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Lecture 08
Computer Communication and Networks
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Signals

• Why We Need For Signals
• One of the major concerns of data communication
  is moving information in the form of
  electromagnetic signals across medium
• Information can be voice, image, numeric data,
  characters or any message that is readable and
  has meaning to the destination user

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Signals

• Why We Need For Signals
• For Example, you cannot roll up a photograph,
  insert it into the wire and transmit it across
  the city.
• You can transmit however an encoded description
  of the photograph.
• The binary digits must be converted into a form
  that transmission medium can accept.
• This is done by conducting energy along a
  physical path, so the data stream of 1s and 0s
  must be turned into energy in the form of EM
  signals

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Analog and Digital

• Both data and signals that represent them, can
  take either analog or digital form
• What is Analog ?
• What is Digital ?
• Analog
• Analog Refers to something that is continuous in
  time
• "Continuous" A set of specific points of data and
  all possible points b/w them
• Digital
• Digital Refers to something that is discrete

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Analog and Digital Data

• Data can be Analog or Digital
• Analog Data
• Example of analog data is human voice, when
  somebody speaks, a continuous wave is created in
  the air.
• This can be captured by a microphone and
  converted to an analog signal
• Digital Data
• Example of digital data is data stored in the
  memory of a computer in the form of 1s and 0s.
• It is usually converted to a digital signal when
  it is transferred from one position to the other
  inside or outside the computer

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Analog and Digital Signals

• Signals can be Analog or Digital
• Signal
• Detectable transmitted energy that can be used to
  carry information
• Analog Signal
• It is a continuous waveform that changes smoothly
  over time.
• As the wave moves from value A to value B,
  it passes through and includes an infinite number
  of values along its path
• Digital Signal
• A digital signal is discrete. It can have only a
  limited number of defined values, often as simple
  as 1s and 0s.
• The transition of a digital signal from value to
  value is instantaneous like a light being
  switched ON and OFF

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Periodic and Aperiodic Signals
Signals (Analog or Digital) can be periodic or
aperiodic Periodic Signals A signal is called
periodic if it completes a pattern within a
measurable time frame called a period and then
repeats that pattern over identical subsequent
periods
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Aperiodic Signals An aperiodic or non-periodic
signal is the one that changes constantly without
exhibiting a pattern or cycle that repeats over
time
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Characteristics Of Signals

• Signals can be described by three
  characteristics
• Amplitude
• Period/Frequency
• Phase
• Amplitude
• Amplitude of a signal is the value of the signal
  at any point on the wave.
• It is equal to the vertical distance from a given
  point on the wave form to the horizontal axis.
• The maximum amplitude of wave is equal to the
  highest value it reaches on the vertical axis.
• Amplitude measured in Volts, Amperes or Watts

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Characteristics Of Signals

• Period Frequency
• Period Amount of time (in seconds) a signal need
  to complete one cycle.
• Frequency Number of cycles completed in one
  second
• Frequency is measured in hertz
• Phase
• Phase describes the position of the waveform
  relative to time zero.
• If we think of the wave as something that can be
  shifted backward or forward along the time axis,
  phase describes the amount of that shift.
• Phase is measured in Degrees or Radians.

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Analog/Digital Conversions

• Analog data are a function of time and occupy a
  limited frequency spectrum such data can be
  represented by an electromagnetic signal
  occupying the same spectrum.
• Digital data can be represented by digital
  signals, with a different voltage level for each
  of the two binary digits
• But these are not the only possibilities
• Digital data can also be represented by analog
  signals by use of a modem (modulator/demodulator).
  
