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Fundamentals of Data and Signals

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Data and signals can be analog or digital ... Examples: (naturally occurring) music and voice ... noise from an analog signal than from a digital signal ... – PowerPoint PPT presentation

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Title: 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


6
Introduction Data and Signals
(continued)


7
Analog versus Digital
  • Analog - continuous waveform
  • Examples (naturally occurring) music and voice



8
Analog versus Digital (continued)
  • Harder to separate noise from an analog signal
    than from a digital signal



9
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


11
Analog versus Digital (continued)
  • If there is too much noise
  • You cannot discern a high voltage from a low
    voltage



12
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



14
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

15
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


21
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


22
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


24
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)



26
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


27
4B/5B Digital Encoding Scheme
(continued)


28
Transmitting Digital Data with Analog
Signals
  • Three basic techniques
  • Amplitude shift keying
  • Frequency shift keying
  • Phase shift keying


29
Amplitude Shift Keying
  • One amplitude encodes a 0 while another amplitude
    encodes a 1 (a form of amplitude modulation)



30
Amplitude Shift Keying (continued)
  • Some systems use multiple amplitudes



31
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)



33
Phase Shift Keying
  • One phase change encodes a 0 while another phase
    change encodes a 1 (a form of phase modulation)



34
Phase Shift Keying (continued)
  • Quadrature Phase Shift Keying
  • Four different phase angles are used
  • 45 degrees
  • 135 degrees
  • 225 degrees
  • 315 degrees


35
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)


37
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

40
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

42
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

49
Spread Spectrum Technology (continued)


50
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


52
EBCDIC



53
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

55
Data and Signal Conversions in Action
Two Examples


56
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

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