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Error Detection
Data Transmission · 4 question types
| Method | Detects | Locates | Corrects | Best for |
|---|---|---|---|---|
| Parity check | Single-bit errors (and other odd numbers of errors) | No | No | Fast, cheap link-level checks |
| Parity block | Single-bit errors | Yes (intersection of row + column) | Single-bit only | Memory chips, simple data blocks |
| Checksum | Most random corruption | No | No | Files, packets, web downloads |
| Echo check | Almost any corruption | No (and ambiguous about direction) | No (use ARQ on top) | Short links where reliability matters more than speed |
| Check digit | Single wrong digit, transpositions, omissions | No | No | Human-entered numerical codes (ISBN, barcodes, bank cards) |
| ARQ | Uses another method (e.g. checksum) | Depends | Yes (via re-transmission) | Combined with detection for reliable delivery |
Choosing the right tool
A typical exam question gives a scenario and asks which method best fits. The decision tree:
- Is the data human-entered numbers? → Check digit.
- Is the data a transmitted byte or packet? → Parity (very fast, weak) or checksum (slower, stronger).
- Does the system need to know exactly which bit is wrong? → Parity block.
- Does the system need to automatically recover from errors, not just detect them? → ARQ on top of any of the above.
The next topic in this chapter, Encryption (topic 6), addresses a different concern: not whether data is correct, but whether it can be read by anyone who intercepts it.