This CCITT Recommendation | International Standard is applicable to the environments where the X.25 packet layer protocol, as standardized in (PLP) ISO/IEC 8208, is operated over a telephone network connection to provide the OSI Connection-mode Network Service (CONS). These environments include a) telephone network leased circuit connection between two DTEs; b) telephone network switched connection between two DTEs; c) DTE access to a PSDN via a telephone network leased circuit connection; and d) DTE access to a PSDN via a telephone network switched connection. This provision is achieved by specifying the mapping of the CONS primitives and parameters to and from the elements of the protocols used.