Error Detection and Correction

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Presentation transcript:

Error Detection and Correction

Error Types

Error Detection and Error Correction The Longitudinal Redundancy Check (LRC) was included as well as the VRC. This is a summing of the 1s in a vertical column of all characters, including the 1s or 0s in each eighth bit location. The sum is now appended at the end of a message frame or packet. This sum was called the block check count (BCC).

Today it may consist of two or four 8-bit sequences and we call it the FCS (frame check sequence), or sometimes the CRC (cyclic redundancy check). At the distant-end receiver, the same addition is carried out and if the sum agrees with the value received, the block is accepted as error-free. If not, it then contains at least one bit error, and a request is sent to the transmit end to retransmit the block (or frame).

LRC Example 11100111 11011101 00111001 10101001 11100111 11011101 00111001 10101001 10101010 LRC 11100111 1101101 00111001 10101001 10101010 Original data plus LRC

Suppose the following block is sent : 10101001 00111001 11011101 11100111 10101010 (LRC) However, it is hit by burst of length eight and some bits are corrupted (Yellow bits are changed) : 10100011 10001001 11011101 11100111 10101010 (LRC) When the receiver checks the LRC, some of the bits are not follow even parity rule and whole block is discarded (the non matching bits are shown in red ) : 10100011 10001001 11011101 11100111 10101010