ps, such as the OSI Network Man- This means that, in theory if not in practice, protocols in one layer agement Forum, and independent conformance testing is carried out can be replaced without affecting other layers. by other bodies, such as the Corporation for Open Systems (COS) Figure 16.10 illustrates this concept of service provision within a in North America, the Standards Promotion and Application Group layer. Layers provide services to its own users and to the layers (SPAG) in Europe, and the Promoting Conference for OSI (POSI) above, using their own functions or those contained within services in Japan. ISO has published the OSI Conformance Testing Metho- provided by the layer below. There are four basic types of service dology and Framework (ISO 9646) which provides a general frame- primitives, request, indication, response and confirm. Primitive full work for conformance testing. It outlines principles which should names consist of a letter, indicating the layer (e.g. S for the Session be applied in developing conformance tests for specific OSI stand- layer) a verb indicating the operation, and the basic type of service ards. primitive, for example 'S- DISCONNECT confirm.' In an actual END SYSTEM END SYSTEM Application layer protocol Application layer Presentation layer protocol Presentation layer Session layer protocol Session layer Transport layer protocol RELAY SYSTEM Transport layer Network layer protocol Network layer protocol Network layer Datalink layer protocol Detalink layer protocol Datalink layer Physical layer protocol Physical layer protocol Physical layer Figure 16.9 The OSI model showing communications between two end systems using an intermediate relay system The OSI model 16/9 are shown in Figure 16.11. This illustrates the choice which exists SERVICES TO LAYER ABOVE when communicating up and down the OSI stack. The standards are of two types: those used in different applications and those which are specific to one type of application. INTERFACE TO MANAGEMENT LAYER PEER TO PEER PROTOCOL