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CS601 - Data Communication - Lecture Handout 14

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Conversions

Introduction

  • 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

  • UNIPOLAR
    • 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

UNIPOLAR

  • 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

Synchronization

  • 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

POLAR

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

POLAR

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

NRZ-I

Summary

  • 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