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A `!packet assembler/disassembler`!, abbreviated `!PAD`! is a communications device which provides multiple `F33f`_`[asynchronous`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Asynchronous_serial_communication]`_`f terminal connectivity to an `F33f`_`[X.25`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=X.25]`_`f (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 `F33f`_`[Frame Relay assembler/disassembler`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Frame_Relay_assembler/disassembler]`_`f (FRAD) is a similar device for accessing `F33f`_`[Frame Relay`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Frame_Relay]`_`f networks.
>>Contents
• `F0af`_`[ITU-T (Triple-X PAD)`#itu-t-triple-x-pad]`_`f
• `F0af`_`[Green Book PAD`#green-book-pad]`_`f
• `F0af`_`[ITP`#itp]`_`f
• `F0af`_`[See also`#see-also]`_`f
• `F0af`_`[References`#references]`_`f
• `F0af`_`[External links`#external-links]`_`f
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>>ITU-T (Triple-X PAD)
The structure of a PAD is defined by the `F33f`_`[ITU-T`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=ITU-T]`_`f in recommendations `F33f`_`[X.3`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=X.3]`_`f, `F33f`_`[X.28`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=X.28]`_`f, 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, `F33f`_`[flow control`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Flow_control_(data)]`_`f, character echo, et al. for a connection to an X.25 host. The X.3 parameters are similar in function to present-day `F33f`_`[Telnet`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Telnet]`_`f 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) the `F33f`_`[Hayes command set`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Hayes_command_set]`_`f. 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.`:cite-ref-x29-1-0[`F5bf`_`[1`#cite-note-x29-1]`_`f]
Connections are established using `F33f`_`[X.121`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=X.121]`_`f 14-digit X.25 addresses.
>>Green Book PAD
One of the UK `F33f`_`[Coloured Book protocols`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Coloured_Book_protocols]`_`f, `!Green Book`!, also defines two PAD protocols. Green Book was developed by (UK) `F33f`_`[Post Office Telecommunications`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Post_Office_Telecommunications]`_`f in the 1970s. Although not identical to Triple-X, Green Book is sufficiently similar to X.3 and X.29 that generally the two will interwork. Green Book also specifies `!TS29`!, a very similar protocol which ran over the Yellow Book Transport Service, which is another of the Coloured Book protocols.
>>ITP
`!ITP`! (Interactive Terminal Protocol) was an early PAD protocol for use over X.25 developed in the 1970s for use with `F33f`_`[UK GPO's`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Post_Office_Telecommunications]`_`f EPSS (Experimental Packet Switching System, the predecessor of `F33f`_`[PSS`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Packet_Switch_Stream]`_`f). ITP predated Triple-X, and is a completely different protocol. `F33f`_`[Science and Engineering Research Council`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Science_and_Engineering_Research_Council]`_`f (SERC) also used ITP on SERCnet and continued developing ITP after EPSS, although it eventually gave way to Triple-X.
>>See also
• `F33f`_`[Terminal node controller`:/page/wikibook/entry.mu`zim=wikipedia_en_all_nopic_2025-08.zim|entry_path=Terminal_node_controller]`_`f
>>References
`:cite-note-x29-1`!1.`! `F0af`_`[↑`#cite-ref-x29-1-0]`_`f "X.29 : Procedures for the exchange of control information and user data between a Packet Assembly/Disassembly (PAD) facility and a packet mode DTE or another PAD". `*www.itu.int`*.
>>External links
• ITP: Protocols in the SERC/NERC Network"
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