Learning Objectives
- Explain why errors can occur during data transmission.
- Describe data loss, data gain and data change.
- Relate interference to altered bit patterns.
- Explain why received data must be checked.
- Distinguish error detection from error correction and retransmission.
Key Terms
- Transmission error
- A difference between the data sent and the data received.
- Interference
- An unwanted signal or disturbance affecting transmitted data.
- Data loss
- One or more transmitted bits or data units fail to arrive.
- Data gain
- Extra bits or data units appear in the received data.
- Data change
- A transmitted value is altered, such as a 0 becoming 1.
- Error detection
- A process that identifies that received data may be incorrect.
Why Transmission Can Introduce Errors
Data transmission uses electrical, radio, optical or other physical signals. These signals can be affected by interference, damaged connections, weak signal strength or temporary faults. The receiver then interprets a signal differently from the value sent.
A small physical disturbance can have a significant logical effect. If a binary 0 is interpreted as 1, a character, instruction or numerical value may change. For this reason, receiving a sequence of bits is not enough; systems need a way to check whether the data is likely to match the original.
The syllabus requires understanding of data loss, data gain and data change. These are categories of outcome rather than names of specific error-detection methods.
Data Loss, Gain And Change
Data loss occurs when expected data does not arrive. A bit, byte, packet or other unit may be missing. In a sequence 101101, losing the third bit changes the length and shifts all later positions.
Data gain occurs when additional data appears. A repeated signal or disturbance may cause an extra bit or unit to be received. This can also change the alignment of all later data.
Data change occurs when the number of received bits is unchanged but one or more values differ. For example, sent data 100101 may be received as 100001 because a 1 has changed to 0.
All three forms can make the result unusable or misleading. A corrupted program instruction may perform the wrong action, while corrupted personal or financial data may produce an incorrect record.
The Purpose Of Error Detection
Error-detection methods add or use checking information. The receiver performs a corresponding check and compares the result with the expected value. A mismatch indicates that an error has probably occurred.
Detection does not always reveal exactly which bit is wrong, and it does not necessarily repair the data. A system may discard the damaged data or request retransmission. The method can also fail to detect certain patterns of multiple errors, so examination answers should avoid saying that a simple check guarantees perfection.
Parity checks, checksums and echo checks detect transmission errors. Check digits are used to detect data-entry errors. Automatic repeat query combines detection with acknowledgements and retransmission. These distinctions are developed in the following lessons.
Choosing A Checking Method
Different checks provide different balances of processing, additional data and detection ability. A parity bit adds very little data but has limitations. A checksum summarises a larger block. An echo check sends received data back for comparison.
The examination may give a method and ask for its process. Answers should describe what the sender does, what the receiver does and how a mismatch reveals an error. Merely writing that the method checks data is not sufficient.
Forms Of Transmission Error
| Error | What Happens | Example |
|---|---|---|
| Data loss | Expected data is missing. | 101101 becomes 10101 after a bit is lost. |
| Data gain | Unexpected extra data appears. | 101101 becomes 1011101 after an extra bit. |
| Data change | A value is altered. | 101101 becomes 100101. |
Detection, Correction And Retransmission
| Process | Meaning |
|---|---|
| Detection | Recognising that received data may contain an error. |
| Correction | Changing data to recover the intended value. |
| Retransmission | Sending the data again after an error or missing response. |
Worked Examples
Classifying An Error
Question: Sent data is 11001010. Received data is 1101010. What type of error occurred?
- Count the bits in each pattern.
- The received pattern has one fewer bit.
Answer: Data loss.
Changed Bit
Question: Sent data is 01101100. Received data is 01100100. Classify the error.
- Both patterns contain eight bits.
- Compare each position.
- One bit value differs.
Answer: Data change.
Why Checking Is Necessary
Question: A bank transmits an account balance. Explain why an error check is needed.
- Interference could alter one or more bits.
- The altered bit pattern could represent a different amount.
- A check can identify that the received record should not be accepted as correct.
Answer: Checking reduces the risk of acting on corrupted transmitted data.
Examination Guidance
- Use the exact terms data loss, data gain and data change.
- Explain the physical disturbance as interference when appropriate.
- Do not say detection always corrects an error.
- When describing a method, include sender and receiver actions.
Common Mistakes
- Calling every transmission error data loss.
- Saying an error-detection method prevents interference.
- Claiming that a detected error is automatically fixed.
- Confusing a data-entry check digit with a packet transmission check.
Knowledge Check
1. What is data change?
2. Why can interference cause errors?
3. What is the purpose of error detection?
4. Does detection always correct data?
5. Name three transmission error outcomes.
