Topic in Computer Networks & Cloud Computing
Data communication is the exchange of digital data between two devices over a transmission medium such as copper wire, optical fibre or radio. Questions here usually start with the five components of a communication system and the simplex, half-duplex and full-duplex modes, then move to signal theory: bandwidth versus throughput, bit rate versus baud rate, and converting a signal-to-noise ratio into decibels. Expect Nyquist and Shannon capacity calculations, baseband and broadband transmission, synchronous and asynchronous framing, modulation schemes like ASK, FSK and PSK, multiplexing with FDM, TDM and WDM, and impairments such as attenuation, noise and intersymbol interference.
Below are 30 practice questions from a pool of 210 Data Communication MCQs, one of 10 topics in Computer Networks & Cloud Computing. Each shows the correct answer with an explanation; when you are ready, take a timed quiz to test recall under exam conditions.
Each question below shows the correct answer with a full explanation. Use these to build conceptual understanding before attempting a timed quiz.
Q1. What is the primary purpose of data communication?
Data communication is the exchange of data between two devices via some form of transmission medium such as a wire cable or wireless.
Q2. Which of the following is NOT a component of a data communication system?
The five components of a data communication system are sender, receiver, message, transmission medium, and protocol. A compiler is not a component of data communication.
Q3. In simplex mode of data transmission, data flows in:
In simplex mode, the communication is unidirectional. Only one device can send and the other can only receive. Example: keyboard to CPU.
Q4. Which transmission mode allows data to flow in both directions simultaneously?
Full-duplex mode allows data to travel in both directions at the same time. Example: telephone communication.
Q5. What does bandwidth refer to in data communication?
Bandwidth is the range of frequencies that a communication channel can pass, typically measured in Hertz (Hz) or bits per second (bps).
Q6. Walkie-talkie is an example of which transmission mode?
A walkie-talkie uses half-duplex mode where both parties can communicate but not at the same time. One must finish before the other can respond.
Q7. Which of the following is a guided transmission medium?
Twisted pair cable is a guided (wired) transmission medium. Radio waves, microwaves, and infrared are unguided (wireless) media.
Q8. What is the unit of data transmission speed?
Data transmission speed is measured in bits per second (bps). Higher-order units include Kbps, Mbps, and Gbps.
Q9. In data communication, what is a protocol?
A protocol is a set of rules that governs data communication. It defines what is communicated, how it is communicated, and when it is communicated.
Q10. Which signal type has a continuous varying pattern?
An analog signal has a continuous varying pattern that changes smoothly over time. Digital signals have discrete values (typically 0 and 1).
Q11. According to the Nyquist theorem, the maximum data rate of a noiseless channel with bandwidth B and L signal levels is:
The Nyquist theorem states that the maximum data rate = 2B log2(L), where B is bandwidth in Hz and L is the number of signal levels.
Q12. What is the Shannon capacity formula for a noisy channel?
Shannon's capacity formula is C = B log2(1 + SNR), where C is capacity in bps, B is bandwidth in Hz, and SNR is the signal-to-noise ratio.
Q13. Which multiplexing technique divides the bandwidth of a link into frequency bands?
Frequency Division Multiplexing (FDM) divides the bandwidth into multiple frequency bands, each carrying a separate signal.
Q14. In PCM (Pulse Code Modulation), what are the three steps in order?
PCM involves three steps: Sampling (measuring the analog signal at regular intervals), Quantization (rounding to nearest level), and Encoding (converting to binary).
Q15. What type of encoding uses the transition at the middle of a bit interval to represent data?
Manchester encoding uses a transition at the middle of each bit interval. A low-to-high transition represents 1, and high-to-low represents 0 (or vice versa in differential Manchester).
Q16. If a signal-to-noise ratio (SNR) is 1000, what is the SNR in decibels?
SNRdB = 10 log10(SNR) = 10 log10(1000) = 10 * 3 = 30 dB.
Q17. Which type of transmission sends bits one at a time over a single channel?
Serial transmission sends data one bit at a time over a single channel. It is used for long-distance communication.
Q18. In asynchronous serial transmission, what is the purpose of start and stop bits?
Start and stop bits in asynchronous transmission provide synchronization at the character level, alerting the receiver to the beginning and end of each character.
Q19. Wavelength Division Multiplexing (WDM) is conceptually similar to which multiplexing technique?
WDM is conceptually the same as FDM but applied to optical fiber channels. It combines multiple light signals at different wavelengths.
Q20. What is throughput in data communication?
Throughput is the actual rate of data transfer, which may be less than the bandwidth due to congestion, protocol overhead, and other factors.
Q21. A channel has a bandwidth of 4 KHz and SNR of 63. What is the maximum achievable data rate according to Shannon's theorem?
C = B log2(1 + SNR) = 4000 * log2(64) = 4000 * 6 = 24,000 bps = 24 Kbps.
Q22. In CDMA (Code Division Multiple Access), if a station's chip sequence is (-1, -1, -1, +1, +1, -1, +1, +1) and the received data is (-1, -1, -3, +1, -1, -3, +1, +1), what is the station's data bit?
The inner product is computed: (1+1+3+1-1+3+1+1)/8 = 10/8 > 0. Since the normalized result is positive, the data bit is +1.
Q23. What is the main disadvantage of Differential Manchester encoding compared to standard Manchester encoding?
Differential Manchester encoding is more complex to implement because the encoding depends on the previous bit's value, requiring state tracking. Both use twice the bandwidth of NRZ.
Q24. In a QAM-16 constellation diagram, how many bits does each signal element represent?
QAM-16 has 16 different signal combinations, so each element represents log2(16) = 4 bits.
Q25. A signal has passed through 3 cascaded amplifiers, each with a gain of 10 dB. If the original signal power is 5 mW, what is the output power?
Total gain = 10 + 10 + 10 = 30 dB. 30 dB means 10^3 = 1000 times amplification. Output = 5 * 1000 = 5000 mW = 5 W.
Q26. In Statistical Time Division Multiplexing (STDM), what is the key advantage over synchronous TDM?
STDM allocates time slots dynamically based on demand rather than pre-assigning them. This avoids wasting capacity when some sources are idle.
Q27. A digitized voice channel is made by sampling at 8000 samples/sec with 8 bits per sample. What is the required data rate?
Data rate = sampling rate * bits per sample = 8000 * 8 = 64,000 bps = 64 Kbps. This is the standard rate for a single PCM voice channel.
Q28. Which scrambling technique replaces 8 consecutive zeros with a specific violation pattern to maintain synchronization?
B8ZS (Bipolar with 8-Zero Substitution) replaces 8 consecutive zeros with a pattern containing intentional violations to ensure transitions for clock recovery.
Q29. What is the relationship between bit rate and baud rate when using 8-PSK modulation?
In 8-PSK, each signal element represents log2(8) = 3 bits, so the bit rate is 3 times the baud rate.
Q30. In spread spectrum communication, what happens to the signal bandwidth compared to the original data bandwidth?
Spread spectrum deliberately spreads the signal over a much wider bandwidth than required. This makes it resistant to interference and eavesdropping.
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