Manchester Encoding Theory and Use By 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
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
Logic 1 is represented by a transition in a particular direction in the center of each bit.
Opposite direction used to represent logic 0
Logic 0 1 to 0 (downward transition at bit center)
Logic 1 0 to 1 (upward transition at bit center)
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
Allows the receiver to extract the clock signal
Correctly decodes the value and timing of each bit
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
Manchester encoding is used to :
Transmit data across systems
Store data on magnetic media, disks etc.
Communicate with Satellites
Based on application different types of encoding can be used
Manchester encoding exhibits poor bit storage density but improved reliability.