X.29 is the ITU-T standard, approved by ISO, specifying procedures for the exchange of control information and user data between a Packet Assembler/Disassembler (PAD), and a remote packet-mode data terminal equipment (DTE) or another PAD. It defines the level 4 (or session layer in the OSI model) for X.25 communications, specifically call establishment and X.3 parameter negotiation.
Character-mode DTEs, which communicate with a PAD in a manner specified by the X.28 standard, are often referred to as X.29 terminals.
References
X.29 Wikipedia(Text) CC BY-SA