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Bài tập Tryền Số Liệu
DATA COMMUNICATIONS AND NETWORKING
Chapter 1:
Review Questions
1. Identify the five components of a data communications system.
2. What are the advantages of distributed processing?
3. What are the three criteria necessary for an effective and efficient network?
4. What are the advantages of a multipoint connection over a point-to-point connection?
5. What are the two types of line configuration?
6. Categorize the four basic topologies in terms of line configuration.
7. What is the difference between half-duplex and full-duplex transmission modes?
8. Name the four basic network topologies, and cite an advantage of each type.
9. For n devices in a network, what is the number of cable links required for a mesh, ring,
bus, and star topology?
10. What are some of the factors that determine whether a communication system is a
LAN or WAN?
1I. What is an internet? What is the Internet?
12. Why are protocols needed?
13. Why are standards needed?
Exercises
14. What is the maximum number of characters or symbols that can be represented by
Unicode?
15. A color image uses 16 bits to represent a pixel. What is the maximum number of
different colors that can be represented?
16. Assume six devices are arranged in a mesh topology. How many cables are needed?
How many ports are needed for each device?
17. For each of the following four networks, discuss the consequences if a connection
fails.
a. Five devices arranged in a mesh topology
b. Five devices arranged in a star topology (not counting the hub)
c. Five devices arranged in a bus topology

d. Five devices arranged in a ring topology
18. You have two computers connected by an Ethernet hub at home. Is this a LAN, a
MAN, or a WAN? Explain your reason.
19. In the ring topology in Figure 1.8, what happens if one of the stations is unplugged?
20. In the bus topology in Figure 1.7, what happens if one ofthe stations is unplugged?
21. Draw a hybrid topology with a star backbone and three ring networks.
22. Draw a hybrid topology with a ring backbone and two bus networks.
23. Performance is inversely related to delay. When you use the Internet, which of the
following applications are more sensitive to delay?
a. Sending an e-mail
b. Copying a file
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c. Surfing the Internet
24. When a party makes a local telephone call to another party, is this a point-to-point
or multipoint connection? Explain your answer.
25. Compare the telephone network and the Internet. What are the similarities? What
are the differences?
Research Activities
26. Using the site \\iww.cne.gmu.edu/modules/network/osi.html, discuss the OSI model.
27. Using the site www.ansi.org, discuss ANSI's activities.
28. Using the site www.ieee.org, discuss IEEE's activities.
29. Using the site www.ietf.org/, discuss the different types of RFCs.
Chapter 2: Network Models
Review Questions
I. List the layers of the Internet model.
2. Which layers in the Internet model are the network support layers?
3. Which layer in the Internet model is the user support layer?
4. What is the difference between network layer delivery and transport layer delivery?
5. What is a peer-to-peer process?

6. How does information get passed from one layer to the next in the Internet
model?
7. What are headers and trailers, and how do they get added and removed?
X. What are the concerns of the physical layer in the Internet model?
9. What are the responsibilities of the data link layer in the Internet model?
10. What are the responsibilities of the network layer in the Internet model?
II. What are the responsibilities of the transport layer in the Internet model?
12. What is the difference between a port address, a logical address, and a physical
address?
13. Name some services provided by the application layer in the Internet model.
14. How do the layers of the Internet model correlate to the layers of the OSI model?
Exercises
15. How are OSI and ISO related to each other?
16. Match the following to one or more layers of the OSI model:
a. Route determination
b. Flow control
c. Interface to transmission media
d. Provides access for the end user
I 7. Match the following to one or more layers of the OSI model:
a. Reliable process-to-process message delivery
b. Route selection
c. Defines frames
d. Provides user services such as e-mail and file transfer
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e. Transmission of bit stream across physical medium
\ 8. Match the following to one or more layers of the OSl model:
a. Communicates directly with user's application program
b. Error correction and retransmission
c. Mechanical, electrical, and functional interface

d. Responsibility for carrying frames between adjacent nodes
I 9. Match the following to one or more layers of the OSI model:
a. Format and code conversion services
b. Establishes, manages, and terminates sessions
c. Ensures reliable transmission of data
d. Log-in and log-out procedures
e. Provides independence from differences in data representation
20. In Figure 2.22, computer A sends a message to computer D via LANl, router Rl,
and LAN2. Show the contents of the packets and frames at the network and data
link layer for each hop interface.
21. In Figure 2.22, assume that the communication is between a process running at
computer A with port address i and a process running at computer D with port
address j. Show the contents of packets and frames at the network, data link, and
transport layer for each hop.
22. Suppose a computer sends a frame to another computer on a bus topology LAN.
The physical destination address of the frame is corrupted during the transmission.
What happens to the frame? How can the sender be informed about the situation?
23. Suppose a computer sends a packet at the network layer to another computer
somewhere in the Internet. The logical destination address of the packet is corrupted.
What happens to the packet? How can the source computer be informed of
the situation?
24. Suppose a computer sends a packet at the transport layer to another computer
somewhere in the Internet. There is no process with the destination port address
running at the destination computer. What will happen?
25. If the data link layer can detect errors between hops, why do you think we need
another checking mechanism at the transport layer?
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Research Activities
26. Give some advantages and disadvantages of combining the session, presentation,

