Fundamentals of Data and Signals

1
Data Communications and Computer Networks A
Business Users Approach Third Edition

• Chapter 2
• Fundamentals of Data and Signals

2
Objectives

• After reading this chapter, you should be able
  to
• Distinguish between data and signals, and cite
  the advantages of digital data and signal over
  analog data and signals
• Identify the three basic components of a signal
• Discuss the bandwidth of a signal and how it
  relates to data transfer speed

3
Objectives (continued)

• Identify signal strength and attenuation, and how
  they are related
• Outline the basic characteristics of transmitting
  analog data with analog signals, digital data
  with digital signals, digital data with analog
  signals, and analog data with digital signals
• List and draw diagrams of the basic digital
  encoding techniques, and explain the advantages
  and disadvantages of each

4
Objectives (continued)

• Identify the different shift keying (modulation)
  techniques and describe their advantages,
  disadvantages, and uses
• Identify the two most common digitization
  techniques and describe their advantages and
  disadvantages
• Discuss the characteristics and importance of
  spread spectrum encoding techniques
• Identify the different data codes and how they
  are used in communication systems

5
Introduction Data and Signals

• Data - entities that convey meaning
• Examples computer file, music on a CD, results
  from a blood gas analysis machine
• Signals - electric or electromagnetic encoding of
  data
• Examples telephone conversation, web page
  download
• Computer networks and data / voice communication
  systems transmit signals
• Data and signals can be analog or digital

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Introduction Data and Signals
(continued)


7
Analog versus Digital

• Analog - continuous waveform
• Examples (naturally occurring) music and voice

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Analog versus Digital (continued)

• Harder to separate noise from an analog signal
  than from a digital signal

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Analog versus Digital (continued)

• Digital - discrete or non-continuous waveform
• Examples computer 1s and 0s

10
Analog versus Digital (continued)

• Despite noise in this digital signal
• You can still discern a high voltage from a low
  voltage

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Analog versus Digital (continued)

• If there is too much noise
• You cannot discern a high voltage from a low
  voltage

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Fundamentals of Signals

• All Signals Have Three Components
• Amplitude
• Frequency
• Phase

13
Fundamentals of Signals (continued)

• Amplitude
• Height of the wave above or below a given
  reference point

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Fundamentals of Signals (continued)

• Frequency
• Number of times a signal makes complete cycle
  within a given time frame
• Spectrum - Range of frequencies that a signal
  spans from minimum to maximum
• Bandwidth - The absolute value of the difference
  between the lowest and highest frequencies of a
  signal

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Fundamentals of Signals (continued)



16
Fundamentals of Signals (continued)

• Frequency (continued)
• For example, consider an average voice
• Average voice has a frequency range of roughly
  300 Hz to 3100 Hz
• The spectrum would thus be 300 - 3100 Hz
• The bandwidth would be 2800 Hz

17
Fundamentals of Signals (continued)

• Phase
• Position of the waveform relative to a given
  moment of time or relative to time zero
• A change in phase can be any number of angles
  between 0 and 360 degrees
• Phase changes often occur on common angles, such
  as 45, 90, 135, etc.

18
Fundamentals of Signals (continued)



19
Loss of Signal Strength

• All signals experience loss (attenuation)
• Denoted as a decibel (dB) loss
• Decibel losses (and gains) are additive

20
Loss of Signal Strength (continued)

• So if a signal loses 3 dB, is that a lot?
• A 3 dB loss indicates the signal lost half of its
  power
• dB 10 log10 (P2 / P1)
• -3 dB 10 log10 (X / 100)
• -0.3 log10 (X / 100)
• 10-0.3 X / 100
• 0.50 X / 100
• X 50

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Converting Data into Signals

• Converting Analog Data into Analog Signals
• Often necessary to modulate analog data onto a
  different set of analog frequencies
• Two common examples are broadcast radio and
  television

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Converting Data into Signals
(continued)


23
Converting Data into Signals
(continued)

• Converting Digital Data into Digital Signals
• Numerous techniques lets examine four
• NRZ-L
• NRZ-I
• Manchester
• Differential Manchester
• Bipolar AMI

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Converting Data into Signals
(continued)


25
Manchester Digital Encoding Schemes

• Note that with a Differential Manchester code,
  every bit has at least one signal change
• Some bits have two signal changes per bit (baud
  rate is twice the bps)

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4B/5B Digital Encoding Scheme

• Converts four bits of data into five-bit
  quantities
• Five-bit quantities are unique
• No five-bit code has more than 2 consecutive
  zeroes
• Five-bit code is then transmitted using an NRZ-I
  encoded signal

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4B/5B Digital Encoding Scheme
(continued)


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Transmitting Digital Data with Analog
Signals

• Three basic techniques
• Amplitude shift keying
• Frequency shift keying
• Phase shift keying

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Amplitude Shift Keying

• One amplitude encodes a 0 while another amplitude
  encodes a 1 (a form of amplitude modulation)

