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Creation: September 18 2018
Modified: January 01 2019

Information Bits Encoding

Encoding of binary data in a signal that the transmission line is able to carry. Assume we are working with two discrete signals: high and low.

Boud rate: the rate of signal transitions on the link. Bit rate: the rate of bits transmission on the link

NRZ

Non-Return to Zero (NRZ) encoding maps the data value 1 onto the high signal and the data value 0 onto the low signal.

Bits    0   1   0   1   1   0   0   1
           ___     _______         ___
NRZ    ___|   |___|       |_______|   

The encoding has two problems caused by long strings of 1s or 0s.

The baud rate is equal to the bit rate.

NRZI

Non-Return to Zero Inverted (NRZI) encoding makes a transition from the current signal to encode a 1 and stay at the current signal to encode a 0.

Bits    0   1   0   1   1   0   0   1
           _______     ___________
NRZI   ___|       |___|           |___

The encoding solves the problem of consecutive ones but not of consecutive zeros.

Manchester

The internal clock signal is XOR-ed with the NRZ-encoded data. The 0 is thus encoded as a low-to-high transition and 1 being encoded as a high-to-low transition.

With Manchester both 0s and 1s encoding results in a transition of the signal.

Bits    0   1   0   1   1   0   0   1
           ___     _______         ___
NRZ    ___|   |___|       |_______|
         _   _   _   _   _   _   _   _ 
Clock  _| |_| |_| |_| |_| |_| |_| |_| |
         ___     ___   _     _   ___   
Manch  _|   |___|   |_| |___| |_|   |_|

The problem is that it doubles the rate at which signal transitions are made on the link. Thus the bit rate is half of the bound rate.

Differential Manchester

A 1 is ancoded with the first half of the signal equal to the last half of the previous bit's signal and 0 is encoded with the first half of the signal opposite to the last half of the previous bit's signal.

Same advantages and disadvantages of the Manchester encoding.

4B/5B

The idea is to insert one extra bit into the bit stream to break long sequences of 0s or 1s. Specifically, every 4 bits of data are encoded in 5-bit code. The 5-bit codes are selected so that each one has no more than one leading 0 and no more than two trailing 0s. Thus, no pair of 5-bit codes results in more than three consecutive 0s being transmitted.

The resulting 5-bit codes are then transmitted using NRZI.

Of the 4B/5B codes, 16 are used to encode the 4-bit payloads, 11111 is used when the line is idle, 00000 corrensponds to a dead line, 00100 is the mean halt. Of the remaining 13 codes, 7 of them are not valid because they violate the constraint rule, and the other 6 represents various control symbols. Some framing protocols (e.g. FDDI) make use of these control symbols.

4B/5B solves the "clock recovery" and the "baseline wander" issues by loosing the 20% of efficiency (4/5=0.8). Baud rate is 5/4 of bit-rate.

Other popular similar encodings are

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