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CS601 - Data Communication - Lecture Handout 11

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Related Content: CS601 - VU Lectures, Handouts, PPT Slides, Assignments, Quizzes, Papers & Books of Data Communication


Need For Signals

  • One of the major concerns of Physical layer is moving information in the form of electromagnetic signals across a TX medium
  • Information can be voice, image, numeric data, characters or any message that is readable and has meaning to the destination user (human or m/c)
  • Generally, the info usable to a person or application is not in a form that can be transmitted over a network
  • 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 TX. Medium can accept
  • TX. Media work 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

Analog and Digital

  • Both data and signals that represent them can take either analog or digital form


    • 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 refers to something that is discrete
    • Discrete– A set of specific points of data with no points in between


  • Data can be Analog or Digital
    • 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
    • An 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
  • Signals can be Analog or Digital


  • 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


  • 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

Analog and Digital Signals

  • We illustrate signals usually by plotting them on a pair of perpendicular axis
  • Vertical axis represent the value or the strength of the signal
  • Horizontal axes represent the passage of time
  • The curve representing the Analog signal is smooth and continuous, passing through an infinite number
  • The vertical lines of the digital signal shows the sudden jump the signal makes from value to value. The flat highs and the lows represent that those values are fixed
  • In short, Analog signal varies continuously w.r.t Time whereas Digital signal varies instantaneous

Periodic and Aperiodic Signals

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
  • The completion of one full pattern is called a CYCLE
  • Period: Time required (in Seconds) to complete one full cycle, represented by ‘T’

Periodic Signals

Aperiodic Signals

  • An Aperiodic or Non-Periodic signal is the one that changes constantly without exhibiting a pattern or cycle that repeats over time

Aperiodic Signals

  • Fourier Transform
    It has been proved by a technique called FOURIER TRANSFORM that any Aperiodic signal can be decomposed into an infinite number of Periodic Signals


  • Analog signals can be classified as Simple or Composite
  • Simple Analog Signal(Sine Wave)
    • Cannot be decomposed into simpler signal
  • Composite Analog Signal
    • Composed of multiple sine waves

Sine Waves

  • Sine Waves are the most fundamental form of Periodic Analog Signals
  • The curve oscillates over the course of a cycle smoothly and consistently
  • Each cycle consists of a single arc above the time axis followed by a single arc below it
  • Sine Waves can be fully 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 the sine wave is equal to the highest value it reaches on the vertical axis
    • Amplitude measured in Volts, Amperes or Watts


Period & Frequency

  • Period: Amount of time ( in seconds) a signal need to complete one cycle
  • Frequency: Number of cycles completed in one second
  • Unit of Period: Period is expressed in seconds

Period & Frequency

  • Communication industry uses 5 units to measure period
  • Frequency is measured in hertz, There are 5 units used in Hertz



  • Signals
  • Analog and Digital
  • Analog and Digital Data & Signals
  • Periodic & Aperiodic Signals
  • Sine Waves and its Characteristics

Reading Sections

  • Section 4.1, 4.2, 4.3“Data Communications and Networking” 4th Edition by Behrouz A. Forouzan