بسم االله الرحمن الرحيم Computer Networks Practice Set I Dr. Hussein Al-Bahadili (1/11)
Q. Circle the right answer. 1. Before data can be transmitted, they must be transformed to. (a) Periodic signals (b) Electromagnetic signals (c) Aperiodic signals (d) Low-frequency sine waves 2. A periodic signal completes one cycle in 0.001 s. What is the frequency? (a) 1 Hz (b) 100 Hz (c) 1 KHz (d) 1 MHz 3. Which of the following can be determined from a frequency-domain graph of a signal? (a) Frequency (b) Phase (c) Power (d) All the above 4. Which of the following can be determined from a frequency-domain graph of a signal? (a) Bandwidth (b) Phase (c) Power (d) All the above 5. In a frequency-domain plot, the vertical axis measures the. (a) Peak amplitude (b) Frequency (c) Phase (d) Slope 6. In a frequency-domain plot, the horizontal axis measures the. (a) Peak amplitude (b) Frequency (c) Phase (d) Slope (2/11)
7. In a time-domain plot, the vertical axis is a measure of. (a) Amplitude (b) Frequency (c) Phase (d) Time 8. In a time-domain plot, the horizontal axis is a measure of. (a) Signal amplitude (b) Frequency (c) Phase (d) Time 9. If the bandwidth of a signal is 5 KHz and the lowest frequency is 52 KHz, what is the highest frequency? (a) 5 KHz (b) 10 KHz (c) 47 KHz (d) 57 KHz 10. What is the bandwidth of a signal that ranges from 40 KHz to 4 MHz? (a) 36 MHz (b) 360 KHz (c) 3.96 MHz (d) 396 KHz 11. When one of the components of a signal has a frequency of zero, the average amplitude of the signal. (a) Is greater than zero (b) Is less than zero (c) Is zero (d) (a) or (b) (3/11)
12. A periodic signal can always be decomposed into. (a) Exactly an odd number of sine waves (b) A set of sine waves (c) A set of sine waves, one of which must have a phase of 0 (d) None of the above 13. As frequency increases, the period. (a) Decreases (b) Increases (c) Remains the same (d) Doubles 14. Given two sine waves A and B, if the frequency of A is twice that of B, then the period of B is that of A. (a) One-half (b) Twice (c) The same as (d) Indeterminate from 15. A sine wave is. (a) Periodic and continuous (b) Aperiodic and continuous (c) Periodic and discrete (d) Aperiodic and discrete 16. If the maximum amplitude of a sine wave is 2 V, the minimum amplitude is V. (a) 2 (b) 1 (c) 2 (d) Between 2 and 2 17. A signal is measured at two different points. The power is P1 at the first point and P2 at the second point. The db is 0. This means. (a) P2 is zero (b) P2 equals P1 (c) P2 is much larger than P1 (d) P2 is much smaller than P1 (4/11)
18. is a type of transmission impairment in which the signal loses strength due to the resistance of the transmission medium. (a) Attenuation (b) Distortion (c) Noise (d) Decibel 19. is a type of transmission impairment in which the signal loses strength due to the different propagation speeds of each frequency that makes up the signal. (a) Attenuation (b) Distortion (c) Noise (d) Decibel 20. is a type of transmission impairment in which an outside source such as crosstalk corrupts a signal. (a) Attenuation (b) Distortion (c) Noise (d) Decibel 21. The has units of meters/second or kilometres/second. (a) Throughput (b) Propagation speed (c) Propagation time (d) (b) or (c) 22. has units of bits/second. (a) Throughput (b) Propagation speed (c) Propagation time (d) (b) or (c) 23. The has units of seconds. (a) Throughput (b) Propagation speed (c) Propagation time (d) (b) or (c) (5/11)
24. When propagation speed is multiplied by propagation time, we get the. (a) Throughput (b) Wavelength of the signal (c) Distortion factor (d) Distance a signal or bit has travelled 25. 43. Propagation time is proportional to distance and proportional to propagation speed. (a) Inversely; directly (b) Directly; inversely (c) Inversely; inversely (d) Directly; directly 26. Wavelength is proportional to propagation speed and proportional to period. (a) Inversely; directly (b) Directly; inversely (c) Inversely; inversely (d) Directly; directly 27. The wavelength of a signal depends on the. (a) Frequencies of the signal (b) Medium (c) Phase of the signal (d) (a) and (b) 28. The wavelength of green light in air is the wavelength of green light in fiber-optic cable. (a) Less than (b) Greater than (c) Equal to (d) None of the above 29. Using the Shannon formula to calculate the data rate for a given channel, if C = B, then. (a) The signal is less than the noise (b) The signal is greater than the noise (c) The signal is equal to the noise (d) Not enough information is given to answer the question (6/11)
Q. Review questions 1. Describe the three characteristics of a sine wave. 2. What is the spectrum of a signal? 3. Contrast an analog signal with a digital signal. 4. A signal has been received that only has values of 1, 0, and 1. Is this an analog or a digital signal? 5. What is the relationship between period and frequency? 6. What are the units of period and frequency? 7. What does the amplitude of a signal measure? 8. What does the frequency of a signal measure? 9. What does the phase of a signal measure? 10. Which type of plot shows the amplitude of a signal at a given time? 11. How can a composite signal be decomposed into its individual frequencies? 12. What is a bit interval, and what is its counterpart in an analog signal? 13. What is bit rate, and what is its counterpart in an analog signal? 14. Name three types of transmission impairment. 15. What does a decibel measure? 16. What is the relationship between propagation speed and propagation time? 17. What is the wavelength of a signal and how is it calculated? 18. What does the Shannon capacity have to do with communications? (7/11)
