Operations and notifications, on an Association, are used in one of the following two modes:
In the synchronous mode, the invoking DIMSE-service-user, on an established Association, requires a response from the performing DIMSE-service-user before invoking another operation or notification.
In the asynchronous mode, the invoking DIMSE-service-user, on an established Association, may continue to invoke further operations or notifications to the performing DIMSE-service-user without awaiting a response. In the asynchronous mode, the performing DIMSE-service-user may respond to the operations or notifications in a different order than they were received.
The mode selection (synchronous or asynchronous) is determined at Association establishment time. The synchronous mode serves as the default mode and shall be supported by all DIMSE-service-users. The asynchronous mode is optional and the maximum number of outstanding operations/notifications is negotiated during Association establishment. This negotiation is accomplished by Application Association Information as defined in Annex D.