nction in the ROOM language between fixed and optional actors. In AML, the boundary of a unit that has a permanent existence shall be drawn as a solid line. If it has a temporary existence, the boundary is dotted or dashed. Note, however, that a unit that has a permanent existence on one integra- tion level, may have a temporary existence on a higher integration level, and vice versa. Event handlers are examples of this: an event handler has a perma- nent existence in its protocol element, but the protocol element may have a tem- porary existence in its logical node. In this case, the event handler also has a temporary existence from the logical node/network's point of view. Colours and different grades of shading may be used to denote different types of units. Note that this cannot be a mandatory requirement, since the symbols for communication relations define the type of unit in an unambiguous way. How- ever, considering the complexity of models that show several structural levels, it is often necessary to use colours and shades to achieve an expressive model. AML provides this viewpoint only as recommendation to toolmakers. For AML users it might, however, be important to establish a company standard for the use of colours. Summary Telecom networks are concrete systems. A concrete network can be disintegrated into two physical network views: a spatial network and a physical network. A physical network designed and operated as a separate physical network is regarded as a suitable object for sysnet modelling. A physical network can be one or a number of physical networks built by interconnecting physical media connec- tion endpoints of physical nodes, thereby creating physical media connections. A telecom system is a logical system that may or may not have physical inter- faces to its environment. Telecom systems are also processing systems, consisting of two major functional components: a resource system and a process control sys- tem. Sysnet modelling supports process control system modelling in general, and 