Manchester Encoding
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Transcript of Manchester Encoding
Manchester Encoding
Theory and UseBy Professor Tom Mavroidis
January 31,2001
What is Manchester Encoding ?
• It is a process by which a Synchronous clock encoding technique is used by the OSI physical Layer to encode the clock and data of a synchronous bit stream.
What is the difference?
• In this technique the binary data to be transmitted over the cable are not sent as a sequence of logical 1 & 0’s known as non return to zero (NRZ), instead the bit are translated into a different format that has a number of advantages over using straight binary.
Where is it used?
• A popular technique used for encoding data on magnetic disks.
• Also known as phase encoding• Problems include poor bit storage
density
Resides in the Physical Layer
• Physical Layer serializes the frame (converts it to a series of bits) and send it across a circuit to the destination.
Three types of encoding discussed
• NRZ - Non Return to Zero• RZ - Return to zero• Manchester encoding
NRZ , Non Return to Zero transmission
• Each data bit is represented by a level. High = logic 1 low = logic 0
Problem with NRZ
• Long runs of consecutive bits with the same value make it impossible to detect bit boundaries by the inability of the receiver to detect the clocking speed.
RZ - Return to zero
• Pulses used to represent bits• logic 1 = pulse logic 0 absence of
pulse
RZ - Advantages
• Clock signals can be extracted from the logic 1 signals.
• Halves the width of the pulse• Polar RZ sends pulses for both
logic 0 and logic 1• Requires an extra voltage level
Manchester Encoding
• Logic 1 is represented by a transition in a particular direction in the center of each bit.
• Opposite direction used to represent logic 0
Representation
• Logic 0 1 to 0 (downward transition at bit center)
• Logic 1 0 to 1 (upward transition at bit center)
Manchester Encoding
• Was named after its birthplace in Manchester, England
• Bit transitions do not always occur at the ‘bit boundaries.
• There is always a transition at the center of the bit
Improved clocking
• Allows the receiver to extract the clock signal
• Correctly decodes the value and timing of each bit
Receiver
• Reassembles the series of bits to form a frame and forwards the frame for processing by the link layer
• Frame is then CRC checked and a retransmission is requested if the checksum is invalid.
Representation of phase shift
Uses
• Manchester encoding is used to :• Transmit data across systems• Store data on magnetic media,
disks etc.• Communicate with Satellites
Conclusion
• Based on application different types of encoding can be used
• Manchester encoding exhibits poor bit storage density but improved reliability.
Links
• http://www.erg.abdn.ac.uk/users/gorry/course/phy-pages/man.html
• http://home.europa.com/~celiac/manch_enc.html• http://csshaun.cs.ru/ac/za/netnotes/netnotes003.html• http://www.pdp8.net/tu56/tu56.shtml