In data communications terminology, what does “baseband” transmission mean, and which description best fits the term in contrast to modulated carrier systems?
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ATransmission of signals without modulation onto a separate carrier (direct baseband over the medium)
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BA signal whose energy is confined within a finite frequency band (bandlimited signal)
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CSimultaneous broadcast of the same data to many receivers (multicast/broadcast)
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DAll of the above
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ENone of the above
Answer
Correct Answer: Transmission of signals without modulation onto a separate carrier (direct baseband over the medium)
Explanation
Introduction: The term baseband distinguishes direct, unmodulated signaling from modulated carrier transmission (passband). Classic Ethernet on copper, for example, is baseband: the digital waveform is placed directly on the wire without up-conversion to a high-frequency carrier. This item asks you to pick the description that accurately captures baseband in communications contexts.
Given Data / Assumptions:
- We contrast baseband versus passband (carrier-modulated) systems.
- Baseband examples include early 10BASE-T/2/5 Ethernet and many serial links.
- We are looking for the most precise definition, not related but different concepts.
Concept / Approach: Baseband transmission sends the original low-frequency (baseband) signal directly over the medium using a line code (e.g., NRZ, Manchester). No separate sinusoidal carrier is used for up/down conversion. In contrast, passband systems (radio, cable TV) modulate the baseband onto a carrier (AM, FM, QAM). While a baseband signal is indeed bandlimited, “bandlimited” is a more general term that applies to many signals (baseband or passband) and is not the definition of baseband itself. Likewise, “simultaneous transmission to many stations” describes broadcast/multicast behavior, not baseband.
Step-by-Step Solution:
Identify the essential property: no modulation onto a separate carrier.Map to examples: 10BASE-T uses line coding directly on twisted pair.Eliminate descriptions that are true of other concepts (bandlimited, broadcast).Select the definition that explicitly says “without modulation.”Verification / Alternative check: Standards documents label early Ethernet as “BASE” (baseband) in names like 10BASE-T, distinguishing it from broadband coax systems that carry multiple RF channels simultaneously via modulation.
Why Other Options Are Wrong:
- Bandlimited signal: true for many signals but not the definition of baseband.
- Simultaneous transmission: broadcast/multicast concept unrelated to baseband vs passband.
- All of the above: incorrect since the other items are not definitions of baseband.
- None of the above: incorrect because one option matches exactly.
Common Pitfalls: Confusing “baseband” with “unfiltered DC content” (some line codes are DC-balanced); thinking baseband implies single-user only—multiple access can be achieved with time slots even at baseband.
Final Answer: Transmission of signals without modulation onto a separate carrier.