distributed agents in service networks Summary This chapter has demonstrated all the basic concepts and modelling methods for creating service network models for process control systems (PCS). Important findings have been: The modelling method is independent of which type of processing system is addressed. The basis for PCS modelling are service resource models. Such models give (partial or complete) abstracted views of the resource system. Resource models are entity-relation type of models. AML provides a general language for such models. Within each engineering domain, however, domain- unique notations should be defined. Service networks can comprise two types of units: service objects and distrib- uted agents. Communication relations between service objects, and between 6.3 Summary service objects and distributed agents, are service relations. Communication relations between distributed agents are modelled as complex relations. A PCS must be specified both as a service network (integration level 0) and as an abstract network (integration level 1). Abstract networks consist of abstract machines that integrate service objects (partly by adding operations that can handle temporary service relations). The behaviour between two abstract machines is described as an abstract proto- col. To describe this behaviour is rather trivial, once the behaviour of the service objects are known. In a service relation, the two service objects are users and providers. Communi- cation between users and providers is based on a service resource model. A PCS defines a hierarchy of service resource models that is called the abstrac- tion hierarchy of the PCS. This hierarchy is defined by the service relations in the service network. A service network that is built on service relations alone is a network that per- forms centralized control. If the network contains distributed agents, it also per- forms, to some extent, decentralized control. Most realistic service networks exhibit a mix of both types of control. As a result of network e