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(This is just a sample only for guide from a previous exam. This semester there will also be fill in
the blanks and shall also include different nature of questions mostly from the text book.
The solutions provided in bold are not all exact and full answers. These are just guides. Consult
the book for more complete answers.)
NAME:
_______________
You must show the calculations for the descriptive questions.
Notes: Any script without a name will receive no points.
_(EXAM1: SECTION A)
1. What three fundamental characteristics determine the effectiveness of a data communication system?
Delivery, accuracy, timeliness.
2. Define the type of topology in the following figure.
hybrid: star backbone connecting two bus
3. Draw two periods of a sine wave with a phase shift of 90 degrees. On the same diagram, draw a sine wave
with the same amplitude and frequency but with a 90 degree phase shift from the first.
4. Draw a hybrid topology with a bus backbone connecting two ring backbones. Each ring backbone connects
three star networks.
5.
Draw the frequency domain representation of the figure below.
6. Can a constellation have 128 points? Why or Why not?
Yes, it ispower of 2.
7. Assuming six devices are arranged in a mesh topology. How many cables are needed?
How many ports are needed for each device?
a. Cable links: n (n – 1) / 2 = (6 × 5) / 2 = 15 b. Number of ports: (n – 1) = 5 ports needed per device
8. Why are protocols needed?
A protocol defines what is communicated, in what way and when. This provides accurate and timely transfer of
information between different devices in a network.
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9. Calculate the baud rate for the given bit rate and type of modulation:
a. 6000 bps, 4-PSK
3000bps
b. 36000 bps, 64-QAM
6000bps
10. The data points in a constellation diagram are shown below.
a. Write the amplitude and phase for each point.
b. Is the modulation ASK, PSK or QAM?
c. How many bits per baud can one send with this constellation
It is 4-QAM (2 amplitudes, 4 phases) with 2 bits per baud. See Figure 5.10
11. What is the difference between network layer delivery and transport layer delivery?
Transport layer: source to destination delivery of entire message
Network Layer: source to destination delivery of individual packet of data message irrespective
their relationship to the each other and entire message.
12. What is the difference between a de facto standard and a de jure standard?
A de jure standard is recognized by an officially recognized body. A de facto stan-dard is established by a
manufacturer seeking to define the functionality of a new product that hasn't been approved by an official
body.
13. Match the following to one of the seven OSI layers. Write the name of the relevant layer.
a. Reliable end-to-end data transmission.
transport
b. User services such as e-mail and file transfer provided.
application
c. Transmission of bit stream across physical medium.
physical
d. Establishes, manages, and terminates session.
session
e. Provides independence from differences in data representation. presentation
f. Synchronizes users.
session
g. Route determination.
network
h. Mechanical, electrical, and functional interface.
physical
i. Reassembly of data packets.
transport
j. Responsibility for information between two adjacent nodes. data link
14. If the bit rate of a signal is 1000 bits/seconds, how many bits can be sent in 100 milliseconds? 100bits
15. What are the functions of a DTE? Give examples
What are the functions of DCE? Give Examples.
A DTE is any device (e.g., a computer) that is a source or destination for binary digital data. A DCE is any device
(e.g., a modem) that transmits or receives data in the form of analog or digital signals through a network.
What is the function of a demodulator?
A modulator converts a digital signal into an analog signal using ASK, FSK, PSK or QAM. A demodulator converts
an analog signal into a digital signal; it reverses the process of modulation.
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16. A periodic composite signal with a bandwidth of 2000 Hz is composed of two sine waves. The first one has
a frequency of 500 Hz with a maximum amplitude of 20 volts; the second one has a maximum amplitude of
5 volts. Draw the frequency spectrum.
100 in the fig to be replaced
by 500 and 2100 to be
replaced by 2500.
17. Compare the two methods of serial transmission. Discuss the advantages and disadvantages of each.
In asynchronous transmission, there is no timing needed at the byte level, because information is received and
translated by agreed-upon patterns. This method is mostly used for low-speed communication. In synchronous
transmission, timing is very important. This method is very fast and used for high-speed transmissions.
