8B6T Line Coding: Advantages and Disadvantages

This page explores the advantages and disadvantages of 8B6T encoding. It details the benefits and drawbacks of this line coding technique.

What is 8B6T Line Coding?

Introduction:

8B6T is a multilevel line coding scheme used to encode “m” data elements into a pattern of “n” signal elements. Specifically, 8B6T converts 8 bits of data into 6 signal elements using three signal levels: positive (+), negative (-), and zero (0). This code finds its application in 100Base-4T cables.

This line code utilizes 2⁸ (256) different data patterns and 3⁶ (729) different signal patterns. This leaves us with approximately 473 redundant signal elements.

The figure above illustrates the encoding of three different data patterns into corresponding signal patterns.

• First 8-bit pattern [00010001]: Encoded as signal pattern [- 0 - 0 + +] with a weight of 0.
• Second 8-bit pattern [01010011]: Encoded as [- + - + + 0] with a weight of +1.
• Third 8-bit pattern [01010000]: Encoded as [+ - - + 0 +] with a weight of +1.

To achieve DC balance, the sender inverts the last bit pattern using a weight of -1. For example:

• The third pattern becomes [- + + - 0 -] before transmission (shown in yellow).
• The receiver easily identifies this inverted pattern and inverts it again before decoding, restoring the pattern to its original form.

The average signal rate (S_ave) is calculated as (1/2) ** N ** (6/8), where N is the data rate. The minimum bandwidth is approximately 6N/8.

Benefits or Advantages of 8B6T Line Coding

The following are the benefits of 8B6T line coding:

• Synchronization and Error Detection: The redundant data provided by 8B6T aids in synchronization and error detection during transmission.
• DC Balance: The redundancy enables DC balance through the use of inverted patterns at the transmitting end.
• Increased Baud Rate: 8B6T increases the number of bits per baud, effectively raising the speed or baud rate.

Drawbacks or Disadvantages of 8B6T Line Coding

The following are the drawbacks of 8B6T line coding:

• Increased Bandwidth: The use of redundant data bits leads to an increase in bandwidth requirements.
• Complexity at the Receiver: The receiver needs to distinguish between three distinct levels (+, -, 0) to accurately decode the data bits.
• Complexity at the Sender: The sender needs to maintain a record of weight and possess the intelligence to determine the weight of groups. If two groups with a weight of “1” are encountered consecutively, the first group is transmitted as is, while the second group is completely inverted to provide a weight of “-1”.