2B1Q Line Coding: Advantages and Disadvantages

This page explores the advantages and disadvantages of 2B1Q encoding, covering its benefits and drawbacks.

What is 2B1Q Line Coding?

Introduction: 2B1Q is a multilevel data encoding scheme used to encode patterns of ‘m’ data elements into patterns of ‘n’ signal elements.

It is classified into different types as “mBnL,” where:

• m = length of binary pattern
• B = binary data
• n = length of signal pattern
• L = number of levels

Here, the first two letters indicate the data pattern, while the second two letters indicate the signal pattern. L=2 means Binary, L=3 means ternary, L=4 means Quaternary, etc.

2B1Q refers to “two binary, one quaternary.” This line coding technique uses data patterns of size 2. It encodes 2-bit patterns as one of the signal elements belonging to a four-level signal.

Other multilevel encoding schemes include 8B6T and 4D-PAM5.

Benefits or Advantages of 2B1Q Line Coding

The following are the benefits of using 2B1Q line coding:

• Increased bits per baud: It helps increase the number of bits per baud, which is desirable in data communication.
• Bandwidth Savings: It can send more than one data bit with a single signal, saving bandwidth. The required average bandwidth equals N/4.
• No Redundant Signals: There are no redundant signal patterns in this technique, making efficient use of signal space.
• DSL Application: It is used in Digital Subscriber Line (DSL) to provide high-speed internet access.
• Voltage Balance: Typical codes balance voltage, leading to less baseline wandering.

Drawbacks or Disadvantages of 2B1Q Line Coding

The following are the drawbacks of using 2B1Q line coding:

• Complexity: It is complex because the receiver has to distinguish between four different thresholds, as 2B1Q uses four different signal levels. This increases the complexity of the receiver design.
• Lack of Self-Synchronization: It lacks self-synchronization for long patterns of similar double bits, potentially leading to timing drift.
• DC Output Problems: Long sequences of zeros or “01” will have a constant DC output. Hence, the spectrum creates very low frequencies, which present problems in certain systems due to DC bias.