Manchester Coding

Line Coding

Manchester Coding

Manchester coding consists of combining the NRZ-L and RZ schemes.In Manchester encoding , the duration of the bit is divided into two halves. The voltage remains at one level during the first half and moves to the other level during the
second half. In other words we can say that every symbol has a level transition in the middle: from high to low or low to high , depending on the values of the bit/symbol. It uses only two voltage levels.

• A ‘One’ is +ve in 1st half and -ve in 2nd half.
• A ‘Zero’ is -ve in 1st half and +ve in 2nd half.

or viceversa.
The transition at the centre of every bit interval is used for synchronization at the receiver. Manchester encoding is called self-synchronizing. Synchronization at the receiving end can be achieved by locking on to the the transitions, which indicate the middle of the bits

Manchester Line Coding Scheme

Advantages:

1. No DC component.
2. Does not suffer from signal droop (suitable for transmission over AC coupled lines).
3. Easy to synchronise.
4. Is Transparent.

Disadvantages: 

1. Because of the greater number of transitions it occupies a significantly large bandwidth. 
2. Does not have error detection capability.

These characteristic make this scheme unsuitable for use in Wide Area Networks. However, it is widely used in Local Area Networks such as Ethernet and Token Ring.

Differential Manchester

Differential Manchester coding consists of combining the NRZ-I and RZ schemes.Every symbol has a level transition in the middle. But the level at the beginning of the symbol is determined by the symbol value. One symbol causes a level change the other does not.

• A'ONE' is represented by absence of transition at the beginning of  bit/symbol interval
• A 'ZERO' is represented by presence of transition at the beginning of bit/symbol interval

In differential Manchester the middle transition is only used for self synchronization and clocking information.

Comparison Between Manchester and Differential Manchester Line Coding Schemes

Modulation rate for Manchester and Differential Manchester is twice the data rate so these schemes are inefficient encoding for long-distance applications.The minimum bandwidth of Manchester and differential Manchester is 2 times that of NRZ. The is no DC component and no baseline wandering. None of these codes has error detection