PAD Standards X.3 X.28 X.29 | Generated by AI
Question: As an extension to the X.25 standard, CCITT proposed a set of three standards related to the Packet Assembler/Disassembler (PAD) facility. Which of the following are they?
Answer:
The correct answer is C. X.3, X.28 and X.29.
Detailed explanation:
The Packet Assembler/Disassembler (PAD) is a network device/function that allows non-packet-mode (character-mode) asynchronous terminals (such as dumb terminals or modems) to access packet-switched networks like those using X.25.
CCITT (now ITU-T) defined three closely related recommendations specifically for PAD operation:
-
X.3 — Packet Assembly/Disassembly facility (PAD) parameters
Defines the functions and configurable parameters of the PAD (e.g., idle timer, line folding, character echoing, data forwarding conditions, editing functions, etc.). It specifies what the PAD actually does and how it can be parameterized. - X.28 — DTE/DCE interface for a start-stop mode DTE accessing the PAD in a public data network situated in the same country
Defines the user interface between an asynchronous terminal (start-stop mode DTE) and the PAD. It covers:- How a terminal connects to the PAD
- PAD command language (e.g., setting parameters with SET? READ? commands)
- Profile selection
- Call establishment/clearing procedures from the terminal side
- X.29 — Procedures for the exchange of control information and user data between a PAD facility and a packet mode DTE or another PAD
Defines the protocol between the PAD and the remote packet-mode host (or another PAD).
It specifies how control information (e.g., parameter setting requests, break signals, reset) and user data are carried inside X.25 data packets using Qualified Data (Q-bit = 1) packets.
These three standards together form the complete PAD framework:
- X.3 = what the PAD does (functions & parameters)
- X.28 = how the terminal talks to the PAD
- X.29 = how the PAD talks to the network/host
Why the other options are incorrect:
- A. X.3, X.25 and X.28 → X.25 is the base packet-layer protocol, not a PAD-specific standard.
- B. X.3, X.25 and X.29 → Again includes X.25 instead of X.28, missing the terminal-to-PAD interface.
- D. X.25, X.28 and X.29 → Includes X.25 (core protocol) but omits X.3, which is the actual definition of PAD parameters/functions.
Memory tip (as provided):
“三分包，28端，29网”
→ Three standards for PAD:
X.3 = PAD itself (包)
X.28 = terminal side (端)
X.29 = network/host side (网)
References:
- None (standard ITU-T/CCITT X-series recommendations for X.25-based packet networks and PAD operation)