18. Assume a data stream is made of three zeros followed by two 1s followed by two 0s, and another three 1s.
Encode the stream using the following encoding scheme.
a. polar NRZ-L.
b. RZ
c. Manchester
19. Suppose you add two new devices to an existing five-device network. If you have a fully connected mesh
topology, how many new cable lines are needed?
If, however, the devices are arranged in a ring, how many new cables are needed?
a. mesh: 11
b. ring: 2
20. What is the difference between a balanced circuit and an unbalanced circuit?
An unbalanced circuit has one line for signal propagation, while a balanced circuit has two lines.
21. Data from a source ranges in value between -1.0 and 1.0. To what do the data points 0.91, -0.25, 0.56, and
0.71 transform if eight bit quantization is used?
a. 0.91 × 127 = 116 ⇒ 01110100 b. –0.25 × 127 = –32 ⇒ 10100000
c. 0.56 × 127 = 71 ⇒ 01000111
22. Using the Nyquest theorem, calculate the sampling rate for the analogue signal with frequencies from 2000
to 8000 Hz.
2 X 8000 = 16000
23. What are the responsibilities of data link layer?
The data link layer is responsible for
a. framing data bits b. providing the physical addresses of the sender/receiver
c. data rate control d. detection and correction of damaged and lost frames
24. Define the syntax and semantics in the following sentence: “The dog drove the car safely to the
destination.” Is the syntax correct? Are the semantics right?
Syntax (the sentence structure) is correct, semantics (the meaning of the sentence) are wrong.
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25. Following figure is the Manchester encoding of a data stream. What is the data stream?
00011100
What would be the data stream if the above figure is an output of differential Manchester coding? 10010010
26. What is the purpose of a null modem?
A null modem is used to connect two compatible digital devices directly over a short distance. Signals do not need to
be modulated, therefore a null modem is a DTE/DTE interface without DCEs.
27. If we want to transmit 1000 ASCII characters (7 bit) asynchrounously. What is the minimum number of
extra bits needed? What is the efficiency in percentage?
For seven-bit ASCII: 7000 bits for data, 1000 stop bits, 1000 start bits, for a total of 9000 bits. This means 78% of
bits transmitted are data (7000/9000). Note that if a parity bit is used to make each character eight bits long, the
calculation would be different.
28. Two signals have the same frequencies. However, when the first signal is at its maximum amplitude, the
second signal has amplitude of zero. What is the phase shift between the two signals?
90 degrees
NAME:
_______________
(EXAM1: SECTION B)
Notes: Any script without a name will receive no points. It is necessary that you mark the correct answer on
scantron page and also on this paper.
Multiple Choices
1. What is the main function of the transport layer?
a. node-to-node delivery b. end-to-end message delivery
c. synchronization
d. updating and maintenance of routing table.
2. Session layer checkpoints _____________
a. allow just a portion of a file to be resent
b. detect and recover errors
c. control the addition of headers
d. are involved in dialog controls.
3. The _______ of a signal is the collection of all its component frequencies.
a. amplitude b. frequency spectrum c. phase d. bandwidth
4. The _______ of a signal is the width of its frequency spectrum.
a. bandwidth b. amplitude c. phase d. frequency
5. A signal is decomposed into three sine waves with frequencies of 10, 20, and 30 Hz. The bandwidth of the signal is
_______ Hz.
a. 10 b. 20 c. 30 d. 60
6. The _______ is the number of bits sent in one second.
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a. bit interval b. bit rate c. baud
d. frequency
7. A bit interval of 10 milliseconds means a bit rate of _______bps.
a. 0.1 b. 1 c. 10 d. 100
8. _______ rate is the number of bits per second; _______ rate is the number of signal units per second.
