Interview questions & answers
Q1. What is multiplexing and why is it used in communication systems?
Multiplexing is the process of combining multiple signals onto a single transmission channel to maximize bandwidth utilization. In a T1 line, 24 voice channels are time-division multiplexed over one 1.544 Mbps link instead of running 24 separate cables. Without multiplexing, the cost of transmission infrastructure would scale linearly with the number of users, making large telecom networks economically unviable.
Follow-up: What happens when the number of users exceeds the available time slots or frequency bands?
Q2. Explain the difference between TDM and FDM.
TDM assigns each user a dedicated time slot in a repeating frame, while FDM assigns each user a dedicated frequency band simultaneously. GSM uses TDM within each 200 kHz carrier, whereas analog cable TV uses FDM to place each channel at a different RF frequency between 54 MHz and 860 MHz. TDM is more flexible for digital systems because slot allocation can be changed in software, while FDM guard bands permanently waste spectrum.
Follow-up: Which of TDM or FDM is more affected by nonlinearity in the channel?
Q3. What is a guard band in FDM and why is it necessary?
A guard band is an unused frequency gap inserted between adjacent channels in FDM to prevent inter-channel interference caused by imperfect filters. In AM broadcasting, a 10 kHz guard band separates channels spaced at 10 kHz centers, even though audio content only uses about 5 kHz. If guard bands are too narrow, adjacent channel leakage bleeds into neighboring carriers and degrades SNR, which is why filter roll-off characteristics directly determine the minimum guard band needed.
Follow-up: How does OFDM eliminate the need for guard bands between subcarriers?
Q4. What is synchronization in TDM and how is it achieved?
Synchronization in TDM means the receiver must identify frame boundaries so it correctly demultiplexes each channel from the right time slot. In T1 framing, a dedicated framing bit at position 1 of each 193-bit frame alternates in a specific pattern that the receiver locks onto. Loss of frame sync causes all channels to be misassigned simultaneously, which is why TDM equipment monitors framing bit errors continuously and triggers alarms on sync loss.
Follow-up: What is the difference between bit synchronization and frame synchronization?
Q5. What is statistical TDM and how does it differ from synchronous TDM?
Statistical TDM allocates time slots dynamically to users who actually have data to send, rather than reserving a fixed slot for every user regardless of activity. A DSL multiplexer uses statistical TDM so that 100 subscribers sharing a 10 Mbps uplink can each burst at higher rates when others are idle. The trade-off is that headers must identify which user owns each slot, adding overhead that synchronous TDM avoids entirely.
Follow-up: What happens to delay and jitter in statistical TDM under heavy load?
Q6. How does WDM work in fiber optic systems?
WDM places each data stream on a separate optical wavelength, effectively making one fiber behave like many parallel fibers. Dense WDM systems like those used by Airtel backbone links carry 80 or more wavelengths spaced 0.8 nm apart on a single single-mode fiber. The key hardware is the arrayed waveguide grating, which separates wavelengths at the receiver with less than 0.5 dB insertion loss per channel.
Follow-up: What limits the number of channels you can pack into a DWDM system?
Q7. What is CDM or CDMA and how does it differ from TDM and FDM?
CDMA gives every user the full bandwidth simultaneously but assigns each a unique spreading code that orthogonalizes them in the code domain. In IS-95 CDMA, each voice channel is spread across 1.25 MHz using a 64-chip Walsh code, and all users transmit at the same time on the same frequency. Unlike TDM or FDM, CDMA capacity degrades gracefully as users are added rather than dropping abruptly when slots or bands run out.
Follow-up: What is the near-far problem in CDMA and how is it solved?
Q8. What is the Nyquist channel capacity and how does it apply to TDM design?
Nyquist capacity states that a noiseless channel of bandwidth B Hz can carry at most 2B symbols per second, which sets the upper bound on how fast individual TDM slots can be filled. For a 4 kHz voice channel sampled at 8 kHz with 8-bit PCM, each user contributes 64 kbps, and the TDM frame rate must sustain 8000 frames per second. Exceeding the Nyquist rate causes inter-symbol interference even before noise is considered.
Follow-up: How does Shannon capacity differ from Nyquist capacity?
Q9. Explain how PCM and TDM work together in telephony.
PCM converts each analog voice channel into a stream of 8-bit samples at 8000 samples per second, and TDM interleaves these sample bytes from multiple channels into a single high-speed serial stream. In E1, 30 voice channels plus two signaling channels produce a 2.048 Mbps stream where each channel occupies one 8-bit time slot in a 256-bit frame repeated 8000 times per second. The frame structure itself is rigid, so a single corrupted framing byte disrupts all 30 channels simultaneously.
