This presentation discusses checksums, which are an error-detecting technique used at the network and transport layers. The document provides the following information: - Checksums work by dividing a message into bits, generating an extra "checksum" bits unit, and comparing the checksum at the destination to detect errors. - An example is provided where numbers are summed and the checksum is the sum, which is compared at the destination to validate the message. - A second example uses ones' complement, where numbers are represented with a set number of bits and extra bits are added for numbers larger than that size. - Diagrams show the procedure and flow of checksum calculation and comparison to detect errors in transmitted data.

1. PRESENTATION ON
CHECKSUM
NAME : POOJA JAISWAL

2. CHECK SUM
 Error-detecting technique
 Applied to a message of any length
 Mostly used at network and transport layer rather
data-link layer.

3. 1. At the source, the message is first divided
into m-bits units
2. The generator then creates the checksum
i.e. an extra m-bits unit
3. At the destination, the checker creates a
new cheksum from the combination of the
message and sent checksum
4. If checksum is all 0s(zeroes),
Message is accepted
Otherwise,
Message is discarded

4. Fig : Checksum

5. Method 1.
Example-
4-bit numbers are: 7, 11, 12, 0, 6
Message to be sent:7,11,12,0,6,36
[7+11+12+0+6=36]
Receiver adds actual nos. and compares with
the sum(36)
If the two are the same
receiver assumes no error, nos. accepted
and sum discarded.
Else
an error somewhere and data are not
accepted.

6. Method 2. (one’s complement)
 Unsigned numbers between 0 and pow((2,m)-1)
are represented using only m bits
 If number has > m bits
extra leftmost bits needed to be added to the
m rightmost bits(wrapping)
Ex- number is 21.
21 in binary is 10101(five bits)
wrapping extra leftmost bit(s) i.e. one bit
we have (0101+1)=0110(in decimal, 6).
6, in one’s complement is 1001 which is 9(in
decimal)
so, complement of 6 is 9.

7. ….Previous ex-

8. Internet Checksum

9. Flow diagram of procedure of
checksum

10. Performance (Checksum)
•Traditional checksum uses a small number of bits (16) to
detect errors in a message of any size (sometimes
thousands of bits).
•However, it is not as strong as the CRC in error-checking
capability.
•For example:
if the value of one word is incremented and the value
of another word is decremented by the same amount, the
two errors cannot be detected because the sum and
checksum remain the same.
•Also, if the values of several words are incremented but the
sum and the checksum do not change, the errors are not
detected.
•Fletcher and Adler have proposed some weighted
checksums that eliminate the first problem.
•However, the tendency in the Internet, particularly in
designing new protocols, is to replace the checksum with a
CRC.