# Expalin the working mechanism of Checksum.

**CHECKSUM**

**The error detection method used by the higher layer protocols is called checksum. It consists of two arts. They are,**

1.
checksum generator

2.
checksum checker

**Checksum Generator:**

In the sender, the checksum generator
subdivides the data unit into equal segments of n bits. These segments are
added with each other by using one’s complement arithmetic in such a way that
the total is also n bits long. That total is then complemented and appended to
the end of the data unit.

**Checksum Checker:**

The receiver subdivides the data unit as above
and adds all segments together and complements the result. If the extended data
unit is intact, the total value found by adding the data segments and the
checksum field should be zero. Otherwise the packet contains an error and the
receiver rejects it.

**EXAMPLE**

**At the sender**

Data unit: 10101001 00111001

10101001

00111001

Sum
1100010

Checksum 00011101

**At the receiver**

1)

Received
data: 10101001 00111001 00011101

10101001

00111001

00011101

Sum 11111111

Complement 00000000

It means that
the patter is ok.

2)

Received
data: 1010111 111001 00011101

10101111

11111001

00011101

Result 11000101

Carry 1

Sum 11000110

Complement 00111001

It means that
the patter is corrupted.

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