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There are different techniques to represent digital data using digital signals. These techniques fall into three divisions viz. line coding, block coding and scrambling. 4B/5B is one of the block coding technique. The other one is 8B/10B encoding.

As block coding technique takes block of "m bits" as input and generates block of "n bits" as output it is known as mB/nB encoding.

### What is 4B/5B encoding?

The Block coding involves three steps viz. division, substitution and combination. Let us understand this with 4B/5B encoding type. It produces more output bits than input bits.
• Step-1 : Initially, input bit sequence is divided into 4 bit groups.
• Step-2 : The 4 bit group is now substituted with 5 bit group as per table.
• Step-3 : Finally, n-bit groups are combined to form output stream.

The 4B/5B encoding technique is designed to work along with NRZ-I line coding. NRZ-I offers better signal rate which is (1/2) of biphase but it suffers from synchronization issue. This issue is solved by incorporating 4B/5B before NRZ-I encoder as shown in figure-1 above. 4B/5B ascertain that NRZ-I encoder does not get long stream of zeros ('0s').

At the receiver, first stream is passed through NRZ-I decoder and later redundant bits are removed using 4B/5B decoder to get original sream of data.

The table above shows corresponding input and their encoded pairs. The 16 input groups will occupy 16 output combinations from total 32 as supported by group of 5 bits. The rest of unused groups are used for control purposes as shown and others are not used at all. The receiver takes care of unused portion of group of bits by considering them as error in data transmission.

### Benefits or advantages of 4B/5B encoding

Following are the benefits or advantages of 4B/5B encoding:
➨It solves problem of synchronization observed in NRZ-I.
➨It offers signal rate less than Biphase scheme.

### Drawbacks or disadvantages of 4B/5B encoding

Following are the drawbacks or disadvantages of 4B/5B encoding:
➨It increases signal rate compare to NRZ-I as redundant bits add 20% more baud.
➨It does not eliminate DC components found in NRZ-I signal waveforms.

If DC components are unacceptable, Biphase or Bipolar encoding is used in place of 4B/5B encoding.