Error Checking
- When data has been received it could be subject to errors and Corruption
Why check for errors?
- Computers expect data in certain formats
- A format is a way of arranging the data so that it can be easily understood by people and by computers
- People agree to certain formats so that systems work more efficiently and there is little chance of misunderstanding each other
- An example of a format is date and time. Date and time can have multiple formats such as:
- 13/04/14 (DD/MM/YY)
- 12/31/2020 (MM/DD/YYYY)
- Jul-04-16 (MMM/DD/YY)
- Computers usually perform processes and calculations on data. If the data is not as expected, things can go wrong. For example, if a receiver expected to receive a date in format DD/MM/YY as 03/04/17 but received 04/03/17, did the sender mean 3rd April 2017 or 4th March 2017?
- An error or corruption occurs when data received is not as expected and therefore is difficult or impossible to process
How is data represented?
- All data is represented in binary as 1’s (high voltage) and 0’s (low voltage)
- For example, the number 67 is represented as 01000011
- An error could cause one of the bits to flip from 1 to 0 or vice versa
- If the sixth most significant bit is flipped, 01000011 -> 01000111, 67 becomes 71
- The number 67 in ASCII represents the uppercase letter ‘C’ whereas the number 71 represents ‘G’
- By flipping a single bit, the meaning of the binary string has changed
- If a book was transmitted over a network, assuming many bits were flipped, it would likely be difficult to read the original text
How can errors cause problems?
- Some errors are small and trivial such as a single swapped letter in a large text
- Other errors, in data such as postage addresses, aerospace coordinates or bank transfers, can be difficult to rectify or be disastrous
- Any job or task that relies on a computer to perform highly sensitive or secure processes must have methods of error checking and correction
How do errors occur?
- Errors can occur using wired or wireless technology due to interference
- Examples of interference include wire degradation or electrical fields changing the signal
- Results of interference include:
- Data loss - data is lost in transmission
- Data gain - additional data is received
- Data change - some bits have been changed or flipped
- Wireless technology uses radio signals or other electromagnetic signals to transmit data
- These signals can be blocked by physical barriers such as buildings, walls, cars or other objects
- Interference can be caused by bad weather such as rain or clouds, or by other wireless signals or electromagnetic radiation
- Wired technology carries more chance of causing an error as physical components can be damaged, degrade or receive interference from outside signals
- Data loss can also occur from interruptions to data transmission such as a blocked signal or if the transmission is intermittent
Worked example
Alex receives an email over a wireless connection from a work colleague containing an important document.
Identify what interference Alex could experience when sending this email and identify the outcomes of interference. Further explain why Alex should check to make sure the document contains no errors.
[4]
- Weather conditions or physical barriers such as building can affect signals, for example bits could be flipped in the document making it hard to understand the original meaning [1]
- Alex should be aware that interference can cause wirelessly received data to contain errors or corruption [1]
- Data could be lost, additional data could be gained or data could be changed [1]
- As Alex received an important work document they need to check for errors so that their work is unaffected and they do not receive incorrect information [1]