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Amplitude Shift Keying (continued)

• Some systems use multiple amplitudes

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Transmitting Digital Data with
Analog Signals (continued)

• Multiple Signal Levels
• Why use multiple signal levels?
• We can represent two levels with a single bit, 0
  or 1
• We can represent four levels with two bits 00,
  01, 10, 11
• We can represent eight levels with three bits
  000, 001, 010, 011, 100, 101, 110, 111
• Note that the number of levels is always a power
  of 2

32
Frequency Shift Keying

• One frequency encodes a 0 while another frequency
  encodes a 1 (a form of frequency modulation)

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Phase Shift Keying

• One phase change encodes a 0 while another phase
  change encodes a 1 (a form of phase modulation)

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Phase Shift Keying (continued)

• Quadrature Phase Shift Keying
• Four different phase angles are used
• 45 degrees
• 135 degrees
• 225 degrees
• 315 degrees

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Phase Shift Keying (continued)



36
Phase Shift Keying (continued)

• Quadrature Amplitude Modulation
• 12 different phases are combined with two
  different amplitudes
• Since only 4 phase angles have 2 different
  amplitudes, there are a total of 16 combinations.
• With 16 signal combinations, each baud equals 4
  bits of information (2 4 16)

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Phase Shift Keying (continued)



38
Higher Data Transfer Rates


How do you send data faster? 1. Use a higher
frequency signal (make sure the medium can handle
the higher frequency) 2. Use a higher number of
signal levels In both cases, noise can be a
problem

39
Maximum Data Transfer Rates

• How do you calculate a maximum data rate?
• Use Shannons equation
• S(f) f log2 (1 W/N)
• Where f signal frequency (bandwidth), W is
  signal power, and N is noise power

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Maximum Data Transfer Rates
(continued)

• For example, what is the data rate of a 3400 Hz
  signal with 0.2 watts of power and 0.0002 watts
  of noise?
• S(f) 3400 x log2 (1 0.2/0.0002)
• 3400 x log2 (1001)
• 3400 x 9.97
• 33898 bps

41
Transmitting Analog Data with Digital
Signals

• To convert analog data into a digital signal,
  there are two basic techniques
• Pulse code modulation (used by telephone systems)
• Delta modulation

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Pulse Code Modulation

• Analog waveform is sampled at specific intervals
• Snapshots are converted to binary values

43
Pulse Code Modulation (continued)

• Binary values are later converted to an analog
  signal
• Waveform similar to original results

44
Pulse Code Modulation (continued)

• The more snapshots taken in the same amount of
  time, or the more quantization levels, the better
  the resolution

45
Pulse Code Modulation (continued)

• Because the human voice has a fairly narrow
  bandwidth
• Telephone systems digitize voice into either 128
  levels or 256 levels
• Called quantization levels
• If 128 levels, then each sample is 7 bits (2 7
  128)
• If 256 levels, then each sample is 8 bits (2 8
  256)

46
Pulse Code Modulation (continued)

• How fast do you have to sample an input source to
  get a fairly accurate representation?
• Nyquist says 2 times the bandwidth
• Thus, if you want to digitize voice (4000 Hz),
  you need to sample at 8000 samples per second

47
Delta Modulation

• An analog waveform is tracked using a binary 1 to
  represent a rise in voltage and a 0 to represent
  a drop

48
Spread Spectrum Technology

• A secure encoding technique that uses multiple
  frequencies or codes to transmit data
• Two basic spread spectrum technologies
• Frequency hopping spread spectrum
• Direct sequence spread spectrum

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Spread Spectrum Technology (continued)


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Spread Spectrum Technology (continued)

• Direct Sequence Spread Spectrum
• This technology replaces each binary 0 and binary
  1 with a unique pattern, or sequence, of 1s and
  0s
• For example, one transmitter may transmit the
  sequence 10010100 for each binary 1, and 11001010
  for each binary 0
• Another transmitter may transmit the sequence
  11110000 for each binary 1, and 10101010 for each
  binary 0

51
Data Codes

• Data Code - set of all textual characters or
  symbols and their corresponding binary patterns
• Two basic data code sets plus a third code set
  that has interesting characteristics
• EBCDIC
• ASCII
• Unicode

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EBCDIC



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ASCII


54
Data and Signal Conversions in Action
Two Examples

• Let us transmit the message Sam, what time is
  the meeting with accounting? Hannah.
• This message first leaves Hannahs workstation
  and travels across a local area network

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Data and Signal Conversions in Action
Two Examples


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Data and Signal Conversions in Action
Two Examples


57
Summary

• Differences between digital and analog data and
  signals
• Components, bandwidth, and data transfer speed of
  signals
• Signal strength and attenuation
• Basic digital encoding techniques
• Shift keying (modulation) techniques
• Spread Spectrum encoding techniques
• Data codes in communication systems