Manchester encoding | check Manchester encoding | no return to zero code

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Manchester Encoding(Manchester encoding), also called phase encoding (PE), is a synchronous clock coding technology used by the physical layer to encode the clock and data of a synchronous bit stream. Manchester encoding is used in Ethernet Media systems. Manchester encoding provides a simple way to encode a simple binary sequence without a long cycle and no conversion level, thus preventing the loss of clock synchronization, or the simulated link bit error from low-frequency displacement in poor compensation. In this technology, binary data is actually transmitted through this cable, instead of being sent as a sequence of logic 1 or 0 (technically called reverse non-zero conversion (NRZ )). On the contrary, these bits are converted to a slightly different format, which has many advantages by using direct binary encoding.

Manchester encoding, commonly used for LAN transmission. In Manchester Coding, there is a hop in the middle of each bit. The hop in the middle of a bit is both a clock signal and a data signal;The jump from high to low indicates "0", and the jump from low to high indicates "1 ". (Disagree! Another statement:"After consulting with an expert, the result is:The leap of Manchester encoding from high to low is from 1 to 0.The same is true for Tsinghua University's "computer communication and network tutorial" and "Computer Network (version 4th .")There is also a differential Manchester code. The hop in the middle only provides the clock timing, and each hop in the beginning indicates "0" or "1 ", A hop changes to "0" and a hop does not change to "1 ".

 

Manchester Encoding(Manchester encoding), also known as Phase Encoding (PE), is commonly used for LAN transmission. In Manchester Coding, there is a hop in the middle of each bit. The hop in the middle is both a clock signal and a data signal. However, in different books, in Manchester encoding, the value of the level beat expression is different, which produces many ambiguities:
1.Network Engineer ExaminationAnd related materials:
Bit CenterThe jump from high to low indicates "0 ";
Bit CenterThe jump from low to high indicates "1 ".
2 《Computer NetworkIn books:
Bit CenterThe jump from high to low indicates "1 ";
Bit CenterThe jump from low to high indicates "0 ".

 

Differential Manchester Encoding

 

The Manchester encoding rules are as follows:

In the letter number, the center jumps from low to high to 1.

In the phone number, the center jumps from high to low to 0.

The differential Manchester encoding rules are as follows:

At the beginning of the letter number, the polarity of the signal is not changed, indicating Series 1.

Changes the polarity of the signal at the beginning of the number, indicating that the logic is 0.

Manchester and differential Manchester are two types of codes with the same principle, and the latter is an improvement of the former. Their feature is that every bit of information transmitted carries a bit synchronization clock, so a single transmission can allow a long data bit.

Each bit of Manchester encoding occupies only half of the clock period. When "1" is transmitted, the first half of the clock period is high, and the last half is low; the opposite is true when "0" is transferred. In this way, each clock cycle will have a hop, which is a bit synchronization signal.

Differential Manchester encoding is an improvement in Manchester encoding. It has a hop in the middle of each clock bit. The transmitted value is "1" or "0", which is determined by whether there is a hop change at the beginning of each clock bit.

Differential Manchester Coding has fewer changes than Manchester Coding, so it is more suitable for transmitting high-speed information and is widely used in Broadband high-speed networks. However, since every clock bit must change once, the efficiency of the two types of codes can only reach 50%.

 

Zero return

 

Digital signals can be transmitted directly using baseband. The so-called baseband refers to the basic band. Baseband transmission is the electrical pulse that directly transmits digital signals in the line. This is the simplest transmission method. The LAN for short-distance communication uses baseband transmission.

During baseband transmission, the Digital Signal Representation of the digital data and the signal synchronization between the two ends must be solved. For digital signal transmission, the simplest and most common method is to use different voltage levels to represent two binary numbers, that is, digital signals are composed of rectangular pulses. According to the digital encoding method, it can be divided into single polarity code and bipolar code. single polarity Code uses positive (or negative) voltage to represent data. Bipolar code is a triplicate code, 1 is a reversal, 0 is to maintain the zero level. Based on whether the signal is set to zero, it can also be divided into zero code and non-zero code. The signal in the middle of the zero code element is returned to the 0 level, instead of the zero code, when the 1 level is flipped, the zero value remains unchanged. Several common basic digital pulse coding schemes are as follows:

The polarity is not zero, and no voltage (that is, the metacurrent) is used to indicate "0", while a constant positive voltage is used to indicate "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 Code does not return zero code. Both "1" code and "0" Code have current, but "1" code is positive current, and "0" code is negative current, it is called a 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.

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.

