escription can be based on either reactive or active states, or a mix of active and reactive states. For the interpreter (e.g. implementer), it is important to know if a state in a state diagram is an active or a reactive state, which is why different state symbols are recommended. An extra bonus with the operation classification system is that it helps to decide what type of states exist in a state diagram. The table in Fig. 7.51 describes how primitive types and different types of states are related. This table can be used to construct tools that control the logical correctness of an AML state diagram. A active state Primitive Initiated by Detected by P pending state inState nextState inState nextState PP provider port Request RS resource state RS,A" Order UP user port RS,A*) Confirm *) Only if the provider Refuse handles interrupts Offer **) Only if the user Reject handles interrupts Invite ReportInit ReportRefuse ReportConfirm Fig. 7.51 The relation between primitive types and states in service relations Feature state: Behaviours are not static. Complex process control systems are continuously extended due to the introduction of new services, which in the process control sys- tem of telecom systems, are often extensions of some existing service. The existing service, of which the new service is an extension, is then a more basic service. Functional enhancements can be added (on the service network level) as new service networks that are connected to other, more basic service networks. For example, in the first version of first-generation mobile networks, authentication of callers was not included. Later, authentication was added. It became a com- 7.5 Behaviour Descriptions pletely new service network that was connected to the existing service network for basic call handling. It may be possible to implement some functional enhancements just by adding one or several new object types to an existing service network. Most common, however, is that functional enhancements comprise new or mod- ified operations in exi