Voiceband communications: Which medium is commonly referred to as a voiceband channel suitable for ordinary speech-frequency transmission?

Introduction / Context:
Voiceband refers to the frequency range used for standard telephone audio (roughly 300 Hz to 3.4 kHz). Data communications historically leveraged this range with modems over public switched telephone networks. This question checks recognition of the voiceband medium.

Given Data / Assumptions:

• Voiceband implies the conventional telephony audio band.
• We are selecting the medium that natively supports that band.
• Focus is on classical telephony and data-over-voice techniques (for example, analog modems).

Concept / Approach:

Traditional analog telephone lines are engineered for voiceband frequencies. While data can move across them via modulation, the physical medium and switching were designed around human speech frequencies, defining the concept of a voiceband channel.

Step-by-Step Solution:

Verification / Alternative check:

Historically, modems (for example, V.32, V.34) transmitted within the voiceband over telephone lines. This validates that the PSTN voice channel is the canonical voiceband medium.

Why Other Options Are Wrong:

• Telegraph line: Associated with low-speed signaling, not a standard voiceband telephony path.
• Coaxial cable: Supports wideband services, not limited to voiceband.
• Microwave systems: High-frequency, wide-area backhaul; not voiceband-limited.
• None of the above: Incorrect because telephone line is correct.

Common Pitfalls:

Equating any copper medium with voiceband. Coax can carry TV and broadband; voiceband is a functional bandwidth concept tied to PSTN analog loops, not just copper presence.

Final Answer:

Telephone line