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Error Detection

This lesson explains why errors happen when data is transmitted and the methods used to detect them. You will learn about parity bits (odd and even), checksums, check digits, echo checks and the Automatic Repeat reQuest (ARQ) system, and you will work through a parity example step by step.

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Why errors occur

How transmission errors happen Sent 1 0 1 1 0 0 1 0 noise Received 1 0 0 1 0 0 1 0 Interference or a poor/long cable can flip a bit a 0 arrives as 1 (or a 1 as 0) and the data is corrupted
  • Errors can occur during transmission because of interference (electrical noise), a poor or damaged connection, or a very long cable weakening the signal.
  • These problems can cause a bit to flip, so a 0 arrives as a 1 or a 1 arrives as a 0, corrupting the data.
  • Error detection methods add extra data before transmission so the receiver can check whether the data arrived correctly.
  • Detecting an error lets the receiver ask for the data to be sent again, keeping the transmitted information reliable.

Check you've got it

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1. A system uses even parity. The seven data bits are 1010011 and a parity bit is added. What must the parity bit be?

2. What is the main limitation of using a single parity bit?

3. Which error detection method adds an extra digit to the end of a number such as a barcode or ISBN, calculated from the other digits?

4. In an Automatic Repeat reQuest (ARQ) system, what happens if the sender receives no acknowledgement before its timer runs out?

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