1. Error Detection Learning Objectives:
5.1.7 Understand the need to detect and correct errors in data transmission

2. Error Detection
Errors can occur whenever information is stored or transmitted. Error detection techniques add extra parity bits to data to determine when errors have occurred.
This activity uses a magic trick to show how to detect when data has been corrupted, and how to correct it.
Click on the image to load the interactive grid.
1. Choose a child to click on the square, so a random mixture of black and white is showing.
Casually click on the add another row/column button, “just to make it a bit harder”.
These squares are the key to the trick. You must choose the extra square to ensure that there is an even number of black cards in each row and column.
2. Get a child to change one square only while you cover your eyes. The row and column containing the changed card will now have an odd number of coloured cards, and this will identify the changed card.
Can the children guess how the trick is done?
Get the students to try the trick for themselves in groups, talk them through setting up the grid.
These videos show the trick in action:

3. A match means that no transmission error has been detected.
Checksums
A checksum is an algorithm used to check for errors in data transmission. It works by calculating a numerical value based on the number of bits in a packet of data. This value is attached to the packet.
When the packet reaches its destination, the same checksum algorithm is applied to the data. If the two checksums don’t match it shows that an error has occurred and the receiving computer asks the sending computer to re-send that package.
A match means that no transmission error has been detected.
Now complete task 7.1.

4. Parity Checking
Parity checking is a simple form of error detection. A parity bit is added to the end of a string of binary code to indicate whether the number of bits in the string with the value 1 is even or odd. There are two type of parity bits:
Even Parity
Odd Parity
Where the bit is set to 1 if the number of bits is odd. The extra parity bit will make the number of 1s even.
Where the parity bit is set to 1 if the number of bits is even. The extra parity bit will make the number of 1s odd.
There are other more sophisticated methods of detecting errors, such as cyclic redundancy checks (CRC).
Now complete task 7.2.

5. Work through these examples as a class. Data being transmitted
Number of 1s
Even parity
Odd parity 3 4 2
Now complete tasks 7.3 to 7.5.