and application layer in the OSI model into one single application layer in the
Internet model.
27. Dialog control and synchronization are two responsibilities of the session layer in
the OSI model. Which layer do you think is responsible for these duties in the
Internet model? Explain your answer.
28. Translation, encryption, and compression are some of the duties of the presentation
layer in the OSI model. Which layer do you think is responsible for these duties in
the Internet model? Explain your answer.
29. There are several transport layer models proposed in the OSI model. Find all of
them. Explain the differences between them.
30. There are several network layer models proposed in the OSI model. Find all of
them. Explain the differences between them.
Physical Layer and Media:
CHAPTER 3
Data and Signals
Review Questions
1. What is the relationship between period and frequency?
2. What does the amplitude of a signal measure? What does the frequency of a signal
measure? What does the phase of a signal measure?
3. How can a composite signal be decomposed into its individual frequencies?
4. Name three types of transmission impairment.
5. Distinguish between baseband transmission and broadband transmission.
6. Distinguish between a low-pass channel and a band-pass channel.
7. What does the Nyquist theorem have to do with communications?
8. What does the Shannon capacity have to do with communications?
9. Why do optical signals used in fiber optic cables have a very short wave length?
10. Can we say if a signal is periodic or nonperiodic by just looking at its frequency
domain plot? How?
11. Is the frequency domain plot of a voice signal discrete or continuous?
12. Is the frequency domain plot of an alarm system discrete or continuous?

13. We send a voice signal from a microphone to a recorder. Is this baseband or broadband
transmission?
14. We send a digital signal from one station on a LAN to another station. Is this baseband
or broadband transmission?
15. We modulate several voice signals and send them through the air. Is this baseband
or broadband transmission?
Exercises
16. Given the frequencies listed below, calculate the corresponding periods.
a. 24Hz
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b. 8 MHz
c. 140 KHz
17. Given the following periods, calculate the corresponding frequencies.
a. 5 s
b. 12 Jls
c. 220 ns
18. What is the phase shift for the foIlowing?
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
19. What is the bandwidth of a signal that can be decomposed into five sine waves
with frequencies at 0, 20, 50, 100, and 200 Hz? All peak amplitudes are the same.
Draw the bandwidth.
20. 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 bandwidth.
21. Which signal has a wider bandwidth, a sine wave with a frequency of 100 Hz or a
sine wave with a frequency of 200 Hz?
22. 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 J-ls
23. 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?
24. What is the bit rate for the signal in Figure 3.34?
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27. A periodic composite signal contains frequencies from 10 to 30 KHz, each with an
amplitude of 10 V. Draw the frequency spectrum.
2K. A non-periodic composite signal contains frequencies from 10 to 30 KHz. The
peak amplitude is 10 V 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.
20. 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?
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30. 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 decibels?
31. The attenuation of a signal is -10 dB. What is the final signal power if it was originally
5 W?
32. 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?
33. If the bandwidth of the channel is 5 Kbps, how long does it take to send a frame of
100,000 bits out of this device?
3cf. The light of the sun takes approximately eight minutes to reach the earth. What is

the distance between the sun and the earth?
35. A signal has a wavelength of 1 11m in air. How far can the front of the wave travel
during 1000 periods?
36. 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?
37. 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?
3X. A file contains 2 million bytes. How long does it take to download this file using a
56-Kbps channel? 1-Mbps channel?
39. A computer monitor has a resolution of 1200 by 1000 pixels. If each pixel uses
1024 colors, how many bits are needed to send the complete contents of a screen?
40. A signal with 200 milliwatts power passes through 10 devices, each with an average
noise of 2 microwatts. What is the SNR? What is the SNRdB?
4 I. If the peak voltage value of a signal is 20 times the peak voltage value of the noise,
what is the SNR? What is the SNRdB?
42. What is the theoretical capacity of a channel in each of the following cases:
a. Bandwidth: 20 KHz SNRdB =40
b. Bandwidth: 200 KHz SNRdB =4
c. Bandwidth: 1 MHz SNRdB =20
43. We need to upgrade a channel to a higher bandwidth. Answer the following
questions:
a. How is the rate improved if we double the bandwidth?
b. How is the rate improved if we double the SNR?
44. We have a channel with 4 KHz bandwidth. If we want to send data at 100 Kbps,
what is the minimum SNRdB? What is SNR?
45. What is the transmission time of a packet sent by a station if the length of the
packet is 1 million bytes and the bandwidth of the channel is 200 Kbps?
46. What is the length of a bit in a channel with a propagation speed of 2 x 108 mls if
the channel bandwidth is

