This Recommendation provides information on key concepts of the Open Systems Interconnection (OSI) Reference Model and how these concepts are applied in various portions of Signalling System No. 7 (SS No.7) and Digital Subscriber Signalling System No. 1 (DSS 1). These concepts form the basis for the development of new application protocols within the SS No. 7 and DSS 1 environments. They apply equally to Operations, Administration and Management protocols. The discussion covers the Application Layer Structure (ALS), the nature of the services provided by the Association Control Service Element in the Application Layer and how it may be adapted to the signalling environment, and the services provided by the OSI Presentation Layer. The application of the concepts is illustrated by means of a detailed discussion of their use in the development of the Intelligent Network Application Part (INAP) for Capability Set 1, Recommendation Q.1218. In addition, this Recommendation contains guidelines for use when an existing protocol is extended. These are presented as two sets. One is for existing, non-OSI structured protocols, and the other is for Remote Operations Service Element (ROSE)-based protocols. ROSE-based protocols are presently the most widely used OSI protocols within telephony signalling systems.