present all requirements in- graphical notation alone. One often has to supplement the model with rules (as the rules for connecting CNUs to routes). 6 Process Control System Modelling SwitchChannel Switch Switch Endpoint Channel Route (SR) Route (CR) connects To connects To Position Internal switchPath Switch Internal connects To Route (IR) Channel connects To connects To AccessChannel Access Access Channel Route (AR) connects To Endpoint connects To **) Connection 1,2,30 Endpoint *) CNU is part of the operator network Exclusive OR Relation set at Relation set by **) CNU is part of a terminal system configuration time connection handling Fig. 6.6 The connection resource system drawn in a general, conceptual modelling notation Comparing the two models of Figs. 6.5 and 6.6, we can probably agree that the first one is much more expressive than the second, which speaks for using domain- specific notations. However, both model types are used in sysnet design. Now that we understand the RS, next we apply a preliminary service network solution to the PCS, see Fig. 6.7. The model structure is based on our general understanding of process control systems (see Sects. 1.4 and 2.2.1) and of logical network structures for telecom systems (see Sect. 3.5.4). The PCS consists of resource controllers for connection handling (the core route managers handles core routes, i.e. both switch routes and internal routes), a number of (unspecified) tele- service resource controllers (TR controllers) and a central process controller. The latter is separated into a teleservice process controller (TEPC) and a connection service process controller (CNPC). This separation of the central process controller functions represents the "call-connection separation". Note that distribution aspects are not considered in this first approach to the PCS. For example, one can expect that the switches may be physically separated, and that the TEPC and the CNPC must be designed as systems that have parts in both the terminal systems and in many physical nod