systems access cellular networks). In order to save resources, users do not have a permanent channel for process control data (as the D-channel in ISDN). When a user on link L1 wants a VC connection, the TPC in his terminal system must first send a service request for a channel for process control data. All such requests use the dedicated channel vci5 in vpi0. 2. The ATM-DNCP selects another virtual channel, in this scenario vc53 in vpi0, and orders the initiating TPC to use this channel in further communication. This VC connection is dedicated to the TPC until the VC connection between the ATMUs is set up. 3. The T