G Overview of the OSI Layer and Services Concepts (Informative)
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# G Overview of the OSI Layer and Services Concepts (Informative)

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.

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:

1. A request primitive in System A induces an indication primitive in System B.

2. 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-2. Service Description Conventions

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