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.