This International Standardized Profile is applicable to interworking units concerned with operating in the Open Systems Interconnection (OSI) environment. It specifies a combination of OSI base standards that collectively provide a Network Relay function for the connectionless-mode Network Service. This part of ISO/IEC ISP 10613 defines the RA51.1121 profile, which is applicable to interworking units operating between CSMA/CD LAN Subnetworks and PSDNs using Virtual Calls over a digital data circuit/CSDN leased line permanent access.