Terminology and Conventions

ISO has developed its own set of jargon to describe the concepts in the model and in the related standards. We have already surreptitiously introduced some of this 'OSIspeak' and it is used extensively in what follows. Some of the important terms are summarised below. A Service definition is a document defining a particular layer service. Usually it defines a set of several services provided by a layer. Each service is described in terms of four Service Primitives: An operation is begun when the local service user generates a request primitive across the local service interface. This should result in the generation of a corresponding indication primitive across the remote service interface. In some cases a reply will be required from the remote service user. This is triggered by the generation of a response primitive across the remote service interface which should eventually result in the generation of a confirm primitive across the local service interface. By way of example, Table #tbtserv#1437> shows the set of primitives that are defined for the OSI Connection Oriented Transport Service (Layer 4).

Table: Transport Service Primitives

In general, the primitives mentioned above have parameters associated with them. For example, a T.CONNECT.request will include the address of the remote host while a T.DATA.request will include data for transfer across the network. Primitive requests cannot be issued in a completely arbitrary way, there are usually rules governing which primitives may follow which and which are legal when the system is in some particular state. In the Transport Service example above, a T.DATA.req cannot be issued in advance of a T.CONNECT.conf. OSI service definitions include a state table specifying these temporal orderings.