Laboratory 10:
Communication
General Engineering
Polytechnic University
Overview
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Objective
Transmission of
Media
LASER
Fiber-Optics
Signal Forms
Number Systems
and Codes
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Conversions
Error Detection
Error Correction
Materials for Lab
Procedure
Written Assignment
Recitation Topics
Closing
Objectives
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Learn the principles of free-space laser
communication systems
Study the fundamentals of analog fiber-optic
communications
Measure the frequency response of a fiber-optic link,
and find its bandwidth
Demonstrate how digital data from a computer can be
transmitted optically
Learn about computer ASCII characters
Learn the difference between analog and digital
message signals
Transmission of Media
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There are two types:
– Guided - Signal travel through a cable
Uses: Telephone
– Non-Guided - Signal travels through air
Uses: AM & FM Radio, Television
LASER (Non-Guided)
L ight
Amplification by
S timulated
E mission of
Radiation
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Powerful: Contains one
frequency of light and the
waves are coherent (in
phase)
– A light bulb has many
frequencies of light and the
waves incoherent (out of
phase), making it weak
LASER (Non-Guided)
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Applications
– Industry
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diamond cutting
shape machine
tools
– Scientific Research
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study molecular
structures of
matter
– Communication
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television
telephone
computers
– Medicine
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surgery
eyes
– Military
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missile guidance
Fiber-Optics (Guided)
Light in
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Light out
Made of glass
The light that travels through the cable is a LED
(light emitting diode)
Uses the principle of Total Internal Reflection
– Light goes through cable by bouncing off the
glass walls
– The light & signal gets reflected back into the
medium
Fiber-Optics (Guided)
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Advantages
– Transmits date faster
than conventional
means
– More than one signal
can be sent at the
same time
– No line of sight
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Disadvantages
– Expensive
– Transmission is lost
if cable is cut or bent
Signal Forms
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Analog - The signal is continuous (infinite
number of states), ranging between a high
and low voltage
Voltage
(V)
Time (sec)
Signal Forms
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Digital - The signal is discreet (only 2 states),
there is no range only “on’s” and “off’s”
– Active High (high=1)
– Active Low (low=1)
Voltage
(V)
0
Active Low 
0
1
0
0
1 1
Time (sec)
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Digitizing - Converting an analog signal to a
digital signal
Number Systems and Code
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American Standard Code
for Information Interchange (ASCII)
– Each alphanumeric character is coded so that it can be stored and
read by the computer
– Each character is given a specific number or code
– An abbreviated chart can be found on page 92
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Number Systems
– Decimal
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Based on 10 possible values; 0 to 9
57610 = (5*102) + (7*101) + (6* 100)
=
500
+
70
+
6
– Binary
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Based on 2 possible values; 0 and 1
= 576
Conversions
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Binary to Decimal
– Similar to expanding a decimal number
– 11012 = (1*23) + (1*22) + (0*21) + (1*20)
=
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8
+ 4
+
0
+
1
= 1310
Decimal to Binary
1310 = 11012
2 ) 3
2 ) 6
2 ) 1310
R1
R1
R0
R1
Read Down
– Continue to divide the decimal number by 2, until
0
the quotient reaches zero
2 ) 1
– The remainder values become the binary value
NOTE: 8 bits = 1 byte
Error Detection
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Whenever a signal is sent, there is possibility
for error
Error can be detected by
– Redundancy - repeating the entire message and
comparing the two transmissions (wastes channel
and storage capacity)
– Parity Bit - A bit added on to a 7-bit character so
the byte has an even or odd number of 1’s (only
can be used to detect a single-bit error)
Error Correction
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Error can be corrected by
– Vertical & Horizontal Parity Check
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Arrange message in 4 x 4 array
Choose parity bits to make all columns and
rows have even (or odd) number of 1’s
– Hamming Code
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Place message bits in overlapping circles
Choose parity bits, so each circle has even
parity
Vertical & Horizontal Parity Check
Message Received
Sent
 Horizontal Parity Bit
Message
1
1
1
0
1  still even
1
0
WRONG
1
0
0  still even
0
1
0
BIT
1
1
1
0
0
1
0  still even
1
ODD!
1110 1010 0011
0111 0101
 Place a 1
to get even
amount
of The
NOTE:
 Place
a0
one’s
parity
bits do
to keep
even
 Place
1
amount
NOTofachange
to
get even
one’s
amount

