Access Control implements exclusive access to services that only the command messages from the controlling client are executed.
Figure 11 Access Control Message Interaction
Operator Control (the client) needs to obtain Session UUID from the Subsystem Control service before sending Request Control to the Access Control service. Access Control Service validates the Session UUID, and compares the Authority Code. It grants access if Session UUID matches and the Authority Code is higher than the current authority code.
The Subsystem Control service constantly updates Access Control service with the current Session UUID by sending the Set Session UUID messages periodically. Access Control service rejects control if the Set Session UUID message times out.