Why Error Detection?
When data is transmitted over a network, errors can occur due to electrical interference, noise, or hardware faults. Error detection methods help identify whether data has been corrupted during transmission.
Important: Cambridge 0478 expects you to know three methods: parity checks, checksums, and echo checks.
Parity Checks
A parity bit is an extra bit added to a group of bits to make the total number of 1s either even (even parity) or odd (odd parity).
How it works:
- Sender and receiver agree on even or odd parity
- Sender counts the 1s in the data and sets the parity bit to make the total even/odd
- Receiver counts 1s including the parity bit — if the total doesn't match, an error has occurred
Worked example (even parity):
Data: 1011001 → five 1s (odd). Add parity bit 1 to make it even: 10110011 (six 1s). ✓
If received as 10110111 (seven 1s — odd), parity fails → error detected.
Limitations:
- Can only detect an odd number of bit errors
- If two bits flip, parity count is still correct → error not detected
- Cannot identify which bit is wrong (error correction not possible)
Checksums
A checksum is a value calculated from the data before transmission. The receiver recalculates and compares.
- Sender adds up all the bytes/values in a block of data to produce a checksum value
- Checksum is sent alongside the data
- Receiver performs the same calculation on received data
- If the calculated checksum matches the sent checksum → data received correctly
- If checksums differ → error detected; receiver requests retransmission
More powerful than parity — can detect multi-bit errors in blocks of data.
Echo Checks (ARQ)
In an echo check, the receiver sends the data back to the sender. The sender compares the echoed data with the original.
- If they match → data transmitted correctly
- If they differ → retransmission is requested
- Simple but doubles the amount of data on the network
- Example: older modem communications, terminal systems
| Method | How it works | Strength | Limitation |
|---|
| Parity bit | Extra bit to make 1s even or odd | Simple, low overhead | Misses even-number bit errors |
| Checksum | Sum of data values compared | Detects multi-bit errors | Doesn't pinpoint which bits |
| Echo check | Data echoed back to sender | Very reliable | Doubles network traffic |
Exam tip: Cambridge commonly asks you to describe or work through a parity check. Know: (1) what even/odd parity means, (2) how to set a parity bit, (3) the limitation (can't detect even-number errors). Also know that error detection can only detect errors — it's error correction methods that fix them.
⚠️ Common Mistakes
- Saying parity detects all errors — it cannot detect errors where an even number of bits flip
- Confusing even and odd parity — the parity bit is set so the TOTAL count of 1s (data + parity) is even or odd
- Saying error detection corrects errors — detection only finds them; you still need retransmission