r. This activity can go on in parallel with connection handling without any interference, since the event-han- dling object gets all state information it needs for checking preconditions from the state object, and it changes state information by communicating with the state object only. To describe the behaviour of a distributed agent therefore is reduced to describing the behaviour of two objects: the event handling object and the ORM manager, since the state object has a predefined behaviour, and the behaviour of the proxy object is trivial. An important conclusion of this discussion is that there are two very different types of communication relations in service networks: 6 Process Control System Modelling In a service network that represents a centralized control solution, all communi- cation relations are service relations of type user-provider (associate to "client- server" in other contexts). These relations define the abstraction hierarchy of the network. Most service networks that represent telecom network control functions also represent some form of decentralized control solution. The service relations (or the abstract protocol) between two distributed agents represent partitioning of a single resource model. Such relations lie between abstraction levels of that hier- archy. Therefore many communication relations in service networks will not represent the abstraction hierarchy, but instead decentralized control decisions. How control responsibility is distributed in a network is something that is modified rather often in a sysnet, while the abstraction hierarchy is something that is kept intact as long as possible. It is therefore very important that a service network model distin- guishes between these two types of relations. The notation suggested by AML, see Fig. 6.32, is to relate distributed agents by complex service relations in service net- work models. Such a relation is an aggregate of several service relations. users Master Slave distributed agents resource controllers Fig. 6.32 Notation for 