a. 1 Mbps?
h. 10 Mbps?
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c. 100 Mbps?
4 7. How many bits can fit on a link with a 2 ms delay if the bandwidth of the link is
a. 1 Mbps?
h. 10 Mbps?
c. 100 Mbps?
48. What is the total delay (latency) for a frame of size 5 million bits that is being sent
on a link with 10 routers each having a queuing time of 2 Ils and a processing time
of 1 Ils. The length of the link is 2000 Km. The speed of light inside the link is 2 x
108 mls. The link has a bandwidth of 5 Mbps. Which component of the total delay
is dominant? Which one is negligible?
CHAPTER 4
Digital Transmission
Review Questions
1. List three techniques of digital-to-digital conversion.
2. Distinguish between a signal element and a data element.
3. Distinguish between data rate and signal rate.
4. Define baseline wandering and its effect on digital transmission.
5. Define a DC component and its effect on digital transmission.
6. Define the characteristics of a self-synchronizing signal.
7. List five line coding schemes discussed in this book.
8. Define block coding and give its purpose.
9. Define scrambling and give its purpose.
10. Compare and contrast PCM and DM.
11. What are the differences between parallel and serial transmission?
12. List three different techniques in serial transmission and explain the differences.
Exercises

13. Calculate the value of the signal rate for each case in Figure 4.2 if the data rate is
1 Mbps and c = 1/2.
14. In a digital transmission, the sender clock is 0.2 percent faster than the receiver clock.
How many extra bits per second does the sender send if the data rate is 1 Mbps?
15. Draw the graph of the NRZ-L scheme using each of the following data streams,
assuming that the last signa11evel has been positive. From the graphs, guess the
bandwidth for this scheme using the average number of changes in the signal level.
Compare your guess with the corresp.onding entry in Table 4.1.
a. 00000000
b. 11111111
c. 01010101
d. 00110011
16. Repeat Exercise 15 for the NRZ-I scheme.
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17. Repeat Exercise 15 for the Manchester scheme.
18. Repeat Exercise 15 for the differential Manchester scheme.
19. Repeat Exercise 15 for the 2B 1Q scheme, but use the following data streams.
a. 0000000000000000
b. 1111111111111111
c. 0101010101010101
d. 0011001100110011
20. Repeat Exercise 15 for the MLT-3 scheme, but use the following data streams.
a. 00000000
b. 11111111
c. 01010101
d. 00011000
22. An NRZ-I signal has a data rate of 100 Kbps. Using Figure 4.6, calculate the value
of the normalized energy (P) for frequencies at 0 Hz, 50 KHz, and 100 KHz.
23. A Manchester signal has a data rate of 100 Kbps. Using Figure 4.8, calculate the

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value of the normalized energy (P) for frequencies at 0 Hz, 50 KHz, 100 KHz.
24. The input stream to a 4B/5B block encoder is 0100 0000 0000 0000 0000 OOOI.
Answer the following questions:
a. What is the output stream?
b. What is the length of the longest consecutive sequence of Os in the input?
c. What is the length of the longest consecutive sequence of Os in the output?
25. How many invalid (unused) code sequences can we have in 5B/6B encoding? How
many in 3B/4B encoding?
26. What is the result of scrambling the sequence 11100000000000 using one of the
following scrambling techniques? Assume that the last non-zero signal level has
been positive.
a. B8ZS
b. HDB3 (The number of nonzero pules is odd after the last substitution)
27. What is the Nyquist sampling rate for each of the following signals?
a. A low-pass signal with bandwidth of 200 KHz?
b. A band-pass signal with bandwidth of 200 KHz if the lowest frequency is
100 KHz?
28. We have sampled a low-pass signal with a bandwidth of 200 KHz using 1024 levels
of quantization.
a. Calculate the bit rate of the digitized signal.
b. Calculate the SNRdB for this signal.
c. Calculate the PCM bandwidth of this signal.
29. What is the maximum data rate of a channel with a bandwidth of 200 KHz if we
use four levels of digital signaling.
30. An analog signal has a bandwidth of 20 KHz. If we sample this signal and send it
through a 30 Kbps channel what is the SNRdB ?
31. We have a baseband channel with a I-MHz bandwidth. What is the data rate for
this channel if we use one of the following line coding schemes?

a. NRZ-L
b. Manchester
c. MLT-3
d. 2B1Q
32. We want to transmit 1000 characters with each character encoded as 8 bits.
a. Find the number of transmitted bits for synchronous transmission.
b. Find the number of transmitted bits for asynchronous transmission.
c. Find the redundancy percent in each case.
CHAPTERS 5
Analog Transmission
5.6 PRACTICE SET
Review Questions
1. Define analog transmission.
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2. Define carrier signal and its role in analog transmission.
3. Define digital-to-analog conversion.
4. Which characteristics of an analog signal are changed to represent the digital signal
in each of the following digital-to-analog conversion?
a. ASK
b. FSK
c. PSK
d. QAM
5. Which of the four digital-to-analog conversion techniques (ASK, FSK, PSK or
QAM) is the most susceptible to noise? Defend your answer.
6. Define constellation diagram and its role in analog transmission.
7. What are the two components of a signal when the signal is represented on a con- .
stellation diagram? Which component is shown on the horizontal axis? Which is
shown on the vertical axis?
8. Define analog-to-analog conversion?

