• With LRC , each bit position has a parity bit. Bo from each character meaasge is XORed with bo from all of the other charcters in the message. Similarly b1,b2 and the rest are XORed with their respective bits from all the characters in the message.
• LRC is the XORing results of characters that makes a message.
• The LRC bit sequence is computed in the transmitter prior to sending the data, then transmitted as though it were the last character of the message . AT the receiver, the LRC is recomputed from the data and recomputed LRC is compared with the LRC transmitted with the message. If they are the same, it is assumed that no transmission errors have occurred.

Note: Block check character (BCC) - is the bit sequence for LRC. Sometimes called Block Check Sequence( BCS)

• VRC used together with LRC will detect the exact bit error

Example(1) Determine the VRC and LRC of the ASCII encoded message : THE CAT. Use odd parity for VRC and even parity for LRC

Example(2) In number 1. If the message is MALIGAYANG SENTENARYO determine the VRC and LRC.

2. CYCLIC REDUNDANCY CHECKING

• Is a special form of multiple modulo remainder which is transmitted as an attachment to the frame or packet (as a checksum) . It is also called the polynomial code.
• As each block of data is received, the attached CRC value is checked against one calculated by the receiver. Any mismatch will trigger a ' request for retransmission' (NAK) of the block .
• CRC approach is much powerful than LRC in that it detects more errors. Is called 'cyclic' check because it effectively uses every bit in the frame several times in computing the checksum characters (LRC uses each bit once). To do this, the bits in the frame are shifted in a register one place at a time, and a recalculation made using a polynomial calculation.-hence the cyclic tag

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