Introduction
Manchester encoding is a bi-phase encoding. It also expresses "0" or "1" by switching between high and low levels, and the level conversion between each bit represents the data code and is also used as a timing signal. Manchester encoding is often used in Ethernet.
Coding rules
In Manchester encoding, there is a transition in the middle of each bit, and the transition in the middle of the bit is used as both a clock signal and a data signal.
Manchester encoding has two opposite conventions.
The first type of agreement was first published by GE Thomas in 1949, and was subsequently used by many writers, such as Andy Tanenbaum. It specifies that for bit 0, the signal level will be low and high (assuming that the data is physically encoded in amplitude)-low in the first half of the bit period and high in the second half. For 1 bit, the signal level will be high-low.
The second convention is also used by many authors (for example, William Stallings), and the low-speed versions of IEEE 802.4 (Token Bus) and IEEE 802.3 (Ethernet) standards follow. It points out that logic 0 is represented by a high-low signal sequence, and logic 1 is represented by a low-high signal sequence.
It is very worth noting that there must be a jump in the "middle" of each bit. According to this rule, the drawing method of the Manchester coded waveform can be drawn. For example: to transmit binary information 0, if we regard 0 as one bit, we use 0 as the center and use dotted lines on both sides to define the range of this bit, and then draw a high-to-low jump in the middle of this bit Change. Each of the following figures can draw the entire waveform by analogy.
Encoding principle
Manchester encoding is to include clock and data in the signal stream. While transmitting code information, it also transmits the clock synchronization signal to the other party. Each symbol of Manchester encoding is modulated into two levels, so the data transmission rate is only 1/2 of the modulation rate.
Decoding
The existence of a guaranteed transition allows the signal to be self-timed and the receiver can also be aligned correctly. The receiver can recognize whether it is misaligned within half a bit period, because there will no longer always be a transition in each bit period. Compared with the simpler NRZ coding scheme, the cost of these benefits is a doubling of bandwidth requirements.
Advantages
The Manchester encoding method has the following advantages: there is a level jump in the middle of 1 bit, and the time interval between the two level jumps can be T/2 or T; the synchronization signal of the sender and receiver can be generated by the level jump; Manchester encoding is a self-synchronizing encoding method, that is, the clock synchronization signal is hidden in the data waveform In. In Manchester encoding, there is a transition in the middle of each bit, which can be used as a clock signal or as a data signal. Therefore, there is no need to send another synchronization signal when sending Manchester coded signals.
Features
The Manchester encoding features are as follows:
(1) The transmission stream rate is twice that of the original data stream, which takes up a wider frequency band.
(2) The signal recovery is simple, as long as the edge of the signal is found for asynchronous extraction.
(3) 10Mb/s Ethernet (Ethernet) uses Manchester code.
Differential Manchester coding
Differential Manchester coding is also a bi-phase code. Unlike Manchester code, the level shift side of the code element in this coding is only used as a timing signal. , Not data. The data indicates whether there is level conversion at the beginning of each bit, with level conversion indicating 0, and without level conversion indicating 1. Differential Manchester codes are used in token ring networks.
Each symbol of these two bi-phase codes must be modulated to two different levels, so the modulation rate is twice the symbol rate. This undoubtedly puts forward higher requirements on the bandwidth of the channel, so it is more difficult and more expensive to implement. However, due to its good anti-noise characteristics and self-timing ability, it is still widely used in local area networks.