9. Which characteristics of an analog signal are changed to represent the lowpass analog
signal in each of the following analog-to-analog conversions?
a. AM
b. FM
c. PM
] 0. Which of the three analog-to-analog conversion techniques (AM, FM, or PM) is
the most susceptible to noise? Defend your answer.
Exercises
11. Calculate the baud rate for the given bit rate and type of modulation.
a. 2000 bps, FSK
b. 4000 bps, ASK
c. 6000 bps, QPSK
d. 36,000 bps, 64-QAM
12. Calculate the bit rate for the given baud rate and type of modulation.
a. 1000 baud, FSK
b. 1000 baud, ASK
c. 1000 baud, BPSK
d. 1000 baud, 16-QAM
13. What is the number of bits per baud for the following techniques?
a. ASK with four different amplitudes
b. FSK with 8 different frequencies
c. PSK with four different phases
d. QAM with a constellation of 128 points.
14. Draw the constellation diagram for the following:
a. ASK, with peak amplitude values of 1 and 3
b. BPSK, with a peak amplitude value of 2
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c. QPSK, with a peak amplitude value of 3
d. 8-QAM with two different peak amplitude values, I and 3, and four different

phases.
15. Draw the constellation diagram for the following cases. Find the peak amplitude
value for each case and define the type of modulation (ASK, FSK, PSK, or QAM).
The numbers in parentheses define the values of I and Q respectively.
a. Two points at (2, 0) and (3, 0).
b. Two points at (3, 0) and (-3, 0).
c. Four points at (2, 2), (-2, 2), (-2, -2), and (2, -2).
d. Two points at (0 , 2) and (0, -2).
16. How many bits per baud can we send in each of the following cases if the signal
constellation has one of the following number of points?
a. 2
b. 4
c. 16
d. 1024
17. What is the required bandwidth for the following cases if we need to send 4000 bps?
Let d = 1.
a. ASK
b. FSK with 2~f =4 KHz
c. QPSK
d. 16-QAM
18. The telephone line has 4 KHz bandwidth. What is the maximum number of bits we
can send using each of the following techniques? Let d = O.
a. ASK
b. QPSK
c. 16-QAM
d.64-QAM
19. A corporation has a medium with a I-MHz bandwidth (lowpass). The corporation
needs to create 10 separate independent channels each capable of sending at least
10 Mbps. The company has decided to use QAM technology. What is the minimum
number of bits per baud for each channel? What is the number of points in

the constellation diagram for each channel? Let d =O.
20. A cable company uses one of the cable TV channels (with a bandwidth of 6 MHz)
to provide digital communication for each resident. What is the available data rate
for each resident if the company uses a 64-QAM technique?
21. Find the bandwidth for the following situations if we need to modulate a 5-KHz
voice.
22. Find the total number of channels in the corresponding band allocated by FCC.
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a. AM
b. FM
CHAPTER 6
Bandwidth Utilization:
Multiplexing and Spreading
Review Questions
1. Describe the goals of multiplexing.
2. List three main multiplexing techniques mentioned in this chapter.
3. Distinguish between a link and a channel in multiplexing.
4. Which of the three multiplexing techniques is (are) used to combine analog signals?
Which of the three multiplexing techniques is (are) used to combine digital signals?
5. Define the analog hierarchy used by telephone companies and list different levels
of the hierarchy.
6. Define the digital hierarchy used by telephone companies and list different levels
of the hierarchy.
7. Which of the three multiplexing techniques is common for fiber optic links?
Explain the reason.
8. Distinguish between multilevel TDM, multiple slot TDM, and pulse-stuffed TDM.
9. Distinguish between synchronous and statistical TDM.
10. Define spread spectrum and its goal. List the two spread spectrum techniques
discussed

