Question

1. Some of the encoding techniques used over fiber are Manchester, 4B5B, and 8B10B. What are these encoding methods?

Answer 1

Manchester encoding is an encoding method we need to follow two rules encoding method.

• To send a logic '0' bit, increase the voltage up from 0 to +v in the middle of the bit period.
     
• To send a logic '1' bit, decrease the voltage down from +v to 0 in the middle of the bit period.

Regardless of the data sequence, there is a consistent voltage level transition in the middle of each bit-time. This frequent voltage level transitions, allow the receiver to easily determine the beginning and end of each bit, since each mid-bit transition can be used as a clock reference. Thus, Manchester encoding is effectively self-clocking

4B/5B is an encoding method that replaces every group of 4 bits by a 5-bit code. The code ensures that at least one voltage level transition occurs for every 5 bits.

let's encode the data stream 0111010000100000

the data is first grouped into 4-bit nibbles. Then a 5-bit code is applied to each separate 4-bit group.

The 8B10B encoding method is similar to 4B5B in that a group of bits are replaced with code words. The difference is that with 8B10B, each group of 8 bits is replaced with a 10-bit code word. Since a 10-bit code word replaces each 8 bits of data, the line speed actually has to be 1.25Gbps for a data speed of 1Gbps.

The 10-bit code words were designed to limit the sequence of 1's or 0's that could occur, to maintain clocking.