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Chapter 9: Information Theory System and Coding: MCQ Practice for the NEC License Exam (AItE)

This chapter contributes 10 of the 100 questions in the IT Engineering license exam. Below are its subtopics, what is usually tested, and 12 free sample questions with answers and explanations.

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Subtopics in this chapter

What is usually tested

Expect questions on the measure of information, entropy, channel capacity and the Shannon-Hartley theorem, source coding with Huffman and Shannon-Fano codes, and the classification of signals and systems. Signalling questions cover PCM, delta modulation, line codes and multiplexing. The chapter also asks about transmission impairments and noise, error detection and correction (parity, CRC, Hamming codes), flow and error control (ARQ, sliding window, HDLC), and analog and digital modulation.

Sample questions with answers

Try each question before opening the answer. These 12 are taken from the 117 questions for this chapter in the practice bank.

1. Entropy of a source is maximum when its symbols are: (9.1 Introduction of information theory and signals)

  1. Highly repeated
  2. Equiprobable
  3. Binary only
  4. Correlated
Show answer
Answer: B. Equiprobable
Uncertainty is highest when all symbols are equally likely.

2. According to information theory, a message carries more information when its probability of occurrence is: (9.1 Introduction of information theory and signals)

  1. Lower
  2. Higher
  3. Exactly one half
  4. Equal to one
Show answer
Answer: A. Lower
Information content is the logarithm of the inverse of the probability, so rare messages carry more information.

3. The three basic steps of pulse code modulation, in the correct order, are: (9.2 Principles of signaling and circuits)

  1. Quantising, sampling, encoding
  2. Sampling, quantising, encoding
  3. Encoding, sampling, quantising
  4. Sampling, encoding, quantising
Show answer
Answer: B. Sampling, quantising, encoding
The analog signal is sampled, each sample is rounded to a quantisation level, and the level is encoded as a binary word.

4. Companding in a PCM system is used to: (9.2 Principles of signaling and circuits)

  1. Remove the need for a clock at the receiver
  2. Lower the sampling rate below the Nyquist rate
  3. Improve the signal-to-quantisation-noise ratio of low-amplitude signals
  4. Convert the PCM signal into delta modulation
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Answer: C. Improve the signal-to-quantisation-noise ratio of low-amplitude signals
Compressing before quantisation gives small amplitudes finer steps; the receiver expands the signal again.

5. Huffman coding is an example of: (9.3 Principles of signaling and source coding)

  1. Lossless compression
  2. Lossy compression
  3. Encryption
  4. Error correction
Show answer
Answer: A. Lossless compression
Huffman coding is a lossless, variable-length prefix code.

6. The main purpose of source coding is to: (9.3 Principles of signaling and source coding)

  1. Keep the data secret from other users
  2. Protect the transmitted data against channel errors
  3. Shift the signal to a higher frequency band
  4. Represent the source output with as few bits as possible
Show answer
Answer: D. Represent the source output with as few bits as possible
Source coding removes redundancy so that the average number of bits per symbol approaches the entropy.

7. Attenuation in a transmission medium refers to the: (9.4 Impairments, error handling and compression techniques)

  1. Change in the shape of the signal
  2. Loss of signal strength as the signal travels
  3. Addition of unwanted signals
  4. Late arrival of the signal
Show answer
Answer: B. Loss of signal strength as the signal travels
The signal loses energy with distance; amplifiers or repeaters are used to restore it.

8. Delay distortion in a guided medium arises because: (9.4 Impairments, error handling and compression techniques)

  1. The transmitter and receiver clocks differ
  2. All frequency components are weakened equally
  3. Different frequency components travel at different speeds
  4. Signals in neighbouring wires couple together
Show answer
Answer: C. Different frequency components travel at different speeds
The components of a signal arrive at different times, which changes the shape of the received signal.

9. The purpose of flow control at the data link layer is to: (9.5 Data link control and protocol)

  1. Stop the sender from overwhelming the receiver with data
  2. Find the shortest route to the destination
  3. Detect bits that were corrupted in transit
  4. Translate logical addresses into physical ones
Show answer
Answer: A. Stop the sender from overwhelming the receiver with data
The receiver has limited buffer space and processing speed, so the sender's rate must be restrained.

10. In stop-and-wait flow control, the sender: (9.5 Data link control and protocol)

  1. Sends frames continuously without waiting for acknowledgements
  2. Sends one frame and waits for its acknowledgement before sending the next
  3. Sends every frame twice to guard against loss
  4. Waits for a fixed time and never expects acknowledgements
Show answer
Answer: B. Sends one frame and waits for its acknowledgement before sending the next
It is simple but slow, because only one frame can be in transit at a time.

11. Modulation differs from encoding in that modulation: (9.6 Data encoding & modulation)

  1. Removes redundant bits from the data
  2. Represents the data directly as voltage pulses
  3. Varies a characteristic of a carrier signal according to the data
  4. Adds check bits to the data
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Answer: C. Varies a characteristic of a carrier signal according to the data
Encoding produces a digital signal; modulation shifts the information onto an analog carrier.

12. One reason for modulating a baseband signal onto a high-frequency carrier before radio transmission is to: (9.6 Data encoding & modulation)

  1. Make the signal travel faster than light
  2. Reduce the bandwidth of the signal to zero
  3. Remove the need for a receiver
  4. Allow an antenna of practical size to be used
Show answer
Answer: D. Allow an antenna of practical size to be used
An efficient antenna is comparable in size to the wavelength, which is very long at baseband frequencies.

Practise this chapter

The full mock tests mix 10 questions from this chapter with 10 from each of the other nine, under the real 2-hour limit. Chapter practice mode lets you work through this chapter alone, untimed, with the explanation shown after every question.

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