Line Codes

A line code is the code used for data transmission of a digital signal over a transmission line. This process of coding is chosen so as to avoid overlap and distortion of signal such as inter-symbol interference.

Properties of Line Coding

Following are the properties of line coding −

·        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.

·        Error detection is done and the bipolar too has a correction capability.

·        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 3 types of Line Coding

• Unipolar
• Polar
• Bi-polar

Unipolar Signaling

Unipolar signaling is also called as On-Off Keying or simply OOK.

The presence of pulse represents a 1 and the absence of pulse represents a 0.

There are two variations in Unipolar signaling −

• Non Return to Zero NRZNRZ
• Return to Zero RZRZ

Unipolar Non-Return to Zero NRZNRZ

In this type of unipolar signaling, a High in data is represented by a positive pulse called as Mark, which has a duration T₀ equal to the symbol bit duration. A Low in data input has no pulse.

The following figure clearly depicts this.

Advantages

The advantages of Unipolar NRZ are −

• It is simple.
• A lesser bandwidth is required.

Disadvantages

The disadvantages of Unipolar NRZ are −

·        No error correction done.

·        Presence of low frequency components may cause the signal droop.

·        No clock is present.

·        Loss of synchronization is likely to occur (especially for long strings of 1s and 0s).