Line coding

1. Line Coding
SUBMITTED BY :
GAGANDEEP SINGH RANDHAWA

2. Line Coding
 In Optical Fiber Communication, Signal Encoding uses a set of rules for
arranging the signal symbols in a particular pattern. This process is called
Channel or line coding.
 Method of converting binary sequence into digital signal.
 Goal is to transmit binary data (e.g., PCM encoded voice, MPEG encoded
video, financial information).

3. Line Coding ( contd. )
 Transmission distance is large enough that communication link bandwidth is
comparable to signal bandwidth.
 Connections between nearby logic gates have bandwidth greater than
switching speed, so no line coding is needed. But longer connections use pulse
shaping.
 Multiple links may be used, with regenerative repeaters.

4. Principle of Line Coding
 To introduce redundancy into the data stream for the purpose of minimizing the
error that results from channel interference effects.
 Depending on the amount of redundancy introduced, any degree of error-free
transmission of digital data can be archived, provided that the data rate that
includes this redundancy is less than the channel capacity. This is called
Shannon channel-coding theory.

5. Line Coding Requirements
 Small transmission bandwidth
 Power efficiency: as small as possible for required data rate and error
probability
 Error detection/correction
 Suitable power spectral density, e.g., little low frequency content
 Timing information: clock must be extracted from data

6. Types of Line Coding
 There are 3 basic types of line coding, that is used for optical fiber
transmission links:
i). NRZ (Non-Return-to-Zero) Format
ii). RZ (Return Zero) Format
iii). PE (Phase Encoded) Format

7. Types of Line Coding

8. Non-Return-to-Zero
 A non-return-to-zero (NRZ) line code is a binary code in which 1s are
represented by one significant condition (positive voltage) and 0s are
represented by some other significant condition (negative voltage), with no
other neutral or rest condition.
 The pulses have more energy than a return-to-zero(RZ) code. Unlike RZ, NRZ
does not have a rest state. NRZ is not inherently a self-clocking signal, thus
some additional synchronization technique must be used for avoiding bit slip.

9. Return-to-Zero
 RZ takes place even if a number of consecutive 0s or 1s occur in the signal.
The signal is self-clocking.
 That a separate clock does not need to be sent alongside the signal, but suffers
from using twice the bandwidth to achieve the same data-rate as compared
to non-return-to-zero format.

10. PE(Phase Encoded) Format
 The mBnB code converts a block of m input bits to a block of n code bits.
 Coding efficiency is determined by the ratio m/n, but generally n is chosen as
m+1.
 The mBnB codes is elimination of the dc component that may exist in the data
source. Hence code words are selected to preserve balance.
 The insertion of transitions to ensure adequate timing recovery. Hence the
selection of code words also attempts to provide at least one transition in each
code and to minimize the distance(number of bits) between transitions.

11. Types of Block Codes

12. Manchester Coding
 Instead of ‘high’ equaling ‘1’ and ‘low’ equaling ‘0’ a timing interval is used to
measure high or low transition.
 Uses inversion at he middle of each bit interval - provides
synchronization - bit representation
 Achieves same level of synchronization as RZ
 Requires only two level of amplitudes representation

13. Advantages of Manchester
Coding
 Error recovery – if part of the signal clipped or distorted
still “intelligence” is available for timing recovery.
 Signal is falling or raising.

14. THANK YOU