(Differential) Manchester Coding and NRZ

Manchester encoding, also known as Phase encode (PE), is a synchronous clock coding technology used by the physical layer to encode the clock and data of a synchronous bit stream. Its application in the Ethernet Media system belongs to the self-synchronization method (the other is the out-of-Sync method) in the Two Bit Synchronization Methods of data communication ), that is, the receiver uses a special code containing a synchronous signal to extract the synchronous signal from the signal itself to lock its own clock pulse frequency for synchronization purposes. Manchester encoding, commonly used for LAN transmission. Manchester Coding includes the clock and data in the data stream. while transmitting code information, it also transmits the clock synchronization signal to the other party. Each encoding has a hop and no DC component, therefore, it has the ability of self-synchronization and good anti-interference performance. However, every code element is adjusted to two levels, so the data transmission rate is only 1/2 of the modulation rate. There are two encoding methodsThe first G. E. Thomas, Andrew S. Tanenbaum proposed in 1949, which stipulates that 0 is represented by the low-high level jump, and 1 is the high-low level jump. In the second IEEE 802.4 (Token Bus) and low-speed IEEE 802.3 (Ethernet) Rules, according to this statement, the low-high hop represents 1, the high-low level jump indicates 0. Because there are two different Representation Methods above, some differences may occur. Of course, this can be overcome in the differential Manchester encoding mode. Differential Manchester code Start TimeChanges the polarity of the signal, indicating that the logic is "0 ". Start time noChanges the polarity of the signal, indicating the logic "1"; a) NRZ (not null) B) Manchester code C) Differential Manchester code Manchester EncodingAlso called Digital dual-phase code. Differential Manchester EncodingAlso called Conditional dual-phase code (CDP code). 3. A simple method to identify the differential Manchester Coding: It mainly looks at two adjacent waveforms. If the last waveform is the same as the previous one, the last waveform represents 0. If the waveform is different, 1. non-return code NRZ

 

One inversion of the signal level represents 1, the level does not change to 0, and after representing a code element, the voltage does not need to return to 0. The non-zero encoding is the most efficient encoding. The disadvantage is that there is a synchronization problem between the sender and the receiver. The polarity is not zero., No voltage (that is, the metacurrent) is used to represent "0", while a constant positive voltage is used to represent "1 ". The intermediate point of each bytecode time is the sampling time, and the decision threshold is a half-amplitude level (that is, 0.5 ). That is to say, if the value of the received signal is between 0.5 and 1.0, it is regarded as "1" code. If it is between O and 0.5, it is determined as "0" code. The number of binary codes sent per second is called "code rate ". Bipolar consistency is not zero, "1" code and "0" Code both have current, but "1" code is positive current, "0" code is negative current, and the amplitude of positive and negative is equal, it is called bipolar code. At this time, the judgment threshold is zero. The receiver uses a zero or positive or negative judge. If the value of the received signal is positive above the zero level, it is judged as "1; if the value is negative below the zero level, the value is 0. Returns the single polarity to zero.When the "1" code is sent, the positive current is sent, but the duration is shorter than the time width of a code element, that is, a narrow pulse is sent; when the "0" code is sent, it still does not send current at all, so this code is called single polarity return code. Bipolar return codeAmong them, "1" code is a narrow pulse, "0" code is a negative narrow pulse, the interval between the two code elements can be greater than the width of each narrow pulse, the sampling time is the center of the target pulse. The non-return code is difficult to determine the end of one digit and the start of the other in transmission. Some method is required to make the timer or synchronization between the transmitter and the receiver. the pulse of the return code is narrow, based on the inverse relationship between the pulse width and the transmission band width, the return code occupies a wider band on the channel. The polarity code will accumulate the DC component, so that the transformer cannot provide good insulation AC coupling between the data communication equipment and the environment, and the DC component will damage the surface plating of the connection point; the DC component of bipolar code is greatly reduced, which is advantageous for data transmission. The above two types of codes are both emitting or not emitting current (single polarity) within the whole time of a code element, and sending positive or negative current (bipolar ). Each bit of encoding occupies the width of all the code elements, so these two types of encoding belong to the full width code, also known as not returning to the zero code NRZ (non return zero ). If the "1" code is repeatedly sent, it is necessary to continuously send positive current; if the "0" code is repeatedly sent, it is necessary to continuously do not send current or continuously send negative current, in this way, there is no gap between a certain code element and its next code element, so it is difficult to distinguish between them. The return code can improve this situation..