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CS601 - Data Communication - Lecture Handout 14

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  • Information must be transformed into signals before it can be transported across the communication media
  • How info is transformed depends on its original format and on the format used by the communication device
  • If you want to send a letter by a smoke signal, you need to know which smoke patterns make which words in your message before building the fire
  • Words are the Information and the puffs of smoke are representation of that information

Introduction to the type of Conversions

  • Data stored in the computer is in the form of 0’s and 1’s. 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 an analog signal to the digital signal
  • For Example, conversion of Telephone conversation to digital signal for a no. of different reasons such as to decrease the effect of noise
  • This is called “Analog-to-Digital Conversion” or “Digitizing an Analog Signal”
  • 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 Tx.
  • The signal should be carried by a higher frequency signal
  • This is called “Analog-to-Analog Conversion” or “Modulating an analog Signal”

Types of Conversions

Types of Conversions

Digital-to-Digital Conversion

  • Digital-to-Digital conversion/encoding is the representation of digital information by digital signal
  • For Example when you Tx data from Computer to the Printer, both original and transmitted data have to be digital
  • In this type of encoding, 1’s and 0’s generated by the computer are translated into voltage pulses that can be propagated over the wire
  • Figure shows the relationship between digital information, digital-to-digital encoding hardware , and the resultant digital signal

Digital-to-Digital Conversion

Types of Digital-to-Digital Encoding

Types of Digital-to-Digital Encoding

    • Encoding is simple , with only one technique in use
    • Simple and Primitive
    • Almost Obsolete Today
    • Study provides introduction to concepts and problems involved with more complex encoding systems


  • Digital Transmission system works by sending voltage pulses on the Tx. Medium
  • One voltage level stands for binary 0 while the other stands for binary 1
  • It is called Unipolar because it uses only one polarity
  • This polarity is assigned to to one of the two binary states usually a ‘1’
  • The other state usually a 0 is represented by zero voltage
  • Figure shows the idea: 1’s are encoded as +ve values, and 0’s are encoded as –ve values

Pros and Cons of Unipolar Encoding

  • PROS
    • Straight Forward and Simple
    • Inexpensive to Implement
  • CONS
    • DC Component
    • Synchronization

DC Component

  • Average Amplitude of a unipolar encoded signal is non-zero
  • This is called DC Component I.e. a component with zero frequency
  • When a signal contains a DC Component, it cannot travel through a Tx. Medium that cannot handle DC components


  • When the signal is unvarying, Rx. Cannot determine the beginning and ending of each bit
  • Synchronization Problem can occur when data consists of long streams of 1’s or 0’s
  • Therefore, Rx has to rely on a TIMER
  • Consider we have a bit rate of the signal to be 1000bps
    • 1000 bits in 1 second
    • 1 bit in 0.001 second
  • So if a +ve voltage lasts 0.005 sec, it reads five 1’s
  • Sometimes it stretches to 0.006 seconds and an extra one bit is read by the Rx
    • Solution:
      • Separate Parallel Line
  • Carries a clock pulse and allows receiver to resynchronize its timer to that of the signal
  • Doubling no. of Tx lines increase Cost and proves uneconomical


Encoding has 3 subcategories:

  • Non Return to Zero (NRZ),-- Return to Zero (RZ)
  • Bi phase
  • Two of which have multiple variations of their own
  • Polar encoding uses two voltage levels
    • One positive and one negative
  • By using two voltage levels, average voltage level on the line is reduced and DC Component problem of unipolar encoding is alleviated


Types of Polar Encoding

Non Return to Zero (NRZ)

  • In NRZ Encoding, the level of signal is either positive or negative

Non Return to Zero (NRZ)

  • NRZ-L
    • Level of the signal depends on the type of bit it represents
    • A +ve voltage usually means the bit is a 1 and a –ve voltage means the bit is a 0 (vice versa)


  • Problem with NRZ-L: When long streams of 0’s or 1’s are there in data, Rx receives a continuous voltage and should determine how many bits are sent by relying on its clock , which may or may not be synchronized with the sender clock


  • NRZ-I
    • The inversion of the level represents a 1 bit
    • A bit 0 is represented by no change
    • NRZ-I is superior to NRZ-L due to synchronization provided by signal change each time a 1 bit is encountered
    • The string of 0’s can still cause problem but since 0’s are not as likely, they are less of a problem



  • Introduction to the Encoding Techniques
  • Digital-To-Digital Encoding
  • Types of Digital-To-Digital Encoding
  • UniPolar Encoding
  • Polar Encoding
    • NRZ

Reading Sections

  • Section 5.1, “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan