Error detection in computer network

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      Error detection techniques

1. Simple Parity check 
Blocks of data from the source are subjected to a check bit or parity bit generator form, where a parity of :
  • 1 is added to the block if it contains odd number of 1’s, and
  • 0 is added if it contains even number of 1’s
This scheme makes the total number of 1’s even, that is why it is called even parity checking.
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2. Two-dimensional Parity check
Parity check bits are calculated for each row, which is equivalent to a simple parity check bit. Parity check bits are also calculated for all columns, then both are sent along with the data. At the receiving end these are compared with the parity bits calculated on the received data.
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3. Checksum
  • In checksum error detection scheme, the data is divided into k segments each of m bits.
  • In the sender’s end the segments are added using 1’s complement arithmetic to get the sum. The sum is complemented to get the checksum.
  • The checksum segment is sent along with the data segments.
  • At the receiver’s end, all received segments are added using 1’s complement arithmetic to get the sum. The sum is complemented.
  • If the result is zero, the received data is accepted; otherwise discarded.

Cyclic Redundancy Check (CRC)

CRC is a different approach to detect if the received frame contains valid data. This technique involves binary division of the data bits being sent. The divisor is generated using polynomials. The sender performs a division operation on the bits being sent and calculates the remainder. Before sending the actual bits, the sender adds the remainder at the end of the actual bits. Actual data bits plus the remainder is called a codeword. The sender transmits data bits as codewords.
CRC
At the other end, the receiver performs division operation on codewords using the same CRC divisor. If the remainder contains all zeros the data bits are accepted, otherwise it is considered as there some data corruption occurred in transit.


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