request DATA request Service provider DATA indication Fig. 3.8 The original OSI RM SAP definition The model defined how a user and a provider communicated over SAPs using abstract service primitives. OSI also defined properties of SAPs, e.g. quality of service parameters (QoS) and addresses (SAPI). OSI launched the concept "service interface" (which SM&AML, in order to be more precise, has renamed network service interface). As can be seen by Fig. 3.8, an OSI-SAP comprised both CSI types of ASPs (CONNECT request, etc.) and SAP types of ASPs (DATA request, etc.). This also implies that the OSI-SAP was originally defined only for connection- oriented (CON) services. These are services provided by packet-switching net- An ASP is a type of communication event. The definition in OSI RM, see ITU-T X.210, Sect. 3.3.4, is: an abstract, atomic, implementation-independent representation of an interaction between service user and its service provider. An ASP describes which operation is referred to, whether the event is a request for something or a response to an earlier request, and which parameter types may be included in the events of the type. Thus, an ASP describes the seman- tics of a type of interaction between two units, without concern for message aspects, such as formatting and encoding of data. ASPs are described as parts of operation specifications, see Sect. 7.4. 3.2 The Communication Service Provider works, where an association between two users (a "connection") has to be ordered and delivered before users can start communicating. As a matter of fact, the OSI RM was based on the understanding of the virtual circuit service that was pro- vided by X.25 networks. The term "connection" is also frequently used in circuit- switched networks, like the ISDN. Since circuit-switched connections and virtual circuits are very different from all points of view, SM&AML specifies the term CN connection for circuit-switched network connections, and CON connection for packet-switched connections. When connection-less (CNL) pac