Q. Exercises 1. Given the frequencies listed below, calculate the corresponding periods. Express the result in seconds, milliseconds, microseconds, nanoseconds, and picoseconds. a. 24 Hz b. 8 MHz c. 140 KHz d. 12 THz 2. Given the following periods, calculate the corresponding frequencies. Express the frequencies in hertz, kilohertz, megahertz, gigahertz, and terahertz. a. 5 s b. 12 μs c. 220 ns d. 81 ps 3. What is the phase shift for the following? a. A sine wave with the maximum amplitude at time zero b. A sine wave with maximum amplitude after 1/4 cycle c. A sine wave with zero amplitude after 3/4 cycle and increasing d. A sine wave with minimum amplitude after 1/4 cycle 4. Show the phase shift in degrees corresponding to each of the following delays in cycles. a. 1 cycle b. 1/2 cycle c. 3/4 cycle d. 1/3 cycle 5. Show the delay in cycles corresponding to each of the following. a. 45 b. 90 c. 60 d. 360 (8/11)
6. Draw the time-domain plot of a sine wave (for only 1 s) with maximum amplitude of 15 V, a frequency of 5, and a phase of 270. 7. Draw two sine waves on the same time-domain plot. The characteristics of each signal are as follows: Signal A: amplitude 40, frequency 9, phase 0; Signal B: amplitude 10, frequency 9, phase 90. 8. Draw two periods of a sine wave with a phase shift of 90. On the same diagram, draw a sine wave with the same amplitude and frequency but with a 90 phase shift from the first. 9. What is the bandwidth of a signal that can be decomposed into four sine waves with frequencies at 0, 20, 50, and 200 Hz? All maximum amplitudes are the same. Draw the frequency spectrum. 10. A periodic composite signal with a bandwidth of 2000 Hz is composed of two sine waves. The first one has a frequency of 100 Hz with a maximum amplitude of 20 V; the second one has a maximum amplitude of 5 V. Draw the frequency spectrum. 11. Show how a sine wave can change its phase by drawing two periods of an arbitrary sine wave with phase shift of 0 followed by the two periods of the same signal with a phase shift of 90. 12. Imagine we have a sine wave called A. Show the negative of A. In other words, show the signal A. Can we relate the negation of a signal to the phase shift? How many degrees? 13. Which signal has a higher bandwidth, a signal that changes 100 times per second or a signal that changes 200 times per second? 14. What is the bit rate for each of the following signals? a. A signal in which 1 bit lasts 0.001 s b. A signal in which 1 bit lasts 2 ms c. A signal in which 10 bits last 20 μs d. A signal in which 1000 bits last 250 ps 15. What is the duration of 1 bit for each of the following signals? a. A signal with a bit rate of 100 bps b. A signal with a bit rate of 200 Kbps c. A signal with a bit rate of 5 Mbps d. A signal with a bit rate of 1 Gbps (9/11)
16. A device is sending out data at the rate of 1000 bps. a. How long does it take to send out 10 bits? b. How long does it take to send out a single character (8 bits)? c. How long does it take to send a file of 100,000 characters? 17. A composite signal contains frequencies from 10 to 30 KHz, each with an amplitude of 10 V. Draw the frequency spectrum. 18. A composite signal contains frequencies from 10 to 30 KHz. The amplitude is zero for the lowest and the highest signals and is 30 V for the 20-KHz signal. Assuming that the amplitudes change gradually from the minimum to the maximum, draw the frequency spectrum. 19. Two signals have the same frequencies. However, whenever the first signal is at its maximum amplitude, the second signal has an amplitude of zero. What is the phase shift between the two signals? 20. What is the mathematical representation of a signal with an amplitude of 10 V, a frequency of 2500 Hz, a phase of 30? 21. Show the frequency domain of the following signal: s(t) = 8 + 3 sin 100πt + 5 sin 200πt 22. What is the period of the following signal? s(t) = 4 sin 628t 23. A cosine wave is a sine wave with a 90 phase shift. Show the equivalent of the following signal in sine format. s(t) = cos (2πft + π) 24. A TV channel has a bandwidth of 6 MHz. If we send a digital signal using one channel, what are the data rates if we use one harmonic, three harmonics, and five harmonics? 25. A signal travels from point A to point B. At point A, the signal power is 100 W. At point B, the power is 90 W. What is the attenuation in decibel? 26. The attenuation of a signal is 10 db. What is the final signal power if it was originally 5 W? 27. A signal has passed through three cascaded amplifiers, each with a 4 db gain. What is the total gain? How much is the signal amplified? (10/11)
28. If the throughput at the connection between a device and the transmission medium is 5 Kbps, how long does it take to send 100,000 bits out of this device? 29. The light of the sun takes approximately eight minutes to reach the earth. What is the distance between the sun and the earth? 30. A signal has a wavelength of 1 μm in air. How far can the front of the wave travel during five periods? 31. A line has a signal-to-noise ratio of 1000 and a bandwidth of 4000 KHz. What is the maximum data rate supported by this line? 32. We measure the performance of a telephone line (4 KHz of bandwidth). When the signal is 10 V, the noise is 5 mv. What is the maximum data rate supported by this telephone line? (11/11)