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

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”

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

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

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

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

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

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