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Advantages of 8B6T Line Coding | Disadvantages of 8B6T Encoding

This page covers advantages and disadvantages of 8B6T encoding. It mentions benefits or advantages of 8B6T line coding and drawbacks or disadvantages of 8B6T line coding.

What is 8B6T line coding?

Introduction:
It is multilevel line coding scheme used to encode "m" data elements into pattern of "n" signal elements. The 8B6T converts 8 bits of data into 6 signal elements using three levels of signal viz. +ve, -ve and zero. This code is used in 100Base-4T cable.

6B8T encoding

This line code uses 28 i.e. 256 different data patterns and 36 (i.e. 729) different signal patterns. There are about 473 redundant signal elements.

The figure depicts encoding of three data patterns using three signal patterns. The three signal levels used are -, 0 and +.
• First 8 bit pattern [00010001] is encoded as signal pattern [- 0 - 0 + +] with weight 0.
• Second 8 bit pattern [01010011] is encoded as [- + - + + 0] with weight +1.
• Third 8 bit pattern [01010000] is encoded as [+ - - + 0 +] with weight +1.
• To create DC balance last bit pattern is inverted using weight -1 by sender.
• Third pattern becomes [- + + - 0 -] before transmission as shown in yellow color.
• The receiver easily recognizes inverted pattern and it does inversion before decoding to bring pattern to normal pattern.

Average signal rate is Save = (1/2)*N*(6/8)
Minimum Bandwidth is close to 6N/8.

Benefits or advantages of 8B6T line coding

Following are the benefits or advantages of 8B6T line coding:
➨Due to redundant data it provides synchronization and error detection.
➨The redundancy is used to provide DC balance using inverted pattern at the transmit end as shown.
➨It increases speed or baud rate as it increases number of bits per baud.

Drawbacks or disadvantages of 8B6T line coding

Following are the drawbacks or disadvantages of 8B6T line coding:
➨It uses redundant data bits with increases bandwidth.
➨Receiver has to distinguish three levels in order to decode the data bits.
➨Sender is complex as it has to keep record of weight and also need to possess intelligence to determine weight of groups. If two groups of weight "1" are encountered consecutively, the first group is sent as it is where as second group is totally inverted to provide weight of "-1".



Line coding techniques

Difference between Unipolar Polar and Bipolar coding    RZ vs NRZ vs Manchester coding    RZ vs NRZ pulse shapes    Advantages and disadvantages of NRZ encoding    RZ encoding    2B1Q coding    8B6T coding    4D PAM5 coding    MLT-3 coding    4B/5B encoding    8B/10B encoding    R8ZS scrambling    HDB3 scrambling   

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