Tag: Checksum Error

Checksum in Networking | Checksum Example

Computer Networks

Error Detection in Computer Networks-

 

Error detection is a technique that is used to check if any error occurred in the data during the transmission.

 

Some popular error detection methods are-

 

 

  1. Single Parity Check
  2. Cyclic Redundancy Check (CRC)
  3. Checksum

 

In this article, we will discuss about Checksum Method.

 

Checksum-

 

Checksum is an error detection method.

Error detection using checksum method involves the following steps-

 

Step-01:

 

At sender side,

  • If m bit checksum is used, the data unit to be transmitted is divided into segments of m bits.
  • All the m bit segments are added.
  • The result of the sum is then complemented using 1’s complement arithmetic.
  • The value so obtained is called as checksum.

 

Step-02:

 

  • The data along with the checksum value is transmitted to the receiver.

 

Step-03:

 

At receiver side,

  • If m bit checksum is being used, the received data unit is divided into segments of m bits.
  • All the m bit segments are added along with the checksum value.
  • The value so obtained is complemented and the result is checked.

 

Then, following two cases are possible-

 

Case-01: Result = 0

 

If the result is zero,

  • Receiver assumes that no error occurred in the data during the transmission.
  • Receiver accepts the data.

 

Case-02: Result ≠ 0

 

If the result is non-zero,

  • Receiver assumes that error occurred in the data during the transmission.
  • Receiver discards the data and asks the sender for retransmission.

 

Checksum Example-

 

Consider the data unit to be transmitted is-

10011001111000100010010010000100

Consider 8 bit checksum is used.

 

Step-01:

 

At sender side,

The given data unit is divided into segments of 8 bits as-

 

 

Now, all the segments are added and the result is obtained as-

  • 10011001 + 11100010 + 00100100 + 10000100 = 1000100011
  • Since the result consists of 10 bits, so extra 2 bits are wrapped around.
  • 00100011 + 10 = 00100101 (8 bits)
  • Now, 1’s complement is taken which is 11011010.
  • Thus, checksum value = 11011010

 

Step-02:

 

  • The data along with the checksum value is transmitted to the receiver.

 

Step-03:

 

At receiver side,

  • The received data unit is divided into segments of 8 bits.
  • All the segments along with the checksum value are added.
  • Sum of all segments + Checksum value = 00100101 + 11011010 = 11111111
  • Complemented value = 00000000
  • Since the result is 0, receiver assumes no error occurred in the data and therefore accepts it.

 

Also Read- Parity Check

 

Important Notes-

 

Note-01:

 

  • Consider while adding the m bit segments, the result obtained consists of more than m bits.
  • Then, wrap around the extra bits and add to the result so that checksum value consists of m bits.

 

Note-02:

 

  • While calculating the checksum, if checksum value is needed, then assume it to be zero.
  • After calculating the checksum value, substitute the checksum value in the checksum field.
  • This will be required during checksum calculation of IP Header, TCP Header and UDP Header.

 

Note-03:

 

  • The checksum is used in the internet by several protocols although not at the data link layer.

 

Also Read- Cyclic Redundancy Check (CRC)

 

PRACTICE PROBLEM BASED ON CHECKSUM ERROR DETECTION METHOD-

 

Problem-

 

Checksum value of 1001001110010011 and 1001100001001101 of 16 bit segment is-

  1. 1010101000011111
  2. 1011111000100101
  3. 1101010000011110
  4. 1101010000111111

 

Solution-

 

We apply the above discussed algorithm to calculate the checksum.

  • 1001001110010011 + 1001100001001101 = 10010101111100000
  • Since, the result consists of 17 bits, so 1 bit is wrapped around and added to the result.
  • 0010101111100000 + 1 = 0010101111100001
  • Now, result consists of 16 bits.
  • Now, 1’s complement is taken which is 1101010000011110
  • Thus, checksum value = 1101010000011110

 

Thus, Option (C) is correct.

 

To gain better understanding about Checksum Method,

Watch this Video Lecture

 

Next Article- Access Control Methods | Introduction

 

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