stsRequest for that. If the entity does not exist, PO responds to the user with stateErrorRepCnf. This error handling function also does not involve EO. However, it considerably simplifies the functions of EO, and thereby the complexity of event specifications. 3. Assuming that the initiating event has passed all controls done by PO, EO will start working on the event data. First, EO performs the remaining state error controls, expressed as preconditions in the EVS. These controls concern rela- 7.7 Concepts and Notations in Abstract Networks tions, properties and cardinallity of data in the SVS. If the actual state is not within the set of states defined by the precondition, the resulting event will be a stateErrorRepCnf, as shown previously 4. The last possibility for operation failure is when EO tries to perform the activity. If needed activity objects are not available, or operations on those objects fail for some reason, EO will, in case of operation classes REQUEST, INVITE and C_REPORT, order PO to initiate the negative response to the operation in ques- tion (Refuse or ReportRefuse). For all other operation classes, users are not notified about the failure. Concepts and Notations in Abstract Networks 7.7.1 Introduction Abstract networks are defined on integration level 1. An abstract network is a collaboration structure of units that integrates service objects. These units are called abstract machines. Connectivity between abstract machines is defined on the protocol network level (integration level 2). It is created by the integrator of the logical nodes in which the abstract machines reside. The behaviour in a communi- cation relation between abstract machines is described as an abstract protocol. Fig- ure 7.62 shows the integration principle (the notation used for abstract network models was defined in Sect. 3.2.4). The figure indicates that the structure of an abstract network (and consequently, the functions that are performed by an abstract machine) is not bound to the structure of the service network