In data communications, what is FDM (Frequency Division Multiplexing) and how does it allow multiple signals to share a single medium?

Introduction / Context:
Frequency Division Multiplexing (FDM) is a classic multiplexing technique used in telecommunications, radio broadcasting, and some networking technologies. It allows several signals to share the same physical medium by separating them in the frequency domain. Interview questions about FDM are common in communications and networking exams to test understanding of how bandwidth is efficiently utilized.

Given Data / Assumptions:

• We have a communication medium such as a copper cable, radio channel, or fiber.
• The medium has a certain total bandwidth (range of frequencies) available.
• Multiple independent signals need to be carried at the same time.
• We can allocate non-overlapping frequency bands to each signal.

Concept / Approach:
FDM divides the available frequency spectrum of a channel into several smaller frequency bands, each assigned to a different signal. Each user's signal is modulated onto a unique carrier frequency so that the signals occupy separate, non-overlapping frequency ranges. At the receiver, filters and demodulators separate the combined signal back into the original components. Because the signals coexist in parallel on different frequencies, they can be transmitted simultaneously without interfering, as long as guard bands and proper filtering are used.

Step-by-Step Solution:
Step 1: Define FDM as a multiplexing scheme that shares a medium by dividing it into multiple frequency bands. Step 2: Explain that each user or channel is assigned a distinct carrier frequency and corresponding band of the spectrum. Step 3: Describe how all modulated signals are combined into a composite signal that travels over the same physical medium. Step 4: At the receiver, state that bandpass filters isolate each frequency band, and demodulators recover the original baseband signals. Step 5: Emphasize that simultaneous transmission is possible because the frequency bands are separated and do not overlap significantly.

Verification / Alternative check:
Real world examples of FDM include traditional analog telephone carrier systems where multiple voice channels share a single trunk line, and cable television systems where different TV channels occupy different RF frequency slots on the same coaxial cable. The presence of distinct, non-overlapping frequency allocations for each channel demonstrates how FDM works in practice. Modern DSL technologies also use forms of FDM (often via discrete multitone) to divide the available frequency range into many narrow subchannels.

Why Other Options Are Wrong:
Option B actually describes Time Division Multiplexing (TDM), where users share the same frequency but in different time slots. Option C conflates FDM with security protocols; while spread spectrum and frequency hopping exist, they are not the same as basic FDM. Option D is unrelated, as cabling standards specify physical characteristics, not how frequencies are shared among signals.

Common Pitfalls:
Students sometimes confuse FDM with TDM or assume multiplexing always involves time slots. Another pitfall is ignoring the need for guard bands and filters; without them, adjacent channels can interfere, causing crosstalk. Understanding that FDM is based on separating signals in the frequency domain rather than in time or code space is essential for mastering basic communication system design.

Final Answer:
FDM is a technique where multiple signals share a single medium by using different frequency bands, allowing simultaneous transmission without interference when properly separated.