In a layered communication model, such as the OSI 7 layer reference model, each layer uses the service provided by the layer immediately below. The operation of a protocol layer on top of the lower layer service provides a new service to the layer above. The service is the “glue” between the layers of protocols.
Services describe the resulting effects of the operation of a protocol without requiring knowledge of the detailed specifications of the protocol itself. A protocol specifies a horizontal dialogue between two computing systems across a network, while a service describes a vertical relationship within a system. See Figure G-1.
Service provided tothe layer above
Protocol within a Layer
Service provided bythe lower layer
Figure G-1 RELATIONSHIP OF SERVICES TO PROTOCOL
The OSI Upper Layer Service is described by a number of service primitives. They each model one of the functional interactions between the service-user in the layer above and the service-provider. In the context of this Standard, the service-user is called the DICOM Application Service Element. The service-provider is called the Upper Layer and performs the Upper Layer Protocol.
Note: The OSI UL Services defined in this standard are provided by the DICOM Upper Layer Protocol for TCP/IP (Section 9).
These service primitives cross the layer boundary at what is called a Service Access Point (SAP). In most cases a direct relationship exists between service primitives in two Application Entities (AEs). This is reflected in the names of these primitives:
a) A request primitive in System A induces an indication primitive in System B.
b) If an indication primitive in System B requires a reply, a response primitive may be issued at the Service Access Point (SAP) in System B. This response primitive will induce a confirmation primitive in System A.
The different types of service primitives and their relationship are shown in Figure G-2. The dotted lines represent the exchange of Protocol Data Units that are triggered by request/response primitives or generated indication/confirmation primitives.
Figure G-2SERVICE DESCRIPTION CONVENTIONS