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