in this chapter.
11. Define FHSS and explain how it achieves bandwidth spreading.
12. Define DSSS and explain how it achieves bandwidth spreading.
Exercises
13. Assume that a voice channel occupies a bandwidth of 4 kHz. We need to multiplex
10 voice channels with guard bands of 500 Hz using FDM. Calculate the required
bandwidth.
14. We need to transmit 100 digitized voice channels using a pass-band channel of
20 KHz. What should be the ratio of bits/Hz if we use no guard band?
15. In the analog hierarchy of Figure 6.9, find the overhead (extra bandwidth for guard
band or control) in each hierarchy level (group, supergroup, master group, and
jumbo group).
16. We need to use synchronous TDM and combine 20 digital sources, each of 100 Kbps.
Each output slot carries 1 bit from each digital source, but one extra bit is added to
each frame for synchronization. Answer the following questions:
a. What is the size of an output frame in bits?
b. What is the output frame rate?
c. What is the duration of an output frame?
d. What is the output data rate?
e. What is the efficiency of the system (ratio of useful bits to the total bits).
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17. Repeat Exercise 16 if each output slot carries 2 bits from each source.
18. We have 14 sources, each creating 500 8-bit characters per second. Since only some
of these sources are active at any moment, we use statistical TDM to combine these
sources using character interleaving. Each frame carries 6 slots at a time, but we need
to add four-bit addresses to each slot. Answer the following questions:
a. What is the size of an output frame in bits?
b. What is the output frame rate?
c. What is the duration of an output frame?

d. What is the output data rate?
19. Ten sources, six with a bit rate of 200 kbps and four with a bit rate of 400 kbps are
to be combined using multilevel TDM with no synchronizing bits. Answer the following
questions about the final stage of the multiplexing:
a. What is the size of a frame in bits?
b. What is the frame rate?
c. What is the duration of a frame?
d. What is the data rate?
20. Four channels, two with a bit rate of 200 kbps and two with a bit rate of 150 kbps, are
to be multiplexed using multiple slot TDM with no synchronization bits. Answer
the following questions:
a. What is the size of a frame in bits?
b. What is the frame rate?
c. What is the duration of a frame?
d. What is the data rate?
21. Two channels, one with a bit rate of 190 kbps and another with a bit rate of 180 kbps,
are to be multiplexed using pulse stuffing TDM with no synchronization bits. Answer
the following questions:
a. What is the size of a frame in bits?
b. What is the frame rate?
c. What is the duration of a frame?
d. What is the data rate?
22. Answer the following questions about a T-1 line:
a. What is the duration of a frame?
b. What is the overhead (number of extra bits per second)?
23. Show the contents of the five output frames for a synchronous TDM multiplexer
that combines four sources sending the following characters. Note that the characters
are sent in the same order that they are typed. The third source is silent.
a. Source 1 message: HELLO
b. Source 2 message: HI

c. Source 3 message:
d. Source 4 message: BYE
24. Figure 6.34 shows a multiplexer in a synchronous TDM system. Each output slot is
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only 10 bits long (3 bits taken from each input plus 1 framing bit). What is the output
stream? The bits arrive at the multiplexer as shown by the arrows.
26. Answer the following questions about the digital hierarchy in Figure 6.23:
a. What is the overhead (number of extra bits) in the DS-l service?
b. What is the overhead (number of extra bits) in the DS-2 service?
c. What is the overhead (number of extra bits) in the DS-3 service?
d. What is the overhead (number of extra bits) in the DS-4 service?
27. What is the minimum number of bits in a PN sequence if we use FHSS with a
channel bandwidth of B =4 KHz and Bss =100 KHz?
28. An FHSS system uses a 4-bit PN sequence. If the bit rate of the PN is 64 bits per
second, answer the following questions:
a. What is the total number of possible hops?
b. What is the time needed to finish a complete cycle of PN?
29. A pseudorandom number generator uses the following formula to create a random
series:
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Ni+1 =(5 +7Ni) mod 17-1
In which Nj defines the current random number and Nj +1 defines the next random
number. The term mod means the value of the remainder when dividing (5 + 7Nj )
by 17.
30. We have a digital medium with a data rate of 10 Mbps. How many 64-kbps voice
channels can be carried by this medium if we use DSSS with the Barker sequence?
CHAPTER 7 : Transmission Media
Review Questions

1. What is the position of the transmission media in the OSI or the Internet model?
2. Name the two major categories of transmission media.
3. How do guided media differ from unguided media?
4. What are the three major classes of guided media?
5. What is the significance of the twisting in twisted-pair cable?
6. What is refraction? What is reflection?
7. What is the purpose of cladding in an optical fiber?
8. Name the advantages of optical fiber over twisted-pair and coaxial cable.
9. How does sky propagation differ from line-of-sight propagation?
10. What is the difference between omnidirectional waves and unidirectional waves?
Exercises
11. Using Figure 7.6, tabulate the attenuation (in dB) of a 18-gauge UTP for the indicated
frequencies and distances.
12. Use the result of Exercise 11 to infer that the bandwidth of a UTP cable decreases
with an increase in distance.
13. If the power at the beginning of a 1 KIn 18-gauge UTP is 200 mw, what is the
power at the end for frequencies 1 KHz, 10 KHz, and 100 KHz? Use the result of
Exercise 11.
14. Using Figure 7.9, tabulate the attenuation (in dB) of a 2.6/9.5 mm coaxial cable for
the indicated frequencies and distances.
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15. Use the result of Exercise 14 to infer that the bandwidth of a coaxial cable
decreases with the increase in distance.
16. If the power at the beginning of a 1 KIn 2.6/9.5 mm coaxial cable is 200 mw, what
is the power at the end for frequencies 1 KHz, 10KHz, and 100 KHz? Use the
result of Exercise 14.
17. Calculate the bandwidth of the light for the following wavelength ranges (assume a
propagation speed of 2 x 108 m):
a. 1000 to 1200 nm

