Laboratory 10: Communication

1
Laboratory 10Communication

• General Engineering
• Polytechnic University

2
Overview

• Objective
• Transmission of Media
• LASER
• Fiber-Optics
• Signal Forms
• Number Systems and Codes

• Conversions
• Error Detection
• Error Correction
• Materials for Lab
• Procedure
• Written Assignment
• Recitation Topics
• Closing

3
Objectives

• 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

4
Transmission of Media

• There are two types
• Guided - Signal travel through a cable Uses
  Telephone
• Non-Guided - Signal travels through air Uses
  AM FM Radio, Television

5
LASER (Non-Guided)
L A S E R
ight mplification by timulated mission of adiation

• 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

6
LASER (Non-Guided)

• Applications
• Industry
• diamond cutting
• shape machine tools
• Scientific Research
• study molecular structures of matter

• Communication
• television
• telephone
• computers
• Medicine
• surgery
• eyes
• Military
• missile guidance

7
Fiber-Optics (Guided)

• 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

8
Fiber-Optics (Guided)

• Advantages
• Transmits date faster than conventional means
• More than one signal can be sent at the same time
• No line of sight

• Disadvantages
• Expensive
• Transmission is lost if cable is cut or bent

9
Signal Forms

• Analog - The signal is continuous (infinite
  number of states), ranging between a high and low
  voltage

Voltage (V)
Time (sec)
10
Signal Forms

• Digital - The signal is discreet (only 2 states),
  there is no range only ons and offs
• Active High (high1)
• Active Low (low1)
• Digitizing - Converting an analog signal to a
  digital signal

Voltage (V)
0
0
0
0
Active Low ?
1
1
1
Time (sec)
11
Number Systems and Code

• 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
• Number Systems
• Decimal
• Based on 10 possible values 0 to 9

57610 (5102) (7101) (6 100)
500 70 6 576

• Binary
• Based on 2 possible values 0 and 1

12
Conversions

• Binary to Decimal
• Similar to expanding a decimal number

11012 (123) (122) (021) (120)
8 4 0 1
1310

• Decimal to Binary
• Continue to divide the decimal number by 2, until
  the quotient reaches zero
• The remainder values become the binary value

0 R1
1 R1
3 R0
1310
11012
6 R1
13
Error Detection
NOTE 8 bits 1 byte

• 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 1s
  (only can be used to detect a single-bit error)

14
Error Correction

• Error can be corrected by
• Vertical Horizontal Parity Check
• Arrange message in 4 x 4 array
• Choose parity bits to make all columns and rows
  have even (or odd) number of 1s
• Hamming Code
• Place message bits in overlapping circles
• Choose parity bits, so each circle has even parity

15
Vertical Horizontal Parity Check
Message Sent 1110 1010 0111 0101
Message
? Place a 1 to get even amount of ones
1
? still even
NOTE The parity bits do NOT change
0
? still even
? Place a 0 to keep even amount of ones
WRONG BIT
? Place a 1 to get even amount of ones
1
ODD!
0
? still even
? Place a 0 to keep even amount of ones
0
0
1
1
? Place a 1 to get even amount of ones
? still even
? still even
? Place a 0 to keep even amount of ones
? Place a 0 to keep even amount of ones
? Place a 1 to get even amount of ones
? still even
ODD!
16
Vertical Horizontal Parity Check
Message
Message Sent 1110 1010 0111 0101
1
0
1
0
0
0
1
1
17
Hamming Code
ODD amount of ones (BAD!) Save circle for later
ODD amount of ones (BAD!) Save circle for later
Even amount of ones (GOOD!) Disregard circle
NOTE Parity bits do NOT change
Message Sent M1 1 M2 0 M3 1 M4 1
0
1
1
1
1
1
WRONG BIT
0
1
0
0
0
18
Hamming Code
Message Sent M1 1 M2 0 M3 1 M4 1
0
1
1
1
0
1
0
19
Materials for Lab

• Function Generator
• 1 BNC connector
• Amplified Speaker
• 2 Coax cables (BNC to alligator clip)
• 4 miniature clip leads
• Computer with LabVIEW Oscilloscope

• Fiber-optic Trainer
• Transmitter
• Receiver
• 5-meter fiber-optic cable

20
Procedure

• Laser Demonstration -
• Performed by Instructors
• Components
• 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

• Laser Demonstration
• Fiber-Optic Communication System - Analog
• TCP/IP Communication System -Digital

21
Procedure

• Laser Demonstration -
• Performed by Instructors

• Laser Demonstration
• Fiber-Optic Communication System - Analog
• TCP/IP Communication System - Digital

22
Procedure

• Laser Demonstration -
• Performed by Instructors

• Laser Demonstration
• Fiber-Optic Communication System - Analog
• TCP/IP Communication System - Digital

23
Procedure

• 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 receivers side
• Unplug the fiber-optic cable from the receiver
  input to observe the visible light beam emitted
  from the cable

• Laser Demonstration
• Fiber-Optic Communication System - Analog
• TCP/IP Communication System - Digital

24
Procedure

• Fiber-Optic Communication System - Analog

• Laser Demonstration
• Fiber-Optic Communication System - Analog
• TCP/IP Communication System - Digital

NOTE Setup Diagram A on page 90
25
Procedure

• 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
• Vout denotes the amplitude of the sinusoidal
  voltage appearing at the receiver's output
• Vin represents the amplitude of the sinusoidal
  transmitter input signal

• Laser Demonstration
• Fiber-Optic Communication System - Analog
• TCP/IP Communication System - Digital

26
Procedure

• Fiber-Optic Communication System - Analog

• Laser Demonstration
• Fiber-Optic Communication System - Analog
• TCP/IP Communication System - Digital

• NOTE Setup Diagram B on page 91

27
Procedure

• 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.
• 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

• Laser Demonstration
• Fiber-Optic Communication System - Analog
• TCP/IP Communication System - Digital

28
Written Assignment

• 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
• Make sure your instructor initials your original
  data
• Include the topics found on the next slide
• Remember to create a title page

29
Written Topics

• 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?

30
Recitation Topics

• 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

31
Closing

• Return all the equipment back to your instructor