Packet assembler/disassembler
A packet assembler/disassembler, abbreviated PAD is a communications device which provides multiple asynchronous terminal connectivity to an X.25 network or host computer. It collects data from a group of terminals and places the data into X.25 packets. A PAD also does the reverse, it takes data packets from packet-switching network or host computer and returns them into a character stream that can be sent to the terminals. A Frame Relay assembler/disassembler is a similar device for accessing Frame Relay networks.
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 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 the Hayes command set. Many commercial PAD products provided completely different enhanced user interfaces.X.29 defines the DTE-P 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.