b. 1000 to 1400 nm
18. The horizontal axes in Figure 7.6 and 7.9 represent frequencies. The horizontal
axis in Figure 7.16 represents wavelength. Can you explain the reason? lfthe propagation
speed in an optical fiber is 2 x 108 ill, can you change the units in the horizontal
axis to frequency? Should the vertical-axis units be changed too? Should the
curve be changed too?
19. Using Figure 7.16, tabulate the attenuation (in dB) of an optical fiber for the indicated
wavelength and distances.
20. A light signal is travelling through a fiber. What is the delay in the signal if the
length of the fiber-optic cable is 10 m, 100 m, and 1 Km (assume a propagation
speed of 2 x 108 ill)?
21. A beam oflight moves from one medium to another medium with less density. The
critical angle is 60°. Do we have refraction or reflection for each of the following
incident angles? Show the bending of the light ray in each case.
a. 40°
b. 60°
c. 800
CHAPTER 8 : Switching
Review Questions
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I. Describe the need for switching and define a switch.
2. List the three traditional switching methods. What are the most common today?
3. What are the two approaches to packet-switching?
4. Compare and contrast a circuit-switched network and a packet-switched network.
5. What is the role of the address field in a packet traveling through a datagram
network?
6. What is the role of the address field in a packet traveling through a virtual-circuit
network?
7. Compare space-division and time-division switches.

8. What is TSI and its role in a time-division switching?
9. Define blocking in a switched network.
10. List four major components of a packet switch and their functions.
Exercises
11. A path in a digital circuit-switched network has a data rate of I Mbps. The exchange
of 1000 bits is required for the setup and teardown phases. The distance between
two parties is 5000 km. Answer the following questions if the propagataion speed is
2 X 108 m:
a. What is the total delay if 1000 bits of data are exchanged during the data transfer
phase?
b. What is the total delay if 100,000 bits of data are exchanged during the data
transfer phase?
c. What is the total delay if 1,000,000 bits of data are exchanged during the data
transfer phase?
d. Find the delay per 1000 bits of data for each of the above cases and compare
them. What can you infer?
12. Five equal-size datagrams belonging to the same message leave for the destination
one after another. However, they travel through different paths as shown in
Table 8.1.
We assume that the delay for each switch (including waiting and processing) is 3,
10, 20, 7, and 20 ms respectively. Assuming that the propagation speed is 2 x 108 m,
find the order the datagrams arrive at the destination and the delay for each. Ignore
any other delays in transmission.
13. Transmission of information in any network involves end-to-end addressing and
sometimes local addressing (such as YCI). Table 8.2 shows the types of networks
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and the addressing mechanism used in each of them.
Answer the following questions:
a. Why does a circuit-switched network need end-to-end addressing during the setup

and teardown phases? Why are no addresses needed during the data transfer phase
for this type of network?
b. Why does a datagram network need only end-to-end addressing during the data
transfer phase, but no addressing during the setup and teardown phases?
c. Why does a virtual-circuit network need addresses during all three phases?
14. We mentioned that two types of networks, datagram and virtual-circuit, need a
routing or switching table to find the output port from which the information
belonging to a destination should be sent out, but a circuit-switched network has
no need for such a table. Give the reason for this difference.
15. An entry in the switching table of a virtual-circuit network is normally created
during the setup phase and deleted during the teardown phase. In other words, the
entries in this type of network reflect the current connections, the activity in the
network. In contrast, the entries in a routing table of a datagram network do not
depend on the current connections; they show the configuration of the network and
how any packet should be routed to a final destination. The entries may remain
the same even if there is no activity in the network. The routing tables, however, are
updated if there are changes in the network. Can you explain the reason for these
two different characteristics? Can we say that a virtual-circuit is a connectionoriented
network and a datagram network is a connectionLess network because of the
above characteristics?
16. The minimum number of columns in a datagram network is two; the minimum number
of columns in a virtual-circuit network is four. Can you explain the reason? Is the
difference related to the type of addresses carried in the packets of each network?
17. Figure 8.27 shows a switch (router) in a datagram network.
Find the output port for packets with the following destination addresses:
Packet 1: 7176
Packet 2: 1233
Packet 3: 8766
Packet 4: 9144
18. Figure 8.28 shows a switch in a virtual circuit network.

