NRZ-L (No return to zero level): this kind of encoding uses negative voltage to represent a binary 1 and positive voltage to represent a binary 0. As shown under: non return to zero is related with the voltage i.e. voltage never returns to a value of zero and the value of the voltage during a bit time is known as level. bit time is related with the amount of time one bit of data occupies.
The NRZ-L is used on very short connections, like the Connection between the computer and an external modem. its main problem is that for long strings of 0's and 1's, the signal's voltage remain negative or positive for extended periods of time, this may lead to a situation called baseline wanders, as this makes difficult to the receiver to properly decode the information. another problem with this is that in order for the sender and receiver to remain in synchronization, frequent changes in the signal are required.
when there are long runs of high or low voltages, the sender and receiver’s clocks may begin to wander so that the two devices are no longer in synchronization. To prevent this separate clock is signal could be used on another channel but this takes up valuable data transmission space. To overcome this NRZI encoding is used.
NRZI (Non Return to Zero Invert on 1):-
It is related with the NRZ-L except the data encoded by either the presence or absence of voltage change at the beginning of the bit time. as shown under: In this category of encoding when the signal changes from high to low voltage or from low to high voltage a binary 1 is encoded. When there is no change in the voltage at the beginning of the current bit time from the last bit time, a binary 0 is encoded. NRZI is also referred as non return to zero inverted.
In this encoding, the point at which the signal changes is used to represent data. as shown under, the locations at which the voltage changes from 0 to a positive value represents a binary 1 and when the voltage changes from positive to Zero, a binary 0 is represented. Manchester Encoding schemes use the edge triggered hardware and changes or transitions are known as rising or falling edges.
When leading edge rises to a positive voltage, a binary 1 is encoded and when the leading edge falls to zero voltage, a binary 0 is encoded. due to this each bit period is divided into two equal intervals. Every bit period has a transition in the middle that makes it easier to the receiver to synchronize with the sender, Manchester Encoding uses a preamble to permit synchronization by the receiver, it will ensure that there is proper synchronization of the time slots used when sampling the signal by receiver.
The preamble is composed of 64 alternating 1's and 0's, which is sent before the frame data. a pattern of alternating 1's and 0's produces a square wave from which the receiver can determine the value of the time slots. due to the preamble the need of external clock for synchronization is got eliminated.
Differential Manchester Encoding:
A variation on standard Manchester encoding is referred to as differential Manchester Encoding. In this a binary 1 is represented by the lack of a transition or voltage change the beginning of sampling interval. A binary 0 is encoded when a transition occurs at the beginning of the sampling interval. A binary 0 is encoded when a transition occurs at the beginning of sampling interval.
Under this encoding more complex hardware is required., but it offers better immunity to influences from outside noises. differential Manchester encoding is used in token ring networks. The figure is showing the Differential Manchester encoding :
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