Which network access standard is classically used for connecting end stations to a public packet-switched network (PSDN)?

Introduction:
In traditional packet-switched data networks (PSDNs), the ITU-T X-series recommendations defined standards for how terminals and hosts accessed and exchanged packets. The question asks which standard specifically governed user access to a packet-switched network.

Given Data / Assumptions:

• We consider classic public packet networks such as those built on X.25.
• “Access” means the interface and procedures between a user device (DTE) and the network (DCE).
• Related X-series recommendations exist for physical interfaces, PAD parameters, and inter-network links.

Concept / Approach:
X.25 specifies the DTE–DCE interface for packet-level access to a PSDN, including virtual circuits (PVC/SVC), packet formats, flow control, and error handling. Other X-standards play supporting roles but are not the end-station packet access protocol itself.

Step-by-Step Solution:

Verification / Alternative check:
Historical PSDNs (e.g., international X.25 networks) used X.25 at the edge for terminals/hosts while X.75 handled network-to-network interconnect.

Why Other Options Are Wrong:

• X.3: parameter set for PADs, not the packet access protocol itself.
• X.21: physical interface/signaling recommendation.
• X.75: inter-PSDN gateway protocol, not user access.
• None of the above: incorrect because X.25 is correct.

Common Pitfalls:
Confusing PAD configuration (X.3) or physical interface (X.21) with the end-to-network packet protocol (X.25).

Final Answer:
X.25.