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Find the output port and the output VCI for packets with the following input port
and input VCI addresses:
Packet 1: 3, 78
Packet 2: 2, 92
Packet 3: 4, 56
Packet 4: 2, 71
19. Answer the following questions:
a. Can a routing table in a datagram network have two entries with the same destination
address? Explain.
b. Can a switching table in a virtual-circuit network have two entries with the same
input port number? With the same output port number? With the same incoming
VCls? With the same outgoing VCls? With the same incoming values (port, VCI)?
With the same outgoing values (port, VCI)?
20. It is obvious that a router or a switch needs to do searching to find information in
the corresponding table. The searching in a routing table for a datagram network is
based on the destination address; the searching in a switching table in a virtualcircuit
network is based on the combination of incoming port and incoming VCI.
Explain the reason and define how these tables must be ordered (sorted) based on
these values.
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21. Consider an n X k crossbar switch with n inputs and k outputs.
a. Can we say that switch acts as a multiplexer if n > k?
b. Can we say that switch acts as a demultiplexer if n < k?
22. We need a three-stage space-division switch with N =100. We use 10 crossbars at
the first and third stages and 4 crossbars at the middle stage.
a. Draw the configuration diagram.
b. Calculate the total number of crosspoints.

c. Find the possible number of simultaneous connections.
d. Find the possible number of simultaneous connections if we use one single crossbar
(100 x 100).
e. Find the blocking factor, the ratio of the number of connections in c and in d.
23. Repeat Exercise 22 if we use 6 crossbars at the middle stage.
24. Redesign the configuration of Exercise 22 using the Clos criteria.
25. We need to have a space-division switch with 1000 inputs and outputs. What is the
total number of crosspoints in each of the following cases?
a. Using one single crossbar.
b. Using a multi-stage switch based on the Clos criteria
26. We need a three-stage time-space-time switch with N = 100. We use 10 TSIs at the
first and third stages and 4 crossbars at the middle stage.
a. Draw the configuration diagram.
b. Calculate the total number of crosspoints.
c. Calculate the total number of memory locations we need for the TSIs.
CHAPTER 9 : Using Telephone and Cable Networks for Data Transmission
Review Questions
1. What are the three major components of a telephone network?
2. Give some hierarchical switching levels of a telephone network.
3. What is LATA? What are intra-LATA and inter-LATA services?
4. Describe the SS7 service and its relation to the telephone network.
S. What are the two major services provided by telephone companies in the United
States?
6. What is dial-up modem technology? List some of the common modem standards
discussed in this chapter and give their data rates.
7. What is DSL technology? What are the services provided by the telephone companies
using DSL? Distinguish between a DSL modem and a DSLAM.
8. Compare and contrast a traditional cable network with a hybrid fiber-coaxial network.
9. How is data transfer achieved using CATV channels?
10. Distinguish between CM and CMTS.

Exercises
11. Using the discussion of circuit-switching in Chapter 8, explain why this type of
switching was chosen for telephone networks.
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12. In Chapter 8, we discussed the three communication phases involved in a
circuitswitched
network. Match these phases with the phases in a telephone call between
two parties.
13. In Chapter 8, we learned that a circuit-switched network needs end-to-end addressing
during the setup and teardown phases. Define end-to-end addressing in a telephone
network when two parties communicate.
14. When we have an overseas telephone conversation, we sometimes experience a
delay. Can you explain the reason?
15. Draw a barchart to compare the different downloading data rates ofcommon modems.
16. Draw a barchart to compare the different downloading data rates of common DSL
technology implementations (use minimum data rates).
17. Calculate the minimum time required to download one million bytes of information
using each of the following technologies:
a. V32 modem
b. V32bis modem
c. V90 modem
18. Repeat Exercise 17 using different DSL implementations (consider the minimum
rates).
19. Repeat Exercise 17 using a cable modem (consider the minimum rates).
20. What type of topology is used when customers in an area use DSL modems for
data transfer purposes? Explain.
21. What type of topology is used when customers in an area use cable modems for
data transfer purposes? Explain.
CHAPTER 10 ; Error Detection and Correction

Review Questions
1. How does a single-bit error differ from a burst error?
2. Discuss the concept of redundancy in error detection and correction.
3. Distinguish between forward error correction versus error correction by retransmission.
4. What is the definition of a linear block code? What is the definition of a cyclic code?
5. What is the Hamming distance? What is the minimum Hamming distance?
6. How is the simple parity check related to the two-dimensional parity check?
7. In CRC, show the relationship between the following entities (size means the number
of bits):
a. The size of the dataword and the size of the codeword
b. The size of the divisor and the remainder
c. The degree of the polynomial generator and the size of the divisor
d. The degree of the polynomial generator and the size of the remainder
8. What kind of arithmetic is used to add data items in checksum calculation?
9. What kind of error is undetectable by the checksum?
10. Can the value of a checksum be all Os (in binary)? Defend your answer. Can the
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value be allIs (in binary)? Defend your answer.
Exercises
11. What is the maximum effect of a 2-ms burst of noise on data transmitted at the
following
rates?
a. 1500 bps
b. 12 kbps
c. 100 kbps
d. 100 Mbps
12. Apply the exclusive-or operation on the following pair of patterns (the symbol EB
means XOR):
a. (10001) (10000)

