A wireless transmit/receive unit (WTRU) may establish a multi-access (MA) protocol data unit (PDU) in accordance with the examples described herein. The WTRU may establish a new packet data network (PDN) connection or identify a suitable existing PDN connection in an evolved packet core (EPC), establish a MA-PDU in a 5G core network (5GC), and associate the existing PDN with the MA-PDU. The WTRU may already have a MA-PDU session established in 5GC with both 3GGP access leg and non-3GPP access leg in 5GC, and the WTRU may replace the 3GGP access leg in 5GC with a suitable PDN connection in EPC. The WTRU may send a request for establishing a single-access PDU session in 5GC via non-3GPP access, and a 5GC network may upgrade a PDU session established for the WTRU to a MA-PDU with 3GGP access leg in EPC.