Subtopics in this chapter
- 9.1Introduction of information theory and signals
- 9.2Principles of signaling and circuits
- 9.3Principles of signaling and source coding
- 9.4Impairments, error handling and compression techniques
- 9.5Data link control and protocol
- 9.6Data encoding & modulation
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
Choose an answer to check it, or open "Show answer". These 12 are taken from the 117 questions for this chapter in the practice bank.
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Entropy of a source is maximum when its symbols are:
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Answer: B. Equiprobable
Uncertainty is highest when all symbols are equally likely.
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According to information theory, a message carries more information when its probability of occurrence is:
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Answer: A. Lower
Information content is the logarithm of the inverse of the probability, so rare messages carry more information.
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The three basic steps of pulse code modulation, in the correct order, are:
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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.
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Companding in a PCM system is used to:
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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.
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Huffman coding is an example of:
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Answer: A. Lossless compression
Huffman coding is a lossless, variable-length prefix code.
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The main purpose of source coding is to:
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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.
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Attenuation in a transmission medium refers to the:
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Answer: B. Loss of signal strength as the signal travels
The signal loses energy with distance; amplifiers or repeaters are used to restore it.
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Delay distortion in a guided medium arises because:
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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.
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The purpose of flow control at the data link layer is to:
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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.
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In stop-and-wait flow control, the sender:
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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.
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Modulation differs from encoding in that modulation:
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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.
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One reason for modulating a baseband signal onto a high-frequency carrier before radio transmission is to:
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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. Go to the mock tests