ITU-T (Triple-X PAD)
The structure of a PAD is defined by the ITU-T in recommendations X.3, X.28, and X.29. Sometimes, this is referred to as a Triple-X PAD, due to the three X series recommendations which define it.
X.3 specifies the parameters for terminal-handling functions such as line speed, flow control, character echo, et al. for a connection to an X.25 host. The X.3 parameters are similar in function to present day Telnet options.
X.28 defines the DTE-C (asynchronous character mode) interface to a PAD, including the commands for making and clearing down connections, and manipulating the X.3 parameters. The commands were very crude, a bit like (but not at all compatible with) Hayes modem commands. Many commercial PAD products provided completely different enhanced user interfaces.
X.29 defines the DTE-P (packet mode) interface to a PAD, i.e. how the PAD encapsulates characters and control information in X.25 packets.
Connections are established using X.121 14-digit X.25 addresses.
Read more about this topic: Packet Assembler/Disassembler