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.
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:
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.
