Manchester Encoding in Computer Network

Prerequisite – Difference between Unipolar, Polar and Bipolar Line Coding Schemes

Manchester encoding is a synchronous clock encoding technique used by the physical layer of the Open System Interconnection [OSI] to encode the clock and data of a synchronous bit stream.  The idea of RZ and the idea of-L are combined in manchester

Different encoding techniques are used in data communication to ensure data security and transmission speed. Manchester encoding is an example of digital encoding. Because each data bit length is defined by default, it differs from other digital encoding schemes. The bit state is defined by the direction of the transition. Bit status is represented in various ways by different systems, although most systems use 1 bit for low to high transitions and 0 bit for high to low transitions.

In manchester duration of a bit is divided into two halves. The voltage remains the same at one level during the first half & moves to the other level.The transition at the middle of the bit provides synchronization.Differential Manchester,on the other hand,combines the idea of RZ and NRZ-I. There is always a transition at the middle of the bit, but the bit values are determined at the beginning of the bit. if next bit is zero there is transition if next bit is 1 there is none. 

Note: Manchester encoding’s main advantage is signal synchronization.

The binary data to be transmitted over the cable are not sent as NRZ [Non-return-to-zero]. 

Non-return-to-zero [NRZ] –
NRZ code’s voltage level is constant during a bit interval. When there is a long sequence of 0s and 1s, there is a problem at the receiving end. The problem is that the synchronization is lost due to a lack of transmissions. 
It is of 2 types: 

1. NRZ-level encoding –
   The polarity of signals changes when the incoming signal changes from ‘1’ to ‘0’ or from ‘0’ to ‘1’. It considers the first bit of data as polarity change.
2. NRZ-Inverted/ Differential encoding –
   In this, the transitions at the beginning of the bit interval are equal to 1 and if there is no transition at the beginning of the bit interval is equal to 0.

Characteristics of Manchester Encoding –

• A logic 0 is indicated by a 0 to 1 transition at the center of the bit and logic 1 by 1 to 0 transition.
• The signal transitions do not always occur at the ‘bit boundary’ but there is always a transition at the center of each bit.
• The Differential Physical Layer Transmission does not employ an inverting line driver to convert the binary digits into an electrical signal. And therefore the signal on the wire is not opposite the output by the encoder.
• The following are the properties of Manchester encoding:
• Each bit is sent at a predetermined rate.
• When a high to low transition happens, a ‘1’ is recorded; when a low to high transition occurs, a ‘0’ is recorded.
• At the mid-point of a period, the transition that is utilized to precisely note 1 or 0 happens.
  The Manchester Encoding is also called Biphase code as each bit is encoded by a positive 90 degrees phase transition or by negative 90 degrees phase transition.
• The Digital Phase Locked Loop (DPLL) extracts the clock signal and deallocates the value and timing of each bit. The transmitted bitstream must contain a high density of bit transitions.
• The Manchester Encoding consumes twice the bandwidth of the original signal.
• The advantage of the Manchester code is that the DC component of the signal carries no information. This makes it possible that standards that usually do not carry power can transmit this information.
• It is a self-clocking protocol, meaning that the receiver can determine the clock frequency from the incoming data.
• The Manchester encoding ensures a constant transition density, making it easier to detect the start and end of a data frame.
• It provides a simple and reliable way to detect errors in the data transmission by checking for a violation of the encoding rules.
• The encoding process adds a redundant bit to the data, enabling error correction in some applications.
• Manchester encoding can also be used for multi-level signaling, where multiple voltage levels are used to represent different data states.

Only drawback is the signal rate.The signal rate is manchester and differential is double that for NRZ. The reason is that there is always one transition at the middle of the bit and maybe one transition at the end of each bit. 

Eg: For 10Mbps LAN the signal spectrum lies between 5 and 20

• Another example to find out the bits by seeing the transitions. 
  

1. GATE-CS-2007 | Question 85
2. GATE IT 2007 | Question 59
3. ISRO CS 2007 | Question 22

Reference :
Book – Computer Networks by Tanenbaum

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