eric units within the process control system, were identified for all major classes of networks. The CSP is based on the concepts defined by the OSI RM almost twenty years ago that were considerably refined in SM&AML. There is no doubt that logical network modelling in SM&AML is heavily influenced by the OSI RM, an excel- lent intellectual effort from 20 years ago. However, modelling has come a long way since then. This is obvious, if one analyses the functional structure of the networks of today and the direction network architecture is going. A general conclusion, 3 Logical Network Modelling therefore, is that there are few ideas from OSI RM which are valid or useful for modelling network architectures today. An outstanding exception is the OSI data- flow model (the PDU-IDU distinction). However, concepts like the OSI layer, the seven-layer structure, the definition of the OSI network layer, to mention a few, cause more confusion than clarity. One challenge for the logical modelling tech- nique presented in this chapter is to encourage people to abandon comparisons with OSI when they describe network concepts. 4 Physical Network Modelling Introduction Physical network models are defined on the highest integration level (lowest abstraction level) in the integration hierarchy. Contrary to lower integration levels (higher abstraction levels), physical network models exhibit almost no addressing transparency. If two physical nodes need to communicate, the sending node must identify a physical media connection endpoint, which is a nearly static identifica- tion. Also contrary to higher abstraction levels, physical nodes exhibit nearly no temporary existence. They either exist in a particular physical network model, or they don't. We say "nearly" because: Models in functional space do not include spatial dimensions. If those dimen- sion are included in a model, a spatial network model is created as well, showing sites, cables, etc. A physical network model can then be mapped on the spatial model, whereby the physical