Which of the following does NOT enable multiple users/devices to share a single communications line?

Introduction / Context:
Sharing a single high-capacity line among many lower-rate users is fundamental in telecom and networking. Devices designed for multiplexing or traffic concentration combine streams; other devices merely manage or interface without providing true line sharing.

Given Data / Assumptions:

• We interpret 'share one communications line' as combining multiple input channels onto a single aggregated link.
• Standard devices: multiplexers (FDM/TDM/StatMux) and concentrators provide sharing.
• Controllers manage or interface but are not inherently multiplexing devices.

Concept / Approach:

A multiplexer combines multiple signals onto one line using time, frequency, code, or statistical methods. A concentrator aggregates many low-duty-cycle devices onto a higher-capacity trunk. A controller (e.g., terminal/line controller) administers connections or protocols but does not itself multiplex signals onto a single line by design.

Step-by-Step Solution:

Verification / Alternative check:

Typical WAN diagrams show CSU/DSU + MUX/Concentrator feeding one T1/E1 or higher line. Controllers appear on edges but do not perform the combining.

Why Other Options Are Wrong:

Multiplexer: Purpose-built for line sharing (TDM/FDM/CDM/statistical multiplexing).

Concentrator: Aggregates many inputs to one trunk, enabling sharing.

Doubleplexer: Nonstandard term; but in the context of the question, the only clearly non-sharing role is 'Controller.'

None of the above: Incorrect because a controller is not a sharing device.

Common Pitfalls:

Assuming any device in a WAN rack performs multiplexing; roles differ between control and aggregation.

Final Answer:

Controller