This article throws light upon the top five schemes of encoding data bytes. The schemes are: 1. Non-return to Zero (NRZ) 2. Non-return to Zero Invert on Ones (NRZI) 3. Bipolar-AMI 4. Manchester Code 5. Differential Manchester Code.
Scheme # 1. Non-Return to Zero (NRZ):
A common way—probably the most common way—to transmit digital signals is to use two different voltage levels to represent the two binary digits being used. All codes that follow this strategy have the common feature that they keep the voltage level constant during a bit interval—there is no return to the zero voltage level.
The presence of a voltage could be used to represent a binary 1 (or a 0) and the absence of a voltage could represent a binary 0 (or a 1). Generally, however, a negative voltage represents one binary value (a 0 or a 1) and a positive voltage represents the other binary value. This encoding scheme is called Non-return to Zero Level (NRZ-L). It is shown along with some of the other encoding schemes in Fig. 4.1.
Scheme # 2. Non-Return to Zero Invert on Ones (NRZI):
This is a variation of NRZI-L discussed above. Like NRZ-L, NRZI retains a constant voltage pulse for the duration of the transmission of a bit. The data is encoded as the presence (or absence) of a signal transition at the start of the bit time. A change, that is high to low or low to high at the start of a bit, denotes a binary 1 for that bit time and no transition indicates a binary 0. NRZI is an example of differential coding.
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The information to be transmitted in differential coding is shown in terms of the changes between successive signal elements and not the signal elements themselves. If the current bit is a binary 0, then it is encoded with the same signal as the immediately previous bit.
On the other hand, if it is a binary 1, then the current bit is encoded with a different signal than the immediately previous bit. The main advantage of this method of encoding is that it is more reliable to detect the transition in the presence of noise and also that if the transmission is complex then it is easy to lose the sense of polarity of the signal. It is shown along with some of the other encoding schemes in Fig. 4.1.
Scheme # 3. Bipolar-AMI:
In the bipolar-AMI scheme, a binary 0 is represented by no line signal and a binary 1 is represented by a positive or negative pulse. The binary 1 pulses must alternate in polarity.
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There are some evident advantages in this scheme:
1. There will be no loss of synchronization if there is a long string of l’s since each 1 introduces a transition and the receiver can resynchronize during that transition. A long string of 0’s may, however, still be a problem.
2. Because the 1 signals alternate in voltage, from positive to negative, there is no net DC component. Further, the bandwidth of the resulting signal is considerably less than the bandwidth for NRZ. Also, the pulse alternation property provides a simple means for error detection.
Any isolated pulse, whether it deletes or adds a pulse, will cause a violation of this property. This encoding scheme is also classified under multilevel binary and they use more than two signal levels. It is shown along with some of the other encoding schemes in Fig. 4.1.
Scheme # 4. Manchester Code:
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This is a bi-phase code and it overcomes some of the limitations of the NRZ codes. This code along with differential Manchester code is in wide use. Let us first consider Manchester code. In Manchester code there is a transition in the middle of each bit period. This becomes important because let us consider where one station sends the bit string 0001000.
Others might falsely interpret it as 10000000 or as 01000000, because there is no way to differentiate between an idle sender and 0 bits, where both represent 0 volt. What is obviously required is for the receivers to unambiguously find the start, end or middle of each bit without reference to any external clock.
In Manchester encoding, each bit period is divided into two equal intervals. A binary 1 bit is sent by keeping the voltage high during the first interval and low in the second interval.
A binary 0 is sent in just the reverse way, that is, in the first interval the voltage is kept low and the second interval it is kept high. This ensures that every bit period has a transition in the middle. This makes it easy for the receiver to synchronize with the sender.
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A disadvantage is that Manchester encoding requires twice the bandwidth as straight binary encoding. It is shown along with some of the other encoding schemes in Fig. 4.1.
Scheme # 5. Differential Manchester Code:
This is a variation of Manchester encoding. In it, a 1 bit is indicated by the absence of a transition at the start of the interval and the 0 bit is indicated by the presence of a transition at the beginning of the interval. But in both cases, there is also a transition in the middle. Differential Manchester encoding needs fairly complex equipment, but it has the advantage of providing better noise immunity.
IEEE’s 802.3 baseband systems use Manchester encoding because of its simplicity. The high signal is usually 0.85 volt and the low signal is —0.85 volt and the DC value is, therefore, 0 volt. It is shown along with some of the other encoding schemes in Fig. 4.1.