Errors in network makes data corrupted as a result bits are changed while sending and receiving the packets of bits, and the placement of changed bits are undetectable errors by checksum.

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## Technical Definition of Checksum:

Error is invisible if a bit inversion is balanced by an opposite bit inversion in the corresponding digit of another segment

At least three types of error cannot be detected by the current checksum calculation.

- If two bits are swapped during transmission e.g. 1001 becomes 1100 means the checksum values are same and undetectable.
- The value of one data item is increased (maliciously
- Third, if one or more data is changed in such a way that the change is a multiple of 216 − 1, the sum or the checksum cannot detect the changes.

## Examples: Errors Undetectable by Checksum

Below we have some of the examples and the reasons, what kind of error is undetectable by checksum?. Why errors are neglected in parity check? and in which case the checksum errors are undetectable.**Example 1 of Checksum:**

Your computer coverts everything to 1's and 0's let's take example of a name **propatel.com**which is equal to

**01010011**.

Suppose the above sum of block of words you are sending to your friend. If, in that block of words a 0 is replaced by 1 or may be 1 is replaced by 0 in the same bit position, a checksum will not catch it.

Sender | Receiver |
---|---|

01010011 | 00110011 |

**Example 2 of Checksum Error:**

As the name indicate it check the sum of data blocks. So if the errors occur in such a way that sum remain same then it can't predict the error.

e.g I take the example of 4-bits blocks

0000

0001

0010

0000

check sum will be 2's complement of (0011) -> 1101

Sender | Receiver |
---|---|

0000 | 0000 |

0001 | 0001 |

0010 | 0001 |

0000 | 0001 |

1101 | 1101 |

receiver will find the sum of first four block.

which is 0011

now he finds 0011 + 1101 = 0000 (Ignoring carry)

the receiver accept the data and think it's correct but actually the receiver data is corrupted. That is the error undetectable by checksum.

**Example 2 of Checksum error:**

Assume out bit looks like this (0101011) Electronic errors. in other words , if the number of 1s in the frame is even and the flag is showing that correctly.Yet everything is looking great. but if receiver receives the frame and it will look like this (1010110), As you can see , The no. of 1s is still even , but the sequence if those bits is wrong. Parity check will still consider this frame as valid while in reality it is not because the order is of bits is wrong.

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