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Line Coding Schemes


September 18, 2016
Published By : Pratik Kataria
Categorised in:

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
    • line-coding2
    • 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. )
    • line-coding3
    • Polar RZ ensures Synchronization, there must be a single transition in each bit.
    • The Return to Zero (RZ) scheme uses three voltage values. +, 0, -.
    • line-coding4
    • 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.
    • line-coding5
  • 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.
    • line-coding6
  • 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.
    • line-coding7

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