Introduction to Networking

# Checksum in Error Detection

A Checksum is a redundancy of bits that are appended with actual data for error detection.

At the sender side, the checksum is generated, and at the receiver side, the checksum is validated. The block diagram of the Checksum is given below.

## 1. Checksum at Sender Side

At the sender side, checksum is generated through the following steps

Step 01: Break the original data into the “K” number of blocks with “N” bits in each block.

Step 02: Sum all the “K” data blocks

Step 03: Add the carry bit if it exists.

Step 04: Find the 1st Complement

The result after 4th step, Checksum, is ready to append with data.

## 2. Checksum at the Receiver Side

For validation of data at the sender side, follow the following steps

Step 01: Sum all the “K” data blocks and Checksum

Step 02: Add carry bits if any

Step 03: If the result is all 1s, ACCEPT data; otherwise, REJECT the data.

### Explain with Example

Consider the data unit 10011001111000100010010010000100. Now apply the above steps

### 1. Checksum Generator at Sender Side

Step No 1: Break the original data into a “K” number of equal-sized blocks.

Step No 2: Sum all “K” Blocks

Step No 3: Add carry if any

Step No 4: Find the 1st Complement = Checksum

### 2. Checksum Validation at the Receiver Side

Step 01: Sum all the “K” data blocks and Checksum

Step 02: Add carry if any

Step 03: If all the resultant bits of step 2 are 1’s, ACCEPT data otherwise, REJECT the data.

As we see, the result of step 2 is all 1’s, so data is accepted.

## Performance of Checksum

• Checksum can detect even or odd bits changing.
• If one or more bits of a data block are changed and corresponding bits or bits of opposite value in a second block are also changed, the sum of those columns will not change. So, an error will not detect.