In telecommunications, ITU-T X.21 defines what type of interface/standard for connecting data terminal equipment (DTE) to a circuit-switched digital network?

Introduction / Context:
Several legacy standards describe how user equipment attaches to carrier networks. Understanding which specification covers the physical/electrical and signaling interface to a circuit-switched digital network helps distinguish access standards from media access control or higher-layer protocols.

Given Data / Assumptions:

• Standard referenced: ITU-T X.21.
• Environment: circuit-switched digital networks.
• Objective: identify what X.21 specifies.

Concept / Approach:
X.21 defines the DTE/DCE interface (pin-out, signaling, and call control) for attaching terminals or computers to a circuit-switched public data network. It is not a shared-medium MAC method, nor a bit-oriented data link protocol like HDLC/SDLC, and it is not an ISO LAN standard. It focuses on access to carrier circuits (e.g., switched virtual circuits in early PDNs).

Step-by-Step Solution:
Recognize X.21 as an ITU-T access specification.Link it to DTE/DCE interface on circuit-switched digital PDNs.Exclude MAC methods and generic data link protocols.Select the description matching a network access standard.

Verification / Alternative check:
X.21 complements other X-series interfaces (e.g., X.25 packet layer with LAPB at link layer); together they enabled connections over carrier-provided circuits before widespread IP.

Why Other Options Are Wrong:
Media access methods (a, b) relate to LAN contention/token techniques, not carrier access interfaces.

Bit-oriented link protocol (c) describes HDLC/SDLC, not X.21.

None of the above: Incorrect because X.21 is indeed a circuit-switched access standard.

Common Pitfalls:
Confusing X.21 (access) with X.25 (packet layer) or HDLC (link control); each addresses different layers/responsibilities.

Final Answer:
A network access standard for connecting stations to a circuit-switched network