Defines the provision of the OSI connection-mode transport service between an end system (the reference end system, RES) which uses a permanent analog or digital circuit to access a packet switched data network (PSDN) and another end system (the compatible end system, CES) which is accessible, by permanent or switched methods, either directly from the same PSDN, or indirectly through the provided OSI connection-mode network service. Also defines the provision of the OSI connection-mode network service between the RES and the CES using the X.25 subnetwork procedures to a PSDN. Is applicable to environments which support the OSI network service.