Placeof
a0
one’s
to
keep even
 Vertical Parity Bit amount of
one’s

Place
a
1
to

even

even
still
Place
ODD!
a 0
to still
Place

 astill
Place
1 to
even
a 0 to
get even
keep even
getamount
even
keep
amount
even
of of
one’s
amount
one’s
of one’samount of one’s
0
1
1
0
ODD
ODD
Evenamount
amount
amountof
ofofones
ones
ones
NOTE: Parity bits doPlace 1 to get
(GOOD!)
(BAD!)
(BAD!)
NOT
change
even amount of
Disregard
circle
one’s in pink
circle
Place 0 Save
Save
to keep
circle for
for later
later
circle
even
amount
of
Message Received
Sent
one’s in gray
circle
M1 1
P2
P1
M1
M2 0
0
1
1
M3 0
1
Only message bit in
1
all the “ODD” circles
WRONG
M4 1
0
M4 1
therefore…
0
M3 BIT
M2
Hamming Code
Place 0 to keep even
amount of one’s in
green circle
0
P3
Materials for Lab
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Function Generator
1 BNC connector
Amplified Speaker
2 Coax cables (BNC
to alligator clip)
4 miniature clip
leads
Computer with
LabVIEW
Oscilloscope
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Fiber-optic Trainer
– Transmitter
– Receiver
– 5-meter fiberoptic cable
Procedure
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•Laser
Demonstration
•Fiber-Optic
Communication
System - Analog
•TCP/IP
Communication
System -Digital
Laser Demonstration Performed by Instructors
– Components:
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Audio Cassette Recorder - Source of the
music, modulates the intensity of the laser
Helium-neon Laser - What the music will
travel along, optical transmitter
Photo-diode - Responds to the light and
produces a electrical signal proportional to
the music signal, recovering the music,
optical receiver, demodulates laser beam
Speaker - Destination of the music
Procedure
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•Laser
Demonstration
•Fiber-Optic
Communication
System - Analog
•TCP/IP
Communication
System - Digital
Laser Demonstration Performed by Instructors
Cassette
Recorder
Music Signal
Helium-Neon
Light Free-Space
Laser
Speaker
Music Signal
Photo-Diode
Procedure
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•Laser
Demonstration
•Fiber-Optic
Communication
System - Analog
•TCP/IP
Communication
System - Digital
Laser Demonstration Performed by Instructors
Procedure
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•Laser
Demonstration
•Fiber-Optic
Communication
System - Analog
•TCP/IP
Communication
System - Digital
Fiber-Optic Communication System - Analog
– Turn on the transmitter and receiver circuit boards
of the fiber-optic trainer & set the slide switch to
“Analog” on both boards
– Connect the boards together with the use of the
fiber-optic cable
– Connect the speaker to the receiver using two
miniature clip leads
– Speak into the microphone on the transmitter
board while your partner listens to the speaker at
the receiver’s side
– Unplug the fiber-optic cable from the receiver input
to observe the visible light beam emitted from the
cable
Procedure
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Fiber-Optic Communication System - Analog
•Laser
Demonstration
•Fiber-Optic
Communication
System - Analog
•TCP/IP
Communication
System - Digital
Analog
Fiber-Optic
Transmitter
(with Microphone)
5-Meter
Critical Fiber
Analog
Fiber-Optic
Receiver
(with Audio
Power Amplifier)
NOTE: Setup Diagram A on page 90
Speaker
Procedure
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•Laser
Demonstration
•Fiber-Optic
Communication
System - Analog
•TCP/IP
Communication
System - Digital
Fiber-Optic Communication System - Analog
– Adjust the function generator to produce 200mV
peak-to-crest (0.2 V), 1 kHz sine wave
– Connect the function generator, the DAQ board
and the circuit boards according to Diagram B on
page 91
– Measure the gain vs. frequency (f) and complete
the data sheet on page 91
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Vout denotes the amplitude of the sinusoidal voltage
appearing at the receiver's output
Vin represents the amplitude of the sinusoidal
transmitter input signal
Procedure
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•Laser
Demonstration
•Fiber-Optic
Communication
System - Analog
Fiber-Optic Communication System - Analog
Function
Generator
200mV peak-tocrest
Sine Wave
Analog
Fiber-Optic
Transmitter
Analog
Fiber-Optic
Critical Fiber Receiver
(with Audio
Power Amplifier)
5-Meter
•TCP/IP
Communication
System - Digital
Vout
Vin
NOTE: Setup Diagram B on page 91
LabVIEW
Oscilloscope
Procedure
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•Laser
Demonstration
•Fiber-Optic
Communication
System - Analog
•TCP/IP
Communication
System - Digital
TCP/IP Communication System - Digital
– Establish a connection between two computers by
using the ‘Talk Active’ and ‘Talk Passive’ VIs
– Type a message (e.g. “HELLO BETH”) and click the
send button on the VI.
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Sent messages are displayed in the “Local” window
Received messages are displayed in the “Remote”
window
– Disconnect the computers and reconnect them
using the ‘Writer’ and ‘Reader’ VIs
– Type in a message (16 characters or less) on the
transmitting computer
– Compare the number displayed on the receiving
computer with the ASCII code on page 92 and find
the corresponding alphanumeric character
Written Assignment
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Full Team Report (one report per team)
Use the guidelines on page 5 for help
Create a graph of gain vs frequency of the table you
completed on page 91
– Be sure to use a log scale for the x-axis
– Find the 3dB point and bandwidth of your
communication system
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Make sure your instructor initials your original data
Include the topics found on the next slide
Remember to create a title page
Written Topics
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Each of the following topics must be addressed in the
full report and should be placed in the proper
sections
– Explain the lab demonstration on the laser
communication system
– What is the significance of the bandwidth measurement
of the frequency response graph?
– From your result would you say your fiber-optic-link is
capable of transmitting video signals from a camcorder
which requires frequencies of about 5 MHz?
– Describe the results obtained with the TCP/IP
connection you set up. Were there any problems?
– Summarize the advantages of fiber-optic systems. Are
there any disadvantages?
Recitation Topics
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Discuss the differences between analog and
digital signals
Discuss the relative strengths and weaknesses of
the three communication media covered in this
lab.
Discuss the relationship between bandwidth and
frequency as they relate to gain, baud rate, and
scan rate
Closing
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Return all the equipment back to your
instructor