s The state diagram is a small excerpt from the diagram of the teleservice process controller (TEPC) from Fig. 6.25. The states AandElnCall and AnotInCall are resource states that are defined in the initial diagram, which is regarded as the Bas- 7 Concepts and Notations for Process Control Systems Modelling icCall diagram. At some point in time, it was decided to extend this function with the possibility for call users to temporarily withdraw from the call. It was also decided that if the withdrawn user did not return within a time T2, the call should be released. Two new operations were therefore defined, suspendREQ and resumeORD. The user who wants to withdraw must initiate a suspendReq and await a suspend- Cnf(ref), and when they want to return, they must initiate a resumeOrd(ref). These new requirements could be defined as a feature service, called the Suspension service. This service was then specified separately, as Fig. 7.52 shows. In the spec- ification for the Suspension service, the relation to the BasicCall service is given by references to resource states of that service. The state in the BasicCall diagram (AandElnCall) in which the new service is ini- tiated is a feature state. A feature state is normally assigned for those resource states where many future service enhancements are expected. There are two special problems that may have to be handled in feature states: Access to a particular feature service may be restricted to users who have sub- scribed to it. That requires special subscription control activities to be associated with feature states. In special in telecom process control systems, there are many dependencies between features. Some features may be mutually exclusive. For example, a user who has activated the service OriginatingCallOnly cannot use the service CallWaiting at the same time. This requires that special feature interaction con- trol activities must be associated with feature states as well. The existence of special feature handling activities is indicated in state diagrams 