The event occurrence period applies to access control units and intrusion detection units
        that were temporarily offline. If a unit is offline in Security
                                Center, but still physically running,
        when the unit is re-connected the events that were stored on the unit while it was offline
        are received in Security
                                    Desk. The occurrence
        period value indicates how long ago the event occurred before the unit came back online, and
        determines what happens when the event is received.
        Only certain types of units (for example, HID VertX, Bosch) keep a record of events that
            occur while it is offline. For units that support offline events, you can configure
            different grace periods in the unit extension in Config
                                    Tool to control how offline events are
            treated in Security
                                Center.
        The four event occurrence values are listed below, with a description of what happens when
            Security
                                Center receives the event:
        Online:  
The event occurred when the unit was online.
 
Grace period:  
The event occurred during the Grace period configured in
                    the unit extension. Security
                                Center
                    treats the event as a live event. It appears in the event list in the
                    Monitoring task. Actions that are associated with the
                    event using event-to-actions are triggered.
 
Offline alarm:  
The event occurred between the 
Grace period and the
                    
Alarm grace period configured in the unit extension.
                    The event does not appear in the event list in the
                    
Monitoring task, unless it is one of the following:
                        - Input alarm activated
 
                        - Intrusion detection area alarm activated
 
                        - Intrusion detection area duress
 
                        - Intrusion detection unit tamper
 
                    
The event is recorded in the database and is available for
                        reporting.
 
 
Offline:  
The event occurred between the Alarm grace period and the
                    Persistence grace period configured in the unit
                    extension. When the event is received in Security
                                Center, it is only recorded in the
                    database for reporting.
 
        Reasons why units can be offline
An access control or intrusion detection unit can be offline in 
Security
                                Center for the following reasons:
                - The unit is rebooting.
 
                - The unit’s firmware is being upgraded.
 
                - The connection between the unit and the Access Manager or Intrusion Manager is lost.
 
                - The connection between the Access Manager or the Intrusion Manager and the
                    Directory is lost. When this happens, the role
                    disconnects from its units until the connection with the Directory is
                    re-established.