This CCITT Recommendation | International Standard is applicable to the environments where Recommendation X.21 or X.21 bis is used and where the X.25 packet layer protocol (PLP), as standardized in ISO/IEC 8208, is operated during the X.21/X.21 bis data phase to provide the OSI Connection-mode Network Service (CONS). These environments include a CSDN and an ISDN Terminal Adaptor complying with Recommendation X.30 which provides an X.21 access. The X.25 PLP operates either in the DTE/DTE or DTE/DCE mode. This provision is achieved by specifying the mapping of the CONS primitives and parameters to and from the elements of the protocols used (i.e. X.25 PLP and X.21).