Interface standards: Which network access standard is used for connecting data terminal equipment (DTE) to a circuit-switched network?

Introduction / Context:
ITU-T X-series recommendations define several data networking standards. Distinguishing between user-network interface standards, packet-switched protocols, and inter-network links is essential when matching equipment to carrier services, especially in legacy and exam contexts.

Given Data / Assumptions:

• We are focusing on access to a circuit-switched network.
• DTE (Data Terminal Equipment) needs a standardized interface to the carrier.
• Options include multiple X-series documents with different scopes.

Concept / Approach:
X.21 specifies the interface between DTE and DCE for circuit-switched digital networks, defining signaling and electrical characteristics. By contrast, X.25 specifies a packet-switched protocol suite over a network (often using LAPB at layer 2), X.75 defines inter-DCE interconnection, and X.3 covers PAD parameters for character-mode access to packet networks—none of which directly define the DTE access to a circuit-switched network.

Step-by-Step Solution:

Verification / Alternative check:
Historical carrier services offering digital circuit switching (e.g., early ISDN-like services) reference X.21 for the customer interface, confirming the match.

Why Other Options Are Wrong:

Common Pitfalls:
Conflating packet-switched and circuit-switched standards; assuming all “X.” recommendations perform the same function.

Final Answer:
X.21.