THE JOURNEY OF LINE ENCODING METHODS
May 20, 2021
What Is Line Encoding ?
Line Encoding is the process of converting digital data to digital signals in waveform pattern of voltage or current. As we consider data, in form of text, number, graphical images, audio etc. are stored in memory as sequences of bits. As of the data transmission of digital signal over transmission line. This process is used to avoid overlap and distortion of signals.
Properties of Line Encoding:-
- As the coding is done to make more bits transmit on a single signal, the bandwidth used is much reduced.
- For a given bandwidth, the power is efficiently used.
- The probability of error is much reduced.
- Power density is much favorable.
- The timing content is adequate.
- Long strings of 1s and 0s is avoided to maintain transparency.
Types of Line Coding:-
There are 5 types of Line Coding:
1)UNIPOLAR:-
- Unipolar Line encoding is a line code which represent positive voltage as binary 1, and zero volts indicates as binary 0.Unipolar encoding uses only one voltage level. Its a simple line code which directly encodes the bitstream, and is analogous to on-off keying in modulation.
But it also has some drawbacks that it is not self-clocking and it has significant DC component, which can be half by using return-to-zero, where the signal returns to zero in middle of bit period. In Unipolar we have one type known NRZ encoding scheme.
NRZ(Non -Return-to-Zero) — Unipolar scheme is designed as a NRZ scheme in which there is bit 1 and bit 0. It is called NRZ because the signal does not return to zero at the middle of the bit ,as instead happens in other line coding schemes such as Manchester code.
2)POLAR:-
Polar encoding is a line code which uses two voltage levels positive and negative. By using both of the levels the average voltage level on the line is reduced & the DC component problem of unipolar encoding is alleviated.
There are 3 popular variation of polar encoding:
- Non Return to Zero (NRZ).
In NRZ encoding the level of signal is always either positive or negative as it does not return to zero. There two most popular methods of NRZ transmission.
a) NRZ-L(NRZ Level):
In NRZ-L encoding the type of signal depends on the type of bit it represents. As if positive voltage usually represent bit as 0, and negative or vice versa. In this scheme the level of the signal is dependent upon the state of the bit.
b) NRZ-I(NRZ Inversion):
Its a method, in which the inversion of the voltage level represent a 1 bit. It is a transition between a positive & a negative voltage, not the voltage themselves that represent a 1 bit. A 0 bit is represented as a no change.
Out of these two methods the NRZ-I is superior to NRZ-L due to the synchronization provided by the signal change each time by 1 bit is encountered
2. Return to Zero (RZ).
RZ (return-to-zero) refers to a form of digital data transmission in which the binary low and high states, represented by numerals 0 and 1, are transmitted by voltage pulses having certain characteristics. The signal state is determined by the voltage during the first half of each data viz 0 or 1.
The signal returns to a resting state ( called zero) during the second half of each bit. The resting is usually of zero volts. In RZ transmission the signal changes not between bits but during the bits .To summarize, RZ uses three values Positive, negative & zero.
3.Biphase Encoding.
The best solution to the problem of synchronization is biphase encoding. In this method the signal changes at the middle of the bit but does not return to zero. Instead it continues to the opposite pole. It has two variation.
a) Manchester Encoding:
In telecommunication and data storage , Manchester coding (also known as phase encoding) is a line code in which the encoding of each data bit has at least one transition and occupies at the same time. It therefore has no DC component, and is self clocking. Manchester encoding uses the inversion at the middle of each bit interval for both synchronization and bit representation.
A negative-to-positive transition represent binary 1 and a positive-to-negative transition represent binary 0.
b) Differential Manchester Encoding:
Its a line code in which data and clock signals are combined to form a single 2-level self — synchronization data stream. It is a differential encoding , using the presence or absence of transition to indicate logical value. In Differential Manchester encoding does not matter weather a logical 1 or 0 is received , but only whether the polarity is same or different from the previous value ; this makes synchronization easier.
In differential Manchester, the inversion at the middle of the bit interval is used for synchronization, but the presence or absence of an additional transition at the beginning of the interval is used to identify the bit.
3)BI-POLAR:
In telecommunication, bipolar encoding is a type of RZ line code, where two non-zero values are used so that the three values are positive, negative and zero. Such signal is called a duobinary signal. The bipolar scheme is an alternative to NRZ , this scheme has the same signal rate but there is no DC component as one bit is represented by voltage zero other alternates every time.
a) Alternate mark inversion (AMI):
AMI is a synchronous clock encoding technique that uses bipolar pulses to represent logical 1. The next logic 1 is represented by a pulse of the opposite polarity. Hence a sequence of logical 1s are represented by a sequence of alternating polarity
4)MULTILEVEL:
Multilevel encoding scheme is desire to increase the data speed or decrease the required bandwidth has resulted in creation of many schemes. The goal is to increase the number of bits per baud by encoding a pattern of m data elements into a pattern of n signal elements.
a) 8B/6T :
It is a very interesting scheme Eight binary, six ternary(8b/6t). This code is used with 100BASE-T4 cable. The idea is to encode pattern of 8 bits as a pattern of six signal elements, where signal has three levels(ternary).
5)MULTITRANSITION:
In Multitransition encoding we have MLT-3 encoding scheme.
MLT-3 encoding (Multi-Level Transmit) is a line code which uses 3 voltage level. MLT-3 interface emits less electromagnetic interference and requires less bandwidth than most other binary or ternary interface that operates at the same bit rate.
MLT-3 cycles sequentially through the voltage levels -1,0,+1. It also has coding efficiency of 1bit/baud, however it requires four transition baud to complete a full cycle.
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SY CS A Group N0. 08
