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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 2 Examinations 2011/
Module Title: Introduction to Data Communications
Module Code: CTEC
School: Science & Informatics
Programme Title: Bachelor of Science (Hons) in Software Development & Computer Networking
Programme Code: KDNET_8_Y
External Examiner(s): Dr Kevin Curran
Internal Examiner(s): Mr J O’Sullivan
Instructions: Answer FOUR questions. All questions carry equal marks.
Duration: Two hours
Sitting: Summer 2012
Requirements for this examination: N/A
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you have received the correct examination paper. If in doubt please contact an Invigilator.
You are expected to present your material in a neat and clear fashion. Start each question on a new page.
Q1 (a) In relation to electrical engineering, explain the difference between voltage and current. State the basic units used to measure each of these entities. [3 marks]
(b) Two types of voltage can be found in electrical circuits, dc voltage and ac voltage. What do the terms dc and ac stand for? Sketch graphs to show the difference between both types of voltage. [3 marks]
(c) The relationship between dc and ac is governed by a factor called the RMS value. Write down in plain language the definition of RMS. [3 marks]
(d) If an ac voltage of peak value 10 V was connected to a 1 k resistor for 5 minutes, determine the power dissipated and the energy consumed. [5 marks]
(e) In a workshop, a carpenter uses a 10 kW circular saw, a plane rated at 5 kW and a low-power sander of 1 kW capacity. Assuming the cost of electricity is €0.1675 per unit, (kWh), incl. VAT, what charge would appear on the carpenter’s electricity bill after a five-day week, if the saw was operated for 15 hours in total, the plane was in use for an average of 2 hours per day and the sander was only used on the final two days for a period of 4 hours each day. [6 marks]
Q2 (a) With the aid of sketches, show how a voltage source can be connected to three resistors (i) in series (ii) in parallel. [2 marks] (b) State the appropriate Kirchoff’s Law for each of the circuits in (a) above. [4 marks]
(c) If the voltage source in (a) above is 12 VDC, and the resistors are 470 , 390 and 560 , calculate the current drawn from the source in both configurations. [6 marks]
(d) If the resistors are reconnected as shown in Figure 1 below, determine the current that would be taken from the source and the current that would flow through the 390 resistor. [8 marks]
Figure 1
12 V
560
390
470
Q5 (a) In your own words what is meant by modulation and why it is used in modern data communications systems? [2 marks]
(b) With the aid of simple sketches show how binary information can be transmitted over a medium using (i) amplitude modulation (ii) phase modulation. [4 marks]
(c) Define the terms bit rate and baud rate and explain briefly how these may be different in a transmission system. [3 marks]
(d) Figure 3 below shows how the two modulation methods in (b) above can be combined to form a QAM signal. Explain what is meant by QAM, state the advantage of using QAM for modulation and determine the relationship between the bit rate and baud rate of this QAM signal – show working. [6 marks]
(e) With reference to a communication system, what is meant by signal-to-noise ratio and why is it normally measured in decibels, (dB). [2 marks]
(f) If the signal level at the output of a communications system is measured as 10 mW and the noise level at this point is measured as 100 W, determine the signal-to-noise ratio in dB. [3 marks]
Figure 3
Q6 (a) With the aid of a simple sketch, explain what is meant by wavelength? What is the wavelength of a 1 GHz electromagnetic wave propagating in free space? [4 marks] (b) What is meant by a transmission line? Why do electromagnetic waves travel slower along a transmission line than when propagating in free space? What effect does this have on the wavelength? [4 marks] (c) With respect to transmission lines, what is meant by characteristic impedance and why is it so important that transmission lines are terminated in their characteristic impedance? [4 marks] (d) Explain briefly how a time domain reflectometer (TDR) is used to detect faults on a transmission line. [4 marks] (e) If a TDR device shows an echo at 350 ns when testing a segment of CAT network cable, what does this imply? Assume the velocity of propagation (VOP) on CAT6 cable is 0.621c – show all calculations. [4 marks]
Useful Formulae
Ohm’s Law R
I V V IR
I
R V
Power P VI P I^2 R R
P V^2
Energy E Pt
Resistors Series: RTOT R 1 R 2 R 3
Parallel: 1 2 3
RTOT R R R
1 2
1 2 R R
RR
RTOT
Potential divider OUT VIN R R
R
V
1 2
2
Instantaneous value v VP sin( 2 ft ) i IP sin( 2 ft )
RMS value RMS P VP
V
V 0. 707
RMS P IP
I
I 0. 707
Reactance X L 2 fL
fC
X C
Impedance Z R^2 X L^2 Z R^2 X C^2 Z R^2 ( XL XC )^2
Phase angle
^
^
R
1 X^ L
tan
^
^
R
1 X^ C
tan
^
^ ^
R
1 X^ L XC
tan
Resonant frequency LC
fr
Gain/attenuation
2
1 10 log (^10) P
P
dB
N
S P
P
SNR 10 log 10
Transmission line C
L
Z 0 v f
MIN
MAX SWR (^) V
V
V
