e service network. Thus, to describe the behaviour of a service network means to: 1. Specify the resource system model. 2. Specify the structure of the service network. The structure is a result of how the representation (the data) of the resource system model is fragmented and abstracted. 3. Specify operations for each type of service relation. 4. Specify the set of possible object states for each service object. 5. Specify the behaviour of each object in terms of a state machine. To describe the behaviour of an abstract network is rather trivial, once the behaviour of the corresponding service networks are known. The behaviour is defined by the abstract protocols. An abstract protocol is primarily the result of how service objects are aggregated in abstract machines. For example, the behav- iour described by the abstract protocol in Fig. 6.4 is the sum of the behaviours of the objects Setup Agent and the DyeRun Controller, modified with respect to the Object Handler operations. In addition, the abstract protocol defines in what order these three objects must be accessed. Normally the Object Handler is accessed first, then the Setup Agent, then the DyeRun Controller and finally, the Object Handler again. Note that the behaviour of an abstract network depends on the service networks it integrates, but the behaviour of a service network is independent of how it is inte- grated in an abstract network. Thus, seen over integration levels 0 and 1, service network models are reusable specifications, abstract network models are not. Simi- larly, seen over integration levels 1 and 2, abstract network models are reusable specifications, protocol network models are not. The main task in specifying a PCS is to specify its service network abstraction. An important, but secondary task is to describe its abstract network abstraction. A Service Network Modelling Case 6.2.1 Introduction As an introduction to service network modelling, we will design the service net- work of a process control system (PCS) that controls the provis