Manchester Encoding in Computer Network

Manchester encoding is a synchronous clock encoding technique used by the physical layer of the Open System Interconnection to encode the clock and data of a synchronous bit stream.  The idea of RZ and the idea of L are combined in manchester encoding. 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 direction of the transition determines the bit state. Different systems represent bit status in various ways, although most systems use 0 bit for low-to-high transitions and 1 bit for high-to-low transitions. In this article, we will learn about the Manchester encoding technique, other approaches to encoding, and the advantages and disadvantages of Manchester encoding.

What is Manchester Encoding?

Manchester encoding is a method of data transmission used in computer networks and telecommunications. It works by combining the clock and data signals into one stream, making it easier to synchronize the data. Each bit of data is represented by a transition; a change from high to low or low to high in the signal. This helps ensure that the data is correctly interpreted by the receiving device. It is widely used in Ethernet technology and other digital communication systems due to its reliability and simplicity.

In Manchester, the 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 in 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 in the middle of the bit, but the bit values are determined at the beginning of the bit. if the next bit is zero there is a 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]. 

Example of Manchester Encoding

In Manchester encoding, each bit of data is represented by a transition in the signal. For example, let's consider a simple binary data sequence: 1101.

• Binary Data: 1101
• Manchester Encoding
  • 1: At First it becomes High to Low transition
  • 1: At second it also becomes High to Low transition
  • 0: It becomes Low to High transition
  • 1: The final time it becomes High to Low transition

This sequence would look like this in Manchester encoded form:

• 1: |¯|_
• 1: |¯|_
• 0: |_|¯
• 1: |¯|_

In this representation:

• |¯|_ signifies a transition from high to low.
• |_|¯ signifies a transition from low to high.

• A binary "1" is represented by a high-to-low transition.
• A binary "0" is represented by a low-to-high transition.

Thus, the binary sequence 1101 would be encoded in Manchester as a series of transitions that ensure synchronization and reliable data transmission

Manchester Encoding vs. Other Approaches to Encoding Methods

Manchester encoding is a method used in digital communication, especially in Ethernet, where each bit is represented by a transition from high to low or low to high. This makes it easy to synchronize data and ensures reliable transmission. Other encoding methods, like Non-Return to Zero (NRZ) and Return to Zero (RZ), handle data differently. NRZ uses different voltage levels without returning to zero, which is efficient but can cause synchronization issues. RZ returns to zero between bits, which helps with synchronization but uses more bandwidth.

Differential Manchester encoding improves noise immunity and error detection by marking data values with transitions at the beginning of each bit. Another method, 4B/5B encoding, turns 4 bits of data into 5-bit codes to ensure enough transitions for synchronization and better error detection. Each encoding method has its pros and cons, making them suitable for different needs. Manchester encoding is known for its reliability and ease of use, while NRZ, RZ, Differential Manchester, and 4B/5B offer other benefits like efficiency and improved error handling, depending on the application.

More Approaches to Data Encoding

Non-Return-to-Zero Encoding [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

• 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.

• 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.
• 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. 

Advantages of Manchester Encoding

• Self-clocking: Manchester encoding is self-clocking, which means that the receiver can synchronize its clock with the transmitter's clock. This ensures that the data is transmitted and received at the same rate, and there is no need for a separate clock signal.

• Reduced DC component: Manchester encoding eliminates the DC component in the transmitted signal, which reduces the risk of errors due to interference from external sources.

• Error detection: Manchester encoding provides a mechanism for detecting errors in the transmitted data. Any change in the voltage level within a time interval indicates a bit error, which can be detected and corrected.

• Simplicity: Manchester encoding is a relatively simple encoding scheme that can be implemented using simple digital circuits.

Disadvantages of Manchester Encoding

• Lower data rate: Manchester encoding has a lower data rate than other encoding schemes, such as non-return-to-zero (NRZ) encoding, which means that it takes more time to transmit the same amount of data.

• Higher bandwidth requirement: Manchester encoding requires a higher bandwidth than other encoding schemes, as each bit requires two voltage transitions within each time interval.

• Clock synchronization: Although Manchester encoding is self-clocking, it still requires the receiver to synchronize its clock with the transmitter's clock, which can be a challenge in some situations.

• Reduced transmission distance: Manchester encoding has a reduced transmission distance compared to other encoding schemes, as the signal loses strength over long distances due to the need for frequent voltage transitions.

Conclusion

Manchester encoding is a reliable and easy-to-synchronize method widely used in digital communication, particularly in Ethernet technology. Its ability to combine clock and data signals into one stream ensures accurate data transmission. However, other encoding methods like Non-Return to Zero (NRZ), Return to Zero (RZ), Differential Manchester, and 4B/5B encoding offer unique benefits, such as improved efficiency, better synchronization, and enhanced error detection. Understanding these different approaches allows for selecting the most suitable method for specific communication needs, ensuring robust and efficient data transfer.

Important Questions on Manchester Encoding in Computer Network

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