PLC error control method and error detection code - Database & Sql Blog Articles

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The basic task of a data communication system is to efficiently and error-freely transmit and process data information, and there are erroneous possibilities for various components of the data communication system. Since the communication device portion can achieve high reliability, it is generally considered that the error of data communication mainly comes from the data transmission channel. Data signals are transmitted over long distances, and are often subject to various interferences, causing errors in the received data signals, causing errors in the sequence of data signals. In practice, random errors and sudden errors are usually present at the same time. of.

1. Error control method

Error control refers to detecting errors and correcting errors in the transmitted data signal. There are four main types of error control methods commonly used in practice:

(1) Automatic error detection retransmission (ARQ) In this mode, the transmitting end adds a redundancy code to the signal code to be transmitted according to the coding rule, and then transmits it. The receiving end performs error detection on the received signal sequence, determines whether there is an error code, and sends the judgment result back to the transmitting end through the feedback channel. If the error code is determined, the sender resends the original data until the receiver considers that there is no error; if the decision is error-free, the sender can continue to transmit the next new data.

(2) Forward Error Correction (FEC) In this manner, the transmitting end adds a redundant code to the signal symbol to be transmitted according to a certain encoding rule to constitute an error correcting code. The receiving end converts the additional redundant symbols according to a certain decoding rule, and detects whether there is an error code in the signal. If there is an error code, the error code position is automatically determined and corrected. The method is simple in physical implementation, does not require a feedback channel, and is suitable for a real-time communication system, but the decoder is generally complicated.

(3) Hybrid error correction mixed error correction mode is a combination of forward error correction and automatic error detection retransmission. The transmitting end sends the symbol with detection and error correction capability, and the receiving end will request the transmitting end through the feedback channel. Send this message. The hybrid error correction method combines the advantages of both ARQ and FEC, but fails to overcome their respective shortcomings, and thus has been limited in practical applications.

(4) Error control without coding Error control without coding means that signal coding of transmitted signal symbols is not required, and redundancy measures are added in the transmission method to reduce errors in transmission.

2. Error detection code

Common error detection codes include parity check codes and cyclic redundancy check (CRC) codes. Since the parity code only needs to add one parity bit for encoding, it is more efficient and thus has been widely used.

The parity code is a verification method in characters. A character is generally composed of 8 bits, the lower 7 bits are the ASCII of the information character, and the highest bit is the parity bit. This bit can be either "0" or "1". The principle is: make the number of "1" in the whole code odd or even. If the number of "1" is odd, it is called odd check. If the number of "1" is even, it is called even check. .

The principle of parity check is: if the odd check is used, the sender sends a character code (including the check code), then the number of "1" must be an odd number, and the number of "1" is counted at the receiving end. If the number of statistical results "1" is even, it must mean that an error has occurred during the transmission. Obviously, if an odd number of bit errors occur, the receiver can find it, but if an even number of bit errors occur, the receiver will not be able to detect it.