ivity, data transfer and addressing can be disregarded. Instead, one can focus on modularization aspects (functional modularization, centralized versus decentralized control, etc.) and abstraction aspects (not every PCS unit needs to have access to all knowledge regarding the RS. Some PCS units may need detailed information, others can operated on high-level abstractions of the RS, etc.). Service networks are the basic functional model type for a PCS. The units of a service network are service objects. Figure 6.3 shows an example. A service net- work can also be built as a hierarchy of substructures (not shown in Fig. 6.3, see Fig. 6.15) in a way similar to physical networks. Service objects in service net- works on the lowest level of that hierarchy represent the smallest functional entities that can be mapped on products defined by an implemented system. Contrary to modelling methods for software systems, the hierarchy of substructures allows service objects to be defined as network-wide functions, as well as objects that reside in a computer in the network. All service objects, no matter which level, are specified in the same way. Thus, service objects are true abstract entities. A service network can represent any kind of function in a sysnet. The functional complexity of a service network is the modeller's choice, within the few modelling restrictions defined by AML. The extent of a service network is chosen to represent some unique function performed by the PCS, being of reasonable complexity. However, once service networks are specified and logically verified, one has to consider processing (P-dimension) and distribution (D-dimension) aspects, which implies defining units that can be implemented as single products, be distributed over a network as single units and, in this case, be refined to protocol elements. Those units are the abstract machines of abstract networks. 6 Process Control System Modelling Run object data handled SetupData by Run object *requiredLevel operation offered by Temp.Contr. *wantAi