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Cambridge IGCSE Computer Science · 0478
Topic 2: Data Transmission — Part 2
Error Detection
The need for error detection
Errors occur during transmission due to interference on the medium, leading to:
- Data loss: some data is not received.
- Data gain: unintended extra data or corruption.
- Data change: bits are "flipped" (e.g. 0 becomes 1).
Error detection methods
- Parity check
- A parity bit is added to make the total count of 1s either even or odd.
- Even parity: parity bit is set so the total number of 1s is an even number.
- Odd parity: parity bit is set so the total number of 1s is an odd number.
- Limitation: if an even number of bits flip (e.g. two bits change), the parity check will not detect the error.
- Parity byte/block check: a whole byte is added to a block of data to check parity across bit positions (columns) as well as rows, allowing for the detection and sometimes correction of bit errors.
- Checksum
- A value calculated from the data using an algorithm (e.g. the modulo function) is appended to the transmission. The receiver recalculates the checksum using the same algorithm; if the values don't match, an error is detected and retransmission is requested.
- Echo check
- The receiver sends the data back to the sender ("echoes" it). The sender compares the echo to the original. Disadvantage: inefficient as it doubles network traffic.
- Check digit
- A single digit calculated from other digits in a set. Used to detect manual data entry errors. Usage: barcodes and ISBN (International Standard Book Numbers).
- Automatic Repeat Query (ARQ)
- Uses acknowledgments and timeouts. If the receiver detects an error (using parity or checksum), it sends a negative acknowledgment. If the sender receives a negative acknowledgment or no response within a set timeout period, it automatically resends the data until it is confirmed correct.
Worked example: Parity byte / block check
A parity byte is added to a block of data to check parity across bit positions (columns) as well as rows, allowing for the detection and sometimes correction of bit errors.
Scenario: Using even parity for three bytes of data.
| Data | Bit 7 | Bit 6 | Bit 5 | Bit 4 | Bit 3 | Bit 2 | Bit 1 | Bit 0 |
|---|---|---|---|---|---|---|---|---|
| Byte 1 | 1 | 1 | 1 | 1 | 0 | 0 | 1 | 1 |
| Byte 2 | 0 | 1 | 0 | 1 | 0 | 1 | 0 | 1 |
| Byte 3 | 1 | 1 | 1 | 0 | 0 | 0 | 0 | 0 |
| Parity byte | 0 | 1 | 0 | 0 | 0 | 1 | 1 | 0 |
Explanation: Each bit in the parity byte is set so that the total number of 1s in that specific column is an even number.
Exam Traps
- Using checksum and check digit interchangeably — check digits detect manual entry errors (ISBN, barcodes); checksums are calculated during transmission.
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