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Line Coding Schemes

September 18, 2016
Published By : Pratik Kataria
Categorised in: Data Communicaiton & Wireless Sensor Networks

Line Coding

• Unipolar
  • Only two voltage levels are used. (1 means sending something, 0 means not sending)
  • All signal levels are on one side of the time axis – either above or below
  • NRZ – Non Return to Zero scheme is an example of this code. The signal level does not return to zero during a symbol transmission.
  • Unipolar NRZ Scheme
    
• Polar
  • The voltages are on the both sides of the time axis. Ex: Polar NRZ
  • Polar alternatives : NRZ, RZ, Manchester, Differential Manchester
  • 
  • Polar NRZ scheme can be implemented with two voltages. E.g. +V for 1 and -V for 0.
  • There are two versions:
    • NZR – Level (NRZ-L) – 1 for low level and 0 for high level
    • NRZ – Inversion (NRZ-I) – the inversion or the lack of inversion determines the value of the bit. (for each one in the bit sequence the signal level is inverted. )
  • 
  • Polar RZ ensures Synchronization, there must be a single transition in each bit.
  • The Return to Zero (RZ) scheme uses three voltage values. +, 0, -.
  • 
  • Manchester coding consists of combining the NRZ-L and RZ schemes.
    • In Manchester encoding there is a mid-bit transition serves as a clocking mechanism and also as data.
    • Low to high represents a 1. High to low represents a 0.
  • Differential Manchester coding consists of combining the NRZ-I and RZ schemes.
    • Always a transition at the middle of the bit
    • Presence of transition in the beginning of a bit represents 0.
  • 
• Bipolar Scheme
  • Three voltage levels- positive, negative, and zero.
  • The voltage level for one data element is at zero, while the voltage level for the other element alternates between positive and negative.
  • Example : AMI and Pseudoternary
• Alternate mark inversion (AMI)
  • A common bipolar encoding scheme is called bipolar alternate mark inversion (AMI). In alternate mark inversion, a neutral zero voltage represents binary O. Binary 1s are represented by alternating positive and negative voltages.
  • Disadv : Synchronization fails when long sequence of 0.
  • 
• Pseudoternary
  • A variation of AMI encoding is called Pseudoternary in which the 1 bit is encoded as a zero voltage and the 0 bit is encoded as alternating positive and negative voltages.
  • 

Scrambling Schemes

• Extension of Bipolar AMI.
• Used in case of long distance applications.
• Goals :
  • No DC Component
  • No Long sequences of 0 level line signal.
  • Minimum increase in bandwidth.
  • Error detection capability.
• Example : B8ZS, HBD3

B8ZS (Bipolar with 8 zero substitution)

HBD3(High Density Bipolar- 3 Zero’s

Pratik Kataria

Pratik Kataria is currently learning Springboot and Hibernate.
Technologies known and worked on: C/C++, Java, Python, JavaScript, HTML, CSS, WordPress, Angular, Ionic, MongoDB, SQL and Android.
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