a. Baud; bit
b. Bit; baud
c. Baud; base
d. Base; baud
9. If the baud rate is 1200 and there are 4 bits for each signal element, then the bit rate is _______.
a. 4800 b. 1200 c. 400 d. 300
10. An 8-QAM modulated signal has a baud rate of 2000; the bit rate is _______ bps.
a. 8000 b. 6000 c. 2000 d. 1000
11. A 128-QAM modulated signal has a baud rate of 2000; there are _______ bits per baud.
a. 2000 b. 1000 c. 7 d. 6
12. The modulation technique most affected by noise is _______.
a. ASK b. PSK c. FSK d. QAM
13. Twisted-pair wire, coaxial cable, and fiber-optic cable are all types of _______.
a. protocols b. messages c. media d. data
14. The key element of a protocol is _______.
a. syntax b. semantics c. timing d. all of the above
15. A _______ is a physical communication pathway that transfers data from one device to another.
a. node b. message c. link d. receiver
16. In a multipoint line configuration, _______ devices share a single link.
a. exactly two b. exactly three c. exactly four d. three or more
17. In a multipoint line configuration, if the devices take turns using the link, we have a _______ shared situation.
a. time b. spatially c. contiguously d. ambiguously
18. Which of the following is a basic network topology?
a. half-duplex b. multipoint c. ring d. all of the above
19. In a _______ relationship, one device controls traffic and the others must transmit through it.
a. peer-to-peer b. point-to-point c. primary-secondary d. full duplex
20. Seven devices are arranged in a mesh topology. _______ physical channels link these devices.
a. Seven b. Six c. Twenty d. Twenty-one
21. In a _______ topology, a dedicated link connects a device to a central controller.
a. ring b. bus c. mesh d. star
22. A _______ topology is a variation of a star topology.
a. ring b. bus c. mesh d. tree
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23. The number of pins, the cable length, and the pin configuration are _______ specifications of the EIA-232 interface
standard.
a. mechanical
b. electrical
c. functional
d. any of the above
24. The data link layer lies between the _______ layer and the network layer.
a. transport
b. session
c. physical
d. application
25. The header added by the transport layer to the packet coming from the upper layer includes the _______ address.
a. logical
b. service-point
c. physical
d. network
26. The performance of data communication network depends on __________________.
a. the number of users b. the transmission media c. the hardware and software d. all of the above
27. Protection of data from a natural disaster such as a tornado is a network _____________ issue.
a. performance
b. reliability
c. security
d. management
28. There are n devices arranged in a ring topology. A device is deleted. There are now _______ links of cable.
a. n - 1 b. n - 2 c. n d. n + 1
29. The _______ is an example of a simplex device.
a. repeater b. tap c. walkie-talkie d. keyboard
30. E-mail is a service handled by the _______ layer.
a. session
b. presentation
c. application
d. data link
31. In the _______ transmission mode, both stations can transmit and receive at the same time.
a. simplex b. half-duplex c. full-duplex d. b and c
32. A _______ is usually privately owned and links the devices in a single office, building, or campus.
a. LAN b. MAN c. WAN d. all of the above
33. _______ refers to the structure or format of the data, the order in which they are presented.
a. Syntax b. Semantics c. Timing d. none of the above
34. _______ refers to the meaning of each section of bits.
a. Syntax b. Semantics c. Timing d. none of the above
35. When a protocol specifies that the address of the sender must occupy the first four bytes of a message, this is a
_______ issue.
a. Syntax b. Semantics c. Timing d. none of the above
36. If the maximum value of a simple sine wave is 10 volts, the voltage values _______.
a. range from 0 to 10, inclusive b. are either 10 or -10 c. range from -10 to 10, inclusive d. are integer values
from -10 to 10
37. The _______ of a signal is its number of cycles per second.
a. amplitude b. frequency c. phase d. any of the above
38. The value of a simple sine wave at time zero is its maximum positive value. The phase shift is therefore _______
degrees.
a. 0 b. 90 c. 180 d. 270
39. A signal with a frequency of 10 MHz has more cycles per second than a signal with a frequency of _______.
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a. 10 KHz b. 10 GHz c. 10 THz d. 1 THz
40. A sine wave has a frequency of 10 Hz. Its period is _______.
a. 10 seconds b. 1 second c. 0.1 second d. 0.01 second
41. A digital signal has its 0 bits represented by 0 volts and its 1 bit represented by -5 volts or 5 volts. This is _______
encoding.
