1. Error Detection and Correction

2. Error Types

6. 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).

7. 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).

8. LRC Example
LRC
Original data plus LRC

9. Suppose the following block is sent :
(LRC)
However, it is hit by burst of length eight and some bits are corrupted (Yellow bits are changed) :
(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 ) :