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Error correction and Detection technique while sending the message
- 1. ERROR DETECTION AND ERROR CORRECTION TECHNIQUE WHILE SENDING THE MESSAGE
- 2. Introduction Data communications is the exchange of data between two devices via some form of transmission medium such as a wire cable. For data communications to occur the communicating devices must be part of a communication system made up of a combination of hardware and software. communication of two devices
- 3. Data communications system depends on three fundamental characteristics: delivery, accuracy and timeliness. A data communications system has five components- Message, sender, Receiver, Medium and Protocol. • Data can be corrupted during transmission. For reliable communication, errors must be detected and corrected. •Error detection and correction are implemented either at data link layer or the transport layer of the OSI model.or reliable communication, errors must be detected and corr
- 5. ERROR DETECTION Error detecting code is to include only enough redundancy to allow the receiver to deduce that an error occurred, but not which error, and have it request a re- transmission. Error detection uses the concept of redundancy, which means adding extra bits for detecting error at the destination.
- 6. Redundancy 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 bit are redundant to the information. They are discarded as soon as the accuracy of the transmission has been determined.
- 7. There are basically four types of redundancy checks. They are: I. VRC (Vertical Redundancy Check). II. LRC (Longitudinal Redund. Check). III. CRC (Cyclical Redundancy Check).
- 8. ERROR DETECTION VERTICAL REDUNDUNCY CHECK LONGITUDINAL REDUNDANCY CHECK CYCLIC REDUNDANCY CHECK
- 9. VERTICAL REDUNDANCY CHECK It is also known as parity check It is least expensive mechanism for error detection In this technique , the redundant bit called parity bit is appended to every data unit so that the total number of 1s in the unit becomes even (including parity bit)
- 11. VERTICAL REDUNDANCY CHECK Example : 1110110 1101111 1110010 - After adding the parity bit 11101101 11011110 11100100
- 13. Advantage : LRC of n bits can easily detect burst error of n bits. Disadvantage : If two bits in one data units are damaged and two bits in exactly same position in another data unit are also damaged , the LRC checker will not detect the error.
- 14. CYCLIC REDUNDANCY CHECK (CRC) In this method , a sequence of redundant bits , called the CRC or the CRC remainder, is appended to the end of the unit so that the resulting data unit become exactly divisible by a second, predetermined binary number. At its destination , the incoming data unit is divided by the same number. If at this step there is no remainder ,the data unit assume to be correct and is accepted, otherwise it indicate that data unit has been damaged in transmission and therefore must be rejected. The redundancy bits is used by CRC are derived by dividing the data unit by a predetermined divisor. The remainder is the CRC.
- 16. ERROR CORRECTION Error correcting code is to include enough redundant information along with each block of data sent to enable the receiver to deduce what the transmitted character must have been. Error Correction must be handled in two ways : When an error is discovered, the receiver can have the sender retransmit the entire data unit. Receiver can use an error correcting code, which automatically corrects certain errors.
- 17. There are two types of Error Correcting techniques : 1. Single bit error correction. 2. Burst error correction. Error Correction can be done with the help of HAMMING CODE.
- 18. HAMMING CODE It is a technique developed by R. W. Hamming. Hamming code can be applied to data units of any length and uses the relationship between data and redundancy bits. For eg.
- 19. A 7 bit ASCII code requires 4 Redundancy bits that can be added to the end of the data unit or interspersed with the original data bits. These bits are placed in positions 1,2,4 and 8. We refer to these bits as r1,r2,r4 and r8. These bits also represented as r in the below fig.