Download Conversions-Data Communication-Lecture Notes and more Study notes Data Communication Systems and Computer Networks in PDF only on Docsity!
LECTURE
Conversions
Introduction o Information must be transformed into signals before it can be transported across the communication media o How info is transformed depends on its original format and on the format used by the communication device o 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 o Words are the Information and the puffs of smoke are representation of that information
Introduction to the type of Conversions
o 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 o This is called “ Digital-to-Digital Conversion ” or “ Encoding digital data into digital signals ” o Sometimes we need to convert an analog signal to the digital signal o For Example, conversion of Telephone conversation to digital signal for a no. of different reasons such as to decrease the effect of noise o This is called “ Analog-to-Digital Conversion ” or “ Digitizing an Analog Signal ” o We might want to send a digital signal coming out of computer through a medium designed for analog signals o For example, To send data from one place to the other using a Telephone line o This is called “ Digital-to-Analog Conversion ” or “ Modulating a digital Signal ” o Often an analog signal is sent over long distances using analog media o 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. o The signal should be carried by a higher frequency signal o This is called “ Analog-to-Analog Conversion ” or “ Modulating an analog Signal ”
Types of Conversions
Digital-to-Digital Conversion o Digital-to-Digital conversion/encoding is the representation of digital information by digital signal
docsity.com
o For Example when you Tx data from Computer to the Printer, both original and transmitted data have to be digital o 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
o Figure shows the relationship between digital information, digital-to-digital encoding hardware , and the resultant digital signal
Types of Digital-to-Digital Encoding
Digital/Digital Encoding
Unipolar Polar Bipolar
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
o Digital Transmission system works by sending voltage pulses on the Tx. Medium o One voltage level stands for binary 0 while the other stands for binary 1 o It is called Unipolar because it uses only one polarity
docsity.com
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) o In NRZ Encoding, the level of signal is either positive or negative
NRZ
NRZ-L NRZ-I
o 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
o NRZ-I
The inversion of the level represents a 1 bit
docsity.com
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
Summary
Introduction to the Encoding Techniques Digital-To-Digital Encoding Types of Digital-To-Digital Encoding UniPolar Encoding Polar Encoding
Reading Sections
Section 5.1, “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan
docsity.com
