. Therefore I also have included an object-ori- ented paradigm as a part of modelling process control systems. This part is represented by two abstraction levels of a process control system model: a service network (where all entities are individual objects) and an abstract network (where entities are aggregates, or packages of objects). A large piece of the puzzle was still missing, however. A telecom system is not just an abstract system, with functionality that is exhaustively described by the pro- cessing system paradigm. As a matter of fact, most engineers in the field would describe it as a network of physical nodes (interconnected by physical media), where the resources are physically distributed and the functions are implemented as distributed systems over that network. They view a physical node as either a small physical network in itself, or a node that consists of a network application that runs on some kind of platform. In the mid-1980s, the OSI Reference model (see the ITU-T recommendation X.200 and associated standards) seemed to be a response to this view. A promising approach to provide a complete, generic system model therefore seemed to be to integrate the processing system paradigm with the OSI- model. After analyzing that track for some years, I realized that although the OSI model provided many useful insights into the structure and generic functions of data com- munication systems, it had to be considerably refined and improved before it could act as a general model for telecom and distributed systems. This brought me to the sysnet concept that allows distributed systems (or networks) to be viewed in three logical dimensions: distribution (protocols), communication (layers) and pro- cessing (platforms). This separation is referred to as functional space. In func- tional space, good OSI concepts have been included and refined ("service", "service access point", "layer", "protocol", etc.), less successful approaches have been omitted (e.g. a fixed number of layers and the definition of several