a. unipolar b. polar c. bipolar d. b and c
42. The DC component is a serious problem for _______ encoding.
a. unipolar b. polar c. bipolar d. b and c
43. _______ encoding is superior to _______ encoding because the problem of the DC component is alleviated.
a. Unipolar; polar b. Polar; bipolar c. Polar; unipolar d. Unipolar; bipolar
44. Ethernet LANs use _______ encoding.
a. RZ b. Manchester c. differential Manchester d. NRZ-I
45. A sampling rate of _______ million samples per second is needed for a signal with components ranging from 10MHz to
100 MHz.
a. 10
b. 90
c. 100
d. 200
46. In _______ the phase and amplitude of the carrier signal is varied based on the information in a digital signal.
a. ASK b. PSK c. FSK d. QAM
47. For _______, the minimum bandwidth required for transmission is equal to the baud rate.
a. ASK b. PSK c. FSK d. a and b
48. In FM the _______ of the information signal modulates the frequency of the carrier signal.
a. amplitude b. frequency c. phase d. any of the above
49. In which type of modulation can the bit rate be four times the baud rate?
a. ASK b. FSK c. PSK d. None of the above
50. In a dibit modulation, the number of points in the constellation is _______.
a. 2 b. 4 c. 8 d. None of the above
51. An 8-QAM constellation is a _________ modulation.
a. dibit
b. tribit c. quadbit d. None of the above
52. Token Ring LANs use _______ encoding.
a. RZ b. Manchester c. differential Manchester d. NRZ-I
53. The frequency of a signal is inversely related to its _______.
a. amplitude b. period c. phase d. any of the above
54. Forty-five physical channels link _______ devices arranged in a mesh topology.
a. nine b. ten c. forty d. forty-five
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55. In _______ the phase and amplitude of the carrier signal is varied based on the information in a digital signal.
a. ASK b. PSK c. FSK d. QAM
56. _______ rate is always less than or equal to _______ rate.
a. Baud; bit
b. Bit; baud
c. Baud; base
d. Base; baud
57. [1 point] In _______ transmission, if n bits are being sent simultaneously, n communication channels are needed.
a. parallel
b. synchronous
c. asynchronous
d. serial
58. _______ transmission features start bits, stop bits, and gaps between data units.
a. Parallel
b. Synchronous
c. Asynchronous
d. Virtual
59. The voltage level and the signal type are _______ specifications of the EIA-232 interface standard.
a. mechanical
b. electrical
c. functional
d. any of the above
60. If two DTEs are to be connected directly, a _______ can be used.
a. null modem
b. modem
c. repeater
d. bridging device
61. The maximum data rate of a full duplex 8-PSK modem using a two-wire regular telephone line is ___________ bps.
a. 1200
b. 3600
c. 7200
d. none of above
62. Radio communication ranges from a frequency of _______ to a frequency of _______.
a. 3 KHz; 300 MHz b. 3 KHz; 300 GHz
c. 3 MHz; 300 GHz
d. 3 GHz; 300 THz
63. The network layer lies between the transport layer and the _______ layer.
a. data
b. physical
c. session
d. data link
64. _______ addresses on headers change as a packet moves from network to network but the _______ do not.
a. Logical; port
b. Logical; network
c. Logical; physical
d. Physical; logical
65. In a __________ connection, more than two devices can share a single link.
a. point-to-point
b. multipoint
c. primary
d. secondary
66. XYZ publishing, with headquarters in London and branch offices through-out Asia, Europe, and South America, is
probably connected by a ___________.
a. LAN
b. MAN
c. WAN
d. none of the above
67. The maximum data rate of a half duplex 8-PSK modem using a two-wire regular telephone line is _____ bps.
a. 1200
b. 2400
c. 7200
d. none of the above
68. If the sending system uses ASCII encoding and the receiving station uses EBCDIC encoding, the _______ layer is
responsible for translation.
a. network
b. presentation
c. transport
d. physical
69. A signal has a constant value of 10 volts. Its frequency is _______ Hz.
a. zero
b. one c. ten d. five
70. A simple sine wave completes one cycle in _______ degrees.
a. 45 b. 90 c. 180 d. 360
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