Returns the single polarity to zero. When a "1" code is sent, a 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 a "0" code is sent, it still does not send current at all, so this code is called single polarity return code.

Bipolar return code, where "1" code returns a positive narrow pulse and "0" code emits 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.

From the above discussion, we can find that another important problem of baseband transmission is synchronization. The data sequence sent from the receiving end and the sending end must be synchronized in time to accurately distinguish and receive each data sent. This requires the receiving end to receive data according to the repetition frequency and start and end time of each code element sent by the sending end, and to continuously calibrate the time and frequency during the receiving process, this process is called a synchronization process. In computer communication and networks, Bit Synchronization and group synchronization are widely used.

1. Bit Synchronization

Bit Synchronization means that the receiver must synchronize each bit of data with the sender. In data communication, bit synchronization is often called "synchronous transmission ". The method of Bit Synchronization can be divided into two types: external synchronization and self-synchronization. In the external synchronization method, the synchronous signal of the receiving end is sent by the sending end in advance, instead of being generated or extracted from the signal. That is, before sending data, the sender sends a series of synchronous clock pulses to the receiver. The receiver locks the receiving frequency of the receiver according to the clock pulse frequency and time sequence, in this way, data is always synchronized with the sender during data receiving.

The self-synchronization method refers to the method that can extract the synchronous signal from the data signal waveform. A typical example is the famous Manchester encoding, which is usually used for LAN transmission. In the Manchester encoding method shown in, there is a hop in the middle of each bit. The hop in the middle of a bit is used as both a clock signal and a data signal: the jump from high to low indicates "1", and the jump from low to high indicates "0 ".

Figure 1

In addition, there is also a differential Manchester encoding, which provides only clock timing for each intermediate hop, and indicates "0" or "1" at the beginning with no hop ", if there is a hop change, it means "0", and if there is no hop change, it means "1 ". It can be seen that the two Manchester coding methods include the clock and data in the signal stream. while transmitting code information, they also transmit the clock synchronization signal to each other, therefore, this encoding is also called self-synchronous encoding.

From the pulse waveforms of Manchester Coding and differential Manchester Coding, we can see that each of these two bipolar codes is modulated into two levels, so the data transmission rate is only 1/2 of the modulation rate, that is, there are higher requirements on the channel bandwidth. However, they have self-synchronization capabilities and good anti-interference performance, and are still widely used in LAN.

2. Group Synchronization

In a group synchronization communication system, the transmitted information is divided into several "groups ". The so-called "group" is generally a unit of characters. Each character is preceded by a start position and a termination position at the end to form a character sequence during data transmission, characters can appear in the bit stream sequentially. The interval between characters is arbitrary, that is, asynchronous timing is used between characters, but each bit in the characters is transmitted at a fixed clock frequency. In data communication, Group Synchronization is often called "asynchronous transmission ". The asynchronous timing between characters and the synchronous timing between characters in bits are the characteristics of group synchronization, that is, asynchronous transmission. In this transmission mode, each character is separated by the Start and Stop bits. Therefore, it is also called "first stop" transmission.

Each character in the Group Synchronization transmission procedure may consist of the following four parts:

(1) 1-digit start position, represented by logic and:

(2) 5 ~ 8-Bit Data bit, that is, the character content to be transmitted;

(3) One odd/even test bit for error detection. This part can be left unselected;

(4) 1 ~ The two-digit Stop bits are represented by logic "1" and used as the interval between characters.

The character format of Group Synchronization is shown in figure 2. As shown in the figure, Group Synchronization relies on the Start bit (logical "0") and stop bit (logical "1") to define the characters and synchronize the bit in the characters. The acceptor obtains the start of a character based on the falling edge of the idle or previous character Stop bits (all logical "1") on the detection link to the start position of the character, then, the number of character bits (5 ~ 8-bit) for receiving by bit, and finally to the agreed algorithm (odd/even verification method) for error detection, complete the transmission of a character. Two agreed clocks in the transmitter and receiver that are similar to the same frequency can be synchronized within a short period of time. In group synchronous transmission, the Start and Stop bits are very important. The start position indicates the start of the character and starts the receiver to synchronize the bit in the character. The stop position is set as the interval between characters without stopping the bit, the descent along the start position of the next character following it may be lost.

The Group synchronization method only needs to keep the start point of each character synchronized. In the group, bitwise reception is performed at the agreed frequency. This method is easy to implement, but additional bit such as start bit, check bit, and stop bit need to be added. Compared with synchronous transmission, the encoding efficiency and channel utilization are low, which is generally used for low-speed data transmission.