of the processing system. Processing system Process Control System (PCS) users Processing Resource system model system (Data representation) services/ Resource operators utilities System (RS) Fig. 6.1 The interfaces of a process control system The RS provides users with the services/utilities they want (data or information transport, or some other type of utility), while the PCS controls service provision- ing. A PCS must perform three major tasks, see Fig. 6.1: 1. Communicate with the users of the processing system: since we may assume that the processing system can deliver many types of services, or services tai- lored to specific needs, the PCS must be able to communicate with processing system users to find out what they want, to tell the user when the service is available and be prepared to receive service release information. 2. Control the state of the RS: to provide a specific type of service means that the PCS must place the resource system in a state where it provides exactly the serv- ice the user has requested. Every instance of service provisioning requires that some resources are allocated. When the service is no longer needed, the PCS must alter the RS state SO that somebody else can use those resources. 3. Communicate with operators of the processing system: we don't expect a PCS to be fully autonomous. When a processing system is in operation, many situa- tion will occur which require that a processing system operator must tell the PCS what to do. Such operator directives may depend on state information delivered by the PCS (fault alarms, undefined states, congestion situations, frauds, etc.), but may also be due to those modifications of the RS that cannot be T. Muth, Modeling Telecom Networks and Systems Architecture © Springer-Verlag Berlin Heidelberg 2001 6 Process Control System Modelling observed by the PCS. The latter case implies that operators must provide infor- mation regarding configuration changes in the RS to the PCS. A primary issue for an operating PCS is therefore that it must, a