Differentiate error detection with error correction. Explain CRC (Cyclic Redundancy Check) method for error detection with a suitable example.

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Error Detection:

Error detection is the method of identifying errors. To identify these errors, it uses some redundancy codes. Redundancy codes are added to actual data, and it has transmitted by the sender. These codes are known as error detection codes.

Types of error detection techniques:

  1. Parity Checking
  2. Cyclic Redundancy Check (CRC)
  3. Checksum

Error Correction

Error correction is more difficult than error detection. Error correction codes are used to detect and correct errors when transmitting data from the sender to the receiver. It can be done in two ways:

  1. Backward Error Correction
  2. Forward Error Correction
  1. Backward Error Correction: If the receiver receives an error in the received data, it tells the sender to retransmit the data again.
  2. Forward Error Correction: If the receiver finds an error in the received data, the error correction codes correct these errors automatically.

Cyclic Redundancy Checks (CRC):

  • Cyclic Redundancy Check (CRC) is an error detection method.
  • It is based on binary division.

CRC technique involves binary division of the data 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 data bits.

Actual data bits plus the remainder is called a codeword.

The sender transmits data bits as a codeword.

At the other end, the receiver performs division operations on the codewords using the same CRC divisor. If the remainder contains all zeros, the data bits are accepted, otherwise, it is considered as some data corruption that occurred in transit.

Example:

Message D = 1010001101 (10 bits)

Predetermined P = 110101 (6 bits)

FCS R = to be calculated 5 bits

Hence, n = 15 K = 10 and (n – k) = 5

5 zeros are added to the actual data bits.

The product is divided by P.

- Hamro CSIT

The remainder is inserted into the actual data bits to provide T = 101000110101110 that is sent.

Suppose that there are no errors, and the receiver gets T perfect. The received frame is divided by P.

- Hamro CSIT

Because of no remainder, there are no errors.

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