On X.25 public data networks, which device enables asynchronous character-mode terminals to access the packet network by assembling and disassembling packets on their behalf?

Introduction / Context:
X.25 was designed for packet-switched public data networks, but many end users connected using asynchronous character-mode terminals. These devices could not speak packet protocols directly, so an intermediary was needed to buffer characters, handle line editing, and translate between character streams and packet flows. This question asks you to name that intermediary device.

Given Data / Assumptions:

• Terminals are asynchronous (start-stop), not synchronous packet devices.
• Network is X.25, using virtual circuits and packet layer procedures.
• A device must translate between terminal I/O and X.25 PLP traffic.

Concept / Approach:
The Packet Assembler/Disassembler (PAD) accepts characters from an asynchronous terminal, assembles them into X.25 packets with appropriate control and addressing, and sends them into the network. In the reverse direction, it disassembles received packets back into characters for the terminal. PAD behavior was standardized (e.g., X.3 for parameters, X.28/X.29 for access procedures) to provide consistent features across providers.

Step-by-Step Solution:

Verification / Alternative check:
Historical network service manuals list PAD nodes as access points for dial-up terminals, showing configuration of PAD parameters such as local echo, editing, idle timers, and packetization thresholds.

Why Other Options Are Wrong:

• Repeater: regenerates signals; no protocol translation.
• Bridges: Layer-2 devices; do not convert async characters to X.25 packets.
• Gateway: generic term; not the specific standardized device for X.25 terminal access.
• None of the above: incorrect because PAD is correct.

Common Pitfalls:
Confusing PADs with modems; assuming “gateway” is specific enough—PAD is the precise, standards-based term in X.25 contexts.

Final Answer:
Packet assembler/disassembler (PAD).