Network must be able to transfer data from one device to another with complete accuracy. Any time data can get corrupted in passage during transmission from one device to another. The ability to detect when a transmission has been changed is called error detection. In most cases when errors are detected, the message is discarded, the sender is notified, and the message is sent again. In other cases when an error is detected, it may actually be fixed without a second transmission. This is called error correction. The sender never knows that the message was damaged and subsequently fixed. The bottom line is that the message eventually is delivered correctly. For reliable communication, errors must be detected and corrected.
  TYPES OF ERRORS
Whenever an electromagnetic signal flows from one device to another, any interference from heat, magnetism, and other forms of electricity, can change the shape or timing of the signal. If the signal is carrying encoded binary, data, such changes can alter the meaning of the data. There are two types of errors:
• Single-bit error
 • Burst error
In a single-bit error, 0 is changed to 1 or 1 is changed to 0. The term single-bit error means that only one bit of a given data unit (such as a byte, character, data unit or packet) is changed from 1 to 0 or from 0 to 1. For example, imagine that each group of eight bits is an ASCCII character with a 0 bit added to the left. For ASCII codes, see Appendix 1.
  0000011 (ASCII ETX) had been sent, but 0001011 was received. Single-bit errors are the least likely type of error in serial data transmission but it can happen often in parallel transmission. For example, if eight wires are used to send all of the eight bits of byte at the same time and one of the wires is noisy, one bit can be corrupted in each byte. Transmission between CPU and memory (in computers) is an example of parallel transmission.

          
        Single-bit error. Burst error means that two or more than two bits in the data unit (or packet) have changed from 1 to 0 or 0 to 1.  

ERROR DETECTION
We transmit the data from one device to another. If the receiving device does not have an original copy of the transmitted data, it is not easy to detect the error without comparison of the receiving data and the original data. One error detection mechanism that would satisfy these requirements would be to send every data unit twice. The receiving device can compare both versions of data bit by bit. Any discrepancy would indicate an error, and an appropriate correction mechanism could be set in place. This procedure would be completely accurate, but it would be very slow because transmission time of data will be doubled and additional time would be needed for the comparison of both versions of data bit by bit.
The concept of including extra information in the transmission solely for the purpose of comparison is a good one. But instead of repeating the entire data stream, a shorter group of bits may be appended to the end of each unit. This technique is called redundancy because the extra bits are redundant to the information; they are discarded as soon as the accuracy of the transmission has been determined.

The sender sends data unit after the addition of group of bits and the receiver checks the complete bit stream through the checking function. Subsequently, the redundant bits are discarded.
Four types of redundancy checks are used in data communication.
(1) Parity check
(2) Longitudinal redundancy check (LRC)
(3) Cyclic redundancy check (CRC)
(4) Checksum
We will explain each of them in detail.

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