In data communications hardware, which device performs both modulation and demodulation to convert between digital data and carrier-based analog signals for transmission?

Introduction:
Digital systems often need to transmit data across channels optimized for analog waveforms, such as radio or legacy telephone circuits. The component that bridges this gap is responsible for modulating a carrier at the transmitter and demodulating it at the receiver—hence the portmanteau “modem.”

Given Data / Assumptions:

• We are discussing physical-layer signal conversions for data transport.
• Digital endpoints must communicate over analog channels in many scenarios.
• Terminology distinguishes medium (fiber, coax) from active signal-processing equipment (modem).

Concept / Approach:
A modem transforms baseband digital data into a passband analog signal by varying a carrier’s amplitude, frequency, or phase (ASK, FSK, PSK, QAM). At the far end, another modem performs the inverse operation to recover the digital bitstream. Cables and satellites are media or platforms, not the active devices that execute modulation/demodulation.

Step-by-Step Solution:
1) Identify the function: convert digital data ↔ analog carrier.2) Map this function to the device class called a “modem.”3) Distinguish from transmission media (fiber, coax) and platforms (satellite), which merely carry signals.4) Conclude that “modem” is the correct answer.

Verification / Alternative check:
Examples include DSL/cable modems, cellular modems, and radio modems—all explicitly named for their modulation/demodulation roles.

Why Other Options Are Wrong:

• Fiber optic / coaxial cable: physical media, not active mod/demod devices.
• Satellite: a communications relay platform; the ground segment still uses modems for waveform conversion.
• None of the above: invalid because a modem fits perfectly.

Common Pitfalls:
Confusing the medium (what the signal travels through) with the modem (what transforms the signal at the endpoints).

Final Answer:
modem