Data Transmission · 4 question types
Past paper frequency (2018 to 2024)
This topic accounts for approximately 5% of your exam marks.
Parity bits, checksums and check digits each appear in most papers. Often 3 to 4 marks.
When data travels along a wire or through the air, the signal can be distorted by physical effects, and a bit that was sent as a 0 can arrive as a 1 (or vice versa). The causes:
| Source of error | Where it bites |
|---|---|
| Electromagnetic interference | Nearby motors, microwave ovens, fluorescent lights, cables carrying other signals |
| Cable degradation | Old cables develop corrosion or damaged insulation; long cables suffer