A method to establish and activate an MA PDU session for data transmission over a selected access is proposed. UE initiates a UE-requested PDU session establishment procedure. Once the UE receives a PDU SESSION ESTABLISHMENT ACCEPT message with ATSSS container IE, the UE can consider the MA PDU session has been activated and user plane resources are successfully established on the selected access. A method to convert an SA PDU session to an MA PDU session for data transmission over a selected access is proposed. UE initiates a UE-requested PDU session modification procedure. Once the UE receives a PDU SESSION MODIFICATION COMMAND message with ATSSS container IE, the UE can consider the MA PDU session has been converted from the SA PDU session and user plane resources are successfully established on the selected access.