Packet assembler/disassembler
A packet assembler/disassembler, abbreviated PAD is a communications device which provides multiple asynchronous terminal connectivity to an X.25 (packet-switching) network or host computer. It collects data from a group of terminals and places the data into X.25 packets (assembly). 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 (disassembly). A FRAD (Frame Relay Assembler/Disassembler} is a similar device for accessing Frame Relay networks.
The structure of a PAD is defined by ITU-T (International Telecommunication Union); 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.
The following is list of X.3 parameters associated with a PAD:
- 1 PAD recall using a character
- 2 Echo
- 3 Selection of data forwarding character
- 4 Selection of idle timer delay
- 5 Ancillary device control
- 6 Control of PAD service signals
- 7 Operation on receipt of break signal
- 8 Discard output
- 9 Padding after carriage return
- 10 Line folding
- 11 DTE speed
- 12 Flow control of the PAD
- 13 Linefeed insertion after carriage return
- 14 Padding after linefeed
- 15 Editing
- 16 Character delete
- 17 Line delete
- 18 Line display
- 19 Editing PAD service signals
- 20 Echo mask
- 21 Parity treatment
- 22 Page wait