Digital communications over telephone lines: What is the device that converts a computer’s digital signals into a form suitable for transmission over an analog telephone line and back again at the other end?

Introduction / Context:
Traditional public switched telephone networks were designed for analog voice. Computers generate digital signals, so a specialized device is required to modulate digital data into an analog waveform for transport and demodulate it back into digital data at the destination.

Given Data / Assumptions:

• Medium: analog telephone line.
• Endpoints: digital computers.
• Need: bidirectional conversion between digital and analog domains.

Concept / Approach:
A modem (MOdulator/DEModulator) maps digital bits to analog symbols (tones, phases, amplitudes) and reverses the process on receive. Standards (e.g., V.34, V.90) defined symbol rates and encoding to approach channel capacity limits on voice-grade lines.

Step-by-Step Solution:

Verification / Alternative check:
Observe that without modulation/demodulation, raw digital levels would not be preserved over analog-only facilities. Modern broadband replaces analog modulation with DSL or all-digital links, but the principle remains.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing multiplexing with modulation, or assuming Ethernet-to-PSTN works without conversion.

Final Answer:
Modem.