service state space: in order to preserve the integrity of the service network, service objects must always be in a resource state that is defined by the service resource models. A service resource model defines all possible states (the service state space) of the abstracted resource that is defined by the model. A single resource state is defined by an instance of the service resource model. Such an instance defines a fixed number of entities and relations, with values given to all properties, all within the range given by the resource model. The two parties in a service relation must agree on how the responsibility for controlling the state space is shared between them. There are two extreme possi- bilities: the provider or the user has the sole responsibility. Normally the user takes responsibility on lower levels in the abstraction hierarchy (as resource controllers do over resources). On higher levels, the provider takes responsibil- ity (as the TEPC should do for the TNW). To economize communication: as was mentioned before, if a user does not have full control over the state space, the service network integrity is not endangered, as long as the provider has full control. If, e.g. the TEPC requests from the CNPC to set up a connection between CNU1 and CNU2 without knowing that the access route to CNU2 is blocked, the CNPC will simply refuse the request, and the integrity of the system is not harmed. However, if the TEPC knew that the access route was blocked, TEPC would not have to communicate with CNPC in the first place. This would decrease the processing load and improve response times. Thus, there is always a need for users to have at least partial knowledge of the resource state. In addition, the user must always know which types of entities, relations and properties exist in the model, SO that users and providers can understand each other. 6 Process Control System Modelling Thus, both the user and provider must have some data representation of their mutual service resource model. We can therefore modi