b. (10001) (10001) (What do you infer from the result?)
c. (11100) (00000) (What do you infer from the result?)
d. (10011) (11111) (What do you infer from the result?)
13. In Table 10.1, the sender sends dataword 10. A 3-bit burst error corrupts the codeword.
Can the receiver detect the error? Defend your answer.
14. In Table 10.2, the sender sends dataword 10. If a 3-bit burst en-or con-upts the first
three bits of the codeword, can the receiver detect the error? Defend your answer.
15. What is the Hamming distance for each of the following codewords:
a. d (10000, 00000)
b. d (10101, 10000)
c. d (11111,11111)
d. d (000, 000)
16. Find the minimum Hamming distance for the following cases:
a. Detection of two en-ors.
b. Correction of two errors.
c. Detection of 3 errors or correction of 2 errors.
d. Detection of 6 errors or correction of 2 errors.
17. Using the code in Table 10.2, what is the dataword if one of the following codewords
is received?
a. 01011
b. 11111
c. 00000
d. 11011
18. Prove that the code represented by Table 10.8 is not a linear code. You need to find
only one case that violates the linearity.
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19. Although it can mathematically be proved that a simple parity check code is a linear
code, use manual testing of linearity for five pairs of the codewords in Table 10.3 to
partially prove this fact.

20. Show that the Hamming code C(7,4) of Table lOA can detect two-bit en-ors but not
necessarily three-bit error by testing the code in the following cases. The character "V"
in the burst en-or means no en-or; the character "E" means an error.
a. Dataword: 0100 Burst error:
b. Dataword: 0111 Burst error:
c. Dataword: 1111 Burst error:
d. Dataword: 0000 Burst en-or:
21. Show that the Hamming code C(7,4) of Table lOA can correct one-bit errors but
not more by testing the code in the following cases. The character "V" in the burst
error means no error; the character "E" means an error.
22. Although it can be proved that code in Table 10.6 is both linear and cyclic, use
only two tests to partially prove the fact:
a. Test the cyclic property on codeword 0101100.
b. Test the linear property on codewords 0010110 and 1111111.
23. We need a dataword of at least 11 bits. Find the values of k and n in the Hamming
code C(n, k) with dmin ::: 3.
24. Apply the following operations on the corresponding polynomials:
a. (x3 + xl + X + 1) + (x4 + xl + x + 1)
b. (x3 + xl + x + 1) - (x4 + xl + x + 1)
c. (x3 + xl) X (x4 + x2 + x + 1)
d. (x3 + x2 + x + 1) / (x2 + 1)
25. Answer the following questions:
a. What is the polynomial representation of 10111O?
b. What is the result of shifting 101110 three bits to the left?
c. Repeat part b using polynomials.
d. What is the result of shifting 101110 four bits to the right?
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e. Repeat part d using polynomials.
26. Which of the following CRC generators guarantee the detection of a single bit

error?
a. x3 + x + 1
b. x4 + xl
c. 1
d. x2 + 1
27. Referring to the CRC-8 polynomial in Table 10.7, answerthe following questions:
a. Does it detect a single error? Defend your answer.
b. Does it detect a burst error of size 6? Defend your answer.
c. What is the probability of detecting a burst error of size 9?
d. What is the probability of detecting a burst error of size 15?
28. Referring to the CRC-32 polynomial in Table 10.7, answer the following questions:
a. Does it detect a single error? Defend your answer.
b. Does it detect a burst error of size 16? Defend your answer.
c. What is the probability of detecting a burst error of size 33?
d. What is the probability of detecting a burst error of size 55?
29. Assuming even parity, find the parity bit for each of the following data units.
a. 1001011
b. 0001100
c. 1000000
d. 1110111
30. Given the dataword 1010011110 and the divisor 10111,
a. Show the generation of the codeword at the sender site (using binary division).
h. Show the checking of the codeword at the receiver site (assume no error).
3 I. Repeat Exercise 30 using polynomials.
32. A sender needs to send the four data items Ox3456, OxABCC, Ox02BC, and
OxEEEE.
Answer the following:
a. Find the checksum at the sender site.
b. Find the checksum at the receiver site if there is no error.
c. Find the checksum at the receiver site if the second data item is changed to

OxABCE.
d. Find the checksum at the receiver site if the second data item is changed to
OxABCE and the third data item is changed to Ox02BA.
33. This problem shows a special case in checksum handling. A sender has two data
items to send: Ox4567 and OxBA98. What is the value of the checksum?
CHAPTER 11 :
Data Link Control
Review Questions
1. Briefly describe the services provided by the data link layer.
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