ACSE means Association Control Service Element, defined by OSI, which is used by DICOM to negotiate an Association. The Association Control Service Element (ACSE) augments the Presentation Layer Service with Association establishment and termination services. In the case of TCP/IP, the full equivalent of ACSE is provided by the DICOM Upper Layer Service.

For the DICOM point-to-point stack, a minimum subset of ACSE is provided by the Session/Transport/Network Service. The communication services specified in PS 3.8 are a proper subset of the services offered by the OSI Presentation Service (ISO 8822) and of the OSI Association Control Service Element (ACSE) (ISO 8649). They are referred to as the Upper Layer Service, which allows peer applications to establish associations, transfer messages and terminate associations.

The OSI Basic Reference Model is used to model the interconnection of medical imaging equipment. DICOM uses the OSI Upper Layer Service to separate the exchange of DICOM Messages at the Application Layer from the communication support provided by the lower layers. This OSI Upper Layer Service boundary allows peer Application Entities to establish Associations, transfer Messages and terminate Associations. For this boundary, DICOM has adopted the OSI Standards (Presentation Service augmented by the Association control Service Element). It is a simple service that isolates the DICOM Application Layer from the specific stack of protocols used in the communication support layers.