onent of the overall system, since ultimately all decisions are taken within the real-world process by humans. The goal with a telecom system, and most other processing systems we build, is to do their processing with a minimum of human intervention. In a processing sys- tem there must therefore exist a (logical) subsystem that takes all decisions. We will call that subsystem the process control system. The remaining part of the processing system consists of resources that process the input and output flows of the processing system. All resources taken together we will regard as the resource system. 1 Introduction to Systems Modelling Human Processing system M/E/I flows Process Resource System Control System resource M/E/I flows control Machine Fig. 1.5 The two major subsystems of a processing system Figure 1.5 depicts this (deceptively) simple logical structure for processing sys- items. The type of resources that are included in the resource system depends, of course, on the type of processing system. If we are talking about a telecom system, typical resources are switches, transmission lines and different types of devices, such as line termination units and answering machines. In any processing system there is a number of interfaces between the process control system and the resource system where resources are controlled. On the low- est level of control, these interfaces will be based on physical interactions (M/E interactions in the Miller sense), through different types of sensors and effectors. A more common way to view the structure of systems is in terms of its major software and hardware components. The model in Fig. 1.6 is taken from Selic et al. (1994), and depicts a process control system structure as a structure of software and hardware components (the resources exist in the "environment"). Sensors Hardware Environment Software interface Effectors Fig. 1.6 The basic elements of a real-time system, according to Selic et al. Selic et al. take this model as a basis for discussing real-time characterist