Connections are set up between connection endpoints. A connection between teleservice elements (TE-UDT and TR) is created by creating switch paths, and by allocating connection endpoints to TE-UDTs and TRs inside CNUs. A switch path is created by interconnecting positions in switches. A normal scenario follows: a connection between TE-UDTs, e.g. between CNU1 and CNU4, is set up by allocating one of the channels in each of the corre- sponding access routes, and one of the channels in the switch route between SW1 and SW2. The corresponding channels are through-connected in the two switches, which establishes the connection between the TE-UDTs in CNU1 and CNU4. The scenario differs somewhat if teleservice resources (TR) are required, which is the case if, e.g. one of the CNUs terminated digital channels, the other an analogue channel. The described scenario considerably simplifies the real process. A number of problems can arise during the establishment process: users might be barred for dif- ferent reasons. All channels in the switch route might be busy. The switches might be congested. Routes and switches might be blocked due to faults. In addition, if an internal teleservice resource is needed, no such resource might be available. It is the responsibility of the PCS to handle not only normal scenarios, but these other situations as well. Considering that the model in Fig. 6.5 represents a network that could be extended with more switches and routes, the model in Fig. 6.6 shows a more generic model of which the previous model is an instance. The model is restricted to the connection resources since we are going to treat those resources and teleserv- ice resources separately. The model is drawn in the general notation used for resource system modelling in AML. The notation consists of only two basic symbols, entities (text strings) and relations (annotated arrows) between entities. Entities and relations can also have properties (not shown here). Note that, contrary to intuitive notations, it is often difficult to 