Applies to interworking units involved with operation in the Open Systems Interconnection (OSI) environment. Defines a combination of OSI base standards for collective provision of a Network Relay function for connectionless-mode Network Service. Defines the RA51.1121 profile, applies to interworking units operating between CSMA/CD LAN subnetworks and PSDNs, using Virtual Calls over a digital data circuit/CSDN leased line permanent access.