Follow-up: Why is the sampling rate 8 kHz specifically for voice channels?
Q10. What is crosstalk in FDM and what causes it?
Crosstalk in FDM is the unwanted coupling of one channel's signal into an adjacent channel, caused by insufficient filter attenuation or amplifier nonlinearity producing intermodulation products. In analog PSTN FDM carrier systems, a 0 dBm tone in one channel could produce -60 dBm crosstalk in adjacent channels if the channel filters had only 40 dB stopband attenuation instead of the required 60 dB. Intermodulation crosstalk is more problematic than linear crosstalk because its products fall at frequencies that filters cannot separate.
Follow-up: How does nonlinearity produce third-order intermodulation products in FDM?
Q11. What is the frame efficiency of a TDM system?
Frame efficiency is the ratio of payload bits to total bits in a frame, accounting for framing overhead, synchronization bits, and signaling slots. In E1, the frame has 256 bits: 248 are payload and 8 are framing and signaling, giving about 96.9% efficiency. Systems that add CRC and alarm bits reduce efficiency further, so designers trade efficiency against the robustness of the management overhead.
Follow-up: How would you calculate the minimum overhead needed to guarantee frame sync recovery within N frames?
Q12. How does OFDM overcome multipath fading compared to single-carrier FDM?
OFDM splits a high-data-rate stream into hundreds of low-rate subcarriers, making each subcarrier's symbol period much longer than the channel delay spread so multipath echoes do not cause inter-symbol interference within a symbol. LTE uses 15 kHz subcarrier spacing with a 66.7 µs symbol period and adds a 4.7 µs cyclic prefix longer than typical urban delay spreads of 1-2 µs. The cyclic prefix is the key: it converts linear convolution with the channel into circular convolution, which a single complex multiply per subcarrier corrects.
Follow-up: What is the cost of the cyclic prefix in terms of spectral efficiency?
Q13. What is the difference between synchronous and asynchronous TDM?
Synchronous TDM reserves a fixed slot for every possible user in every frame regardless of whether that user has data, while asynchronous TDM only creates a slot when a user actually has data and tags it with an address. SONET is synchronous TDM: an OC-3 frame always has exactly 2430 bytes regardless of traffic load. ATM is asynchronous: 53-byte cells are emitted only when a virtual circuit has payload, so lightly loaded connections consume zero bandwidth.
Follow-up: Why does synchronous TDM guarantee bounded latency while asynchronous TDM does not?
Q14. What is a multiplexer hierarchy and give an example?
A multiplexer hierarchy stacks multiple levels of TDM so that lower-speed tributaries are first multiplexed, then those outputs are multiplexed again to reach higher line rates. The PDH hierarchy combines four 2.048 Mbps E1 streams into one 8.448 Mbps E2, four E2s into one E3 at 34.368 Mbps, and four E3s into one E4 at 139.264 Mbps. Justification bits are added at each level to absorb clock differences between tributaries, which is why PDH bit rates are not exact multiples of lower levels.
Follow-up: Why did SDH/SONET replace PDH despite PDH being simpler?
Q15. What is subband coding and how is it related to FDM?
Subband coding splits a wideband signal into frequency subbands using a filter bank, encodes each band independently, then recombines them — it is essentially an FDM applied to a single source signal for compression purposes. In MP3 audio, a polyphase filter bank divides audio into 32 subbands of 689 Hz each, and bits are allocated to each band based on psychoacoustic masking models. Treating each subband as an independent FDM channel allows the encoder to discard masked subbands entirely, which is impossible in wideband time-domain coding.
Follow-up: How does the bit allocation across subbands change with signal content?
Common misconceptions
Misconception: TDM and FDM cannot be used together in the same system.
Correct: GSM simultaneously uses FDM to divide the spectrum into 200 kHz carriers and TDM to divide each carrier into 8 time slots, so both are routinely combined.
Misconception: Guard bands in FDM are wasted spectrum that could be avoided with better design.
Correct: Guard bands are physically necessary because real bandpass filters have finite roll-off and cannot achieve a brick-wall cutoff at the channel edge.
Misconception: CDMA is a form of TDM because users take turns transmitting.
Correct: In CDMA all users transmit simultaneously on the same frequency; orthogonality is achieved through spreading codes, not time or frequency separation.
Misconception: Increasing the number of TDM channels always reduces per-channel latency.
Correct: Adding channels increases frame length or frame rate, which increases the waiting time each channel experiences before its slot arrives, so latency grows with channel count.