• Similarly, analog data can be represented by
  digital signals. The device that performs this
  function for voice data is a codec (coder-decoder)

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Analog/Digital Conversions

• Data stored in the computer is in the form of 0s
  and 1s. To be carried from one place to the
  other, data is usually converted to digital
  signals.
• This is called Digital-to-Digital Conversion or
  Encoding digital data into digital signals
• Sometimes we need to convert analog data to the
  digital signal, for example, conversion of
  telephone conversation to digital signal for a
  no. of different reasons.
• This is called Analog-to-Digital Conversion or
  Digitizing an Analog Signal

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Analog/Digital Conversions

• We might want to send a digital signal coming out
  of computer through a medium designed for analog
  signals, for example, to send data from one place
  to the other using a telephone line.
• This is called Digital-to-Analog Conversion or
  Modulating a digital Signal
• Often an analog signal is sent over long
  distances using analog media, for example, voice
  or music from a radio station which is an analog
  signal is transmitted through the air, however
  the frequency of voice or music is not, suitable
  for this kind of transmission.
• The signal should be carried by a higher
  frequency signal. This is called
  Analog-to-Analog Conversion or Modulating an
  analog Signal

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Analog/Digital Conversions
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Signal To be transmitted, data must be
transformed to electromagnetic signals.
Data can be analog or digital. The term analog
data refers to information that is continuous
digital data refers to information that has
discrete states. Analog data take on continuous
values. Digital data take on discrete values.
In data communications, we commonly use periodic
analog signals and nonperiodic digital signals.
Topics discussed in this section
Analog and Digital DataAnalog and Digital
SignalsPeriodic and Nonperiodic Signals
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PERIODIC ANALOG SIGNALS
Periodic analog signals can be classified as
simple or composite. A simple periodic analog
signal, a sine wave, cannot be decomposed into
simpler signals. A composite periodic analog
signal is composed of multiple sine waves.
Topics discussed in this section
Sine WaveWavelengthTime and Frequency
DomainComposite Signals Bandwidth
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Figure A sine wave
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Figure Two signals with the same phase and
frequency, but different amplitudes
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Figure Two signals with the same amplitude and
phase, but different
frequencies
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Example -1
The power we use at home has a frequency of 60
Hz. The period of this sine wave can be
determined as follows Ms 10 3 milliseconds
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Frequency and period are the inverse of each
other.
Frequency is the rate of change with respect to
time. Change in a short span of time means high
frequency. Change over a long span of time
means low frequency.
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If a signal does not change at all, its frequency
is zero. If a signal changes instantaneously, its
frequency is infinite.
Phase describes the position of the waveform
relative to time 0.
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Figure Three sine waves with the same amplitude
and frequency, but
different phases
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Figure Wavelength and period
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Figure The time-domain and frequency-domain
plots of a sine wave
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A complete sine wave in the time domain can be
represented by one single spike in the frequency
domain.
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Example-2
Figure 9 shows a periodic composite signal with
frequency f. This type of signal is not typical
of those found in data communications. We can
consider it to be three alarm systems, each with
a different frequency. The analysis of this
signal can give us a good understanding of how to
decompose signals.
Figure -9 A composite periodic signal
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Figure Decomposition of a composite periodic
signal in the time and
frequency domains
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Figure The time and frequency domains of a
nonperiodic signal
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The bandwidth of a composite signal is the
difference between the highest and the lowest
frequencies contained in that signal.
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Figure The bandwidth of periodic and nonperiodic
composite signals
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Example-3
If a periodic signal is decomposed into five sine
waves with frequencies of 100, 300, 500, 700, and
900 Hz, what is its bandwidth? Draw the spectrum,
assuming all components have a maximum amplitude
of 10 V. Solution Let fh be the highest
frequency, fl the lowest frequency, and B the
bandwidth. Then
The spectrum has only five spikes, at 100, 300,
500, 700, and 900 Hz.
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Figure The bandwidth for Example-4
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DIGITAL SIGNALS
In addition to being represented by an analog
signal, information can also be represented by a
digital signal. For example, a 1 can be encoded
as a positive voltage and a 0 as zero voltage. A
digital signal can have more than two levels. In
this case, we can send more than 1 bit for each
level.
Topics discussed in this section
Bit RateBit LengthDigital Signal as a Composite
Analog Signal Application Layer
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Figure Two digital signals one with two signal
levels and the other
with four signal levels
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Figure The time and frequency domains of
periodic and nonperiodic
digital signals
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Figure Baseband transmission
A digital signal is a composite analog signal
with an infinite bandwidth.
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Figure Bandwidths of two low-pass channels
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Figure Baseband transmission using a dedicated
medium