Checksum is an error detection method used by the TCP/IP protocol suite to detect errors in a block of data. Like LRC, checksum works on the entire data block. However, instead of using parity (like in the case of LRC), checksum uses addition, making it more reliable.

**A checksum is an error detection code, calculated by the sender and transmitted along with the original data. **This calculated checksum is compared with the transmitted checksum by receiver. It helps the receiver identify errors or corrupt datablocks.

**NOTE: The checksum code is added as redundant bits to the data to be transmitted.**

## Calculation of Checksum:

- The original message is broken into ‘k’ number of blocks with ‘n’ bits each. Typically ‘n’ is 16bits.

- All the ‘k’ number of data blocks are added.

- If there is any carry, it is added to the sum.

- 1’s compliment of the sum is then calculated. This is called the checksum.

**NOTE: This checksum is also of ‘n’ bits.**

**NOTE**:** This addition is binary addition. Here are some rules of binary addition:**

### Let’s take an example:

Suppose data to be transmitted is: **11001010 00101100 10111001 **

**Step1:**Divide the original data into k blocks of n bits each.

In the above example, say, we decide to divide the data into 3 blocks of 8 bits each:

Thus, k = 3 and n = 8.

11001010 | 00101100 | 10111001 |

**Step2:**Sum all the ‘k’ data blocks:

In this case k = 3. We sum all three data blocks together

1 | 1 | 0 | 0 | 1 | 0 | 1 | 0 | |

0 | 0 | 1 | 0 | 1 | 1 | 0 | 0 | |

1 | 0 | 1 | 1 | 1 | 0 | 0 | 1 | |

1 | 1 | 0 | 1 | 0 | 1 | 1 | 1 | 1 |

**Step 3:**Add carry, if any.

The final checksum is of ‘n’ bits. In this case, n = 8. Whereas, our result is of 9 bits. Thus , there is an extra bit in the result of the addition. This extra bit is the carry we got duing the addition.

As there is a carry, we add the carry to the sum.

1 | 0 | 1 | 0 | 1 | 1 | 1 | 1 |

1 | |||||||

1 | 0 | 1 | 1 | 0 | 0 | 0 | 0 |

This results in a sequence of ‘n = 8’ bits. The sequence we got is **10110000**

**Step 4:** 1’s compliment of the result of addition and carry.

Calculating the 1’s compliment of the final sequence after addition gives us the **checksum code.**

**NOTE: To get 1’s complement, we invert the bit. So:**

In this case:

1 | 0 | 1 | 1 | 0 | 0 | 0 | 0 |

0 | 1 | 0 | 0 | 1 | 1 | 1 | 1 |

Final checksum is :** 01001111**

## Transmission of Data with Checksum

The final checksum is appended to the left of the data. This data is then transmitted to the receiver such that, the checksum is the last block to be sent.

In this example, the checksum code, ** 01001111**, is appended such that it is on the left of the original data blocks:

01001111 | 11001010 | 00101100 | 10111001 |

This data is then sent in the order such that the checksum is the last data block received.

## Receiver’s Side

On receiving the transmitted data, the receiver:

- Collects all the datablocks, including the checksum

- Adds all the datablocks together

- If there is any carry, it is added to the result.

- If the final result is all 1 bits, the data is accepted. If however, this is not the case, the data was corrupted during network transmission.

Let’s assume in our example, that data is transmitted over the network with no errors. Then the receiver would do the following calculation:

Data block 1 | 1 | 1 | 0 | 0 | 1 | 0 | 1 | 0 |

Data block 2 | 0 | 0 | 1 | 0 | 1 | 1 | 0 | 0 |

Data block 3 | 1 | 0 | 1 | 1 | 1 | 0 | 0 | 1 |

Checksum | 0 | 1 | 0 | 0 | 1 | 1 | 1 | 1 |

1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 |

**111111110**as the result.

This leaves a carry. This carry is added back to the answer:

Result | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 0 |

Carry | 1 | |||||||

Final Result | 1 | 1 | 1 | 1 | 1 | 1 | 1 | 1 |

As the final result consists of all 1-bits, the receiver knows that no error occured during network transmission. It thus, accepts the data.

## Advantages of Checksum:

**Efficiency:**Checksums provide a quick and efficient method of error detection, requiring minimal computational resources.

**Simplicity:**Checksum calculations are relatively straightforward, making them widely applicable and compatible with different systems and protocols.

**Wide Adoption:**Checksums are extensively used in various domains, including networking, file transfers, and storage systems, ensuring data integrity across diverse environments.

## Limitations of Checksum:

**Limited Error Detection:**Checksums may fail to detect certain types of errors, especially when they result in the same checksum value as the correct data.

**Inability to Correct Errors:**Checksums can only detect errors but lack the capability to correct them. Additional mechanisms are required for error correction.

## FAQs

**Can checksum work on messages of any length?**

Yes. Checksum can work on messages of any length. It would break the message into ‘k’ blocks of ‘n’ length each and then continue with it’s normal protocol.

**What value of ‘n’ is usually used?**

16 bits. ‘n’ is the length of bits in each segment of data. So we can say, data is divided into ‘k’ number of datablocks of 16 bits each.

**What are some various checksum algorithms used for error****detection**?

Internet Checksum, CRC (Cyclic Redundancy Check), Adler-32 Checksum, Fletcher’s Checksum, MD5 and SHA-1 are some checksum algorithms

**Is checksum more reliable than LRC?**

Yes. As checksum uses addition over parity used in LRC, checksum is better than LRC.

**Is checksum more reliable than VRC?**

Yes. As checksum uses addition over parity used in VRC, checksum is better than VRC. Checksum error detection detects both odd and even number of bits error.

**Is checksum more reliable than CRC?**

No. The error detection capabilities of a CRC make it better than checksum.

**Can a checksum correct errors?**

No. Checksums can only detect errors. It can’t correct them.

**Is checksum and crc the same thing?**

No. CRC is a type of checksum algorithm.

**Is md5 a checksum?**

Yes, MD5 (Message Digest Algorithm 5) is commonly used as a checksum in addition to its cryptographic applications.

**Is SHA1 a checksum?**

Yes. While SHA-1 (Secure Hash Algorithm 1) is primarily designed as a cryptographic hash function, it can be used as a form of checksum for error detection purposes.

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