Line Coding

SUBMITTED BY :

GAGANDEEP SINGH RANDHAWA

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).

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.

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.

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

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

Types of Line Coding

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.

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.

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.

Types of Block Codes

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

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.

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