iGCSE Computer Science - Distance Learning

Tutor Marked Assignment 6
Tutor Marked Assignment 8 (Programming)

Understanding Error Checking Methods

In this lesson, we are going to extend our knowledge of error checking methods using your knowledge from last lesson to look at how we can detect and fix errors in the transmission of data using checksums, and also detecting missed data using automatic repeat requests (ARQ).

As part of our understanding of checksums we will be looking at binary addition. In order to complete this lesson, you will require a pen and paper. There is no need for a calculator, and it is useful to practice this without one as there is no calculator allowed in the exam.

Last lesson, we looked at how parity could be used to detect errors. However, one major flaw with simply using parity is that errors with an even number of corruptions will be missed as they will remain in the correct parity. For example, if the bit pattern 1011101 was to be transmitted this would have an odd parity. If the first 2 digits became corrupted it would become 0111101 which is also in odd parity and so the error would be missed.

One of the ways that we can avoid this is by using checksums. A checksum takes the bytes of data within a data packet and performs binary addition on the bits being transmitted. This additional some of data is then transmitted along with data packet. The receiving device performs the same binary addition and compares the two numbers – if they are not identical, an error has occurred.

Often, check sums are used in combination with parity allowing the receiving device to identify both the row and possible column that’s an error has occurred. If the device can identify both the row and the column then the bit can be reversed and potentially correct the error.