Error Detection types
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Certain types of errors cannot be detected by error detection algorithms. 1. Two-dimensional parity cannot detect few 4-bit errors or errors of 5 bits or more. CRC can detect errors up to 32 bits. 2. Checksums cannot detect errors where two data items are exchanged or a data item value is increased and another is decreased by the same amount. 3. CRC cannot detect all burst errors affecting an even number of bits, or burst errors longer than the polynomial degree. More complex algorithms like CRC are needed for safe transmission.
Error Detection types
- 1. Kamran Ashraf MSCs 1st Semester 13-NTU-4009 National Textile University Page 1 Q: What type of errors cannot be detected in all covered detection algorithms and why? What can be done to detect such types of errors? Explain above in all one by one error detection algorithms. 1- Errorsnot detectable by Two Dimensional Parity: a. Few 4-bit errors are not detectable b. All 5-bit and above errors are not detectable Example: We have Four 7 bit character packet with even parity Two-Dimensional Parity didn’t detect 4 bit error shown below: Sent Packet Even parity 0110100 1 1011010 0 0010110 1 1110101 1 1001011 0 1000110 1 Solution: Instead of Two dimensional Parity we can use CRC to detect errors upto 32 bit. 2- Errorsnot detectable by the checksum: a. If somehow two data items gets exchanged during transmission, error is not detected because sum and the checksum value will remain same. b. If somehow value of one data item is increased and the value of any other data item is decreased by the same amount, then the sum and the checksum remain same and error cannot be detected. c. If a zero value byte is inserted or deleted, the changes are undetectable. 4-bit Error not detected 0110100 1 1011010 0 0000111 1 1100100 1 1001011 0 1000110 1
- 2. Kamran Ashraf MSCs 1st Semester 13-NTU-4009 National Textile University Page 2 Example: Four data items got error as the bits got changed but checksum value is same, the error is not detected by checksum. Original Data Data Item in binary Checksum Value 0001 1 0010 2 0011 3 0001 1 Total 7 Solution: For safe transmission of data more complex algorithms are needed. Cyclic redundancy check (CRC) is more sophisticated method to detect these kinds of errors. 3- Errorsnot detectable by the Cyclic RedundancyChecks: CRC can detect broad range for errors however all kind of errors are not detectable. a. CRC detect burst error upto CRC size only. b. CRC cannot detect all burst errors that affect an even number of bits. c. CRC may not detect burst errors of length greater than the degree of the polynomial. After undetected Error Data Item in binary Checksum Value 0011 3 0000 0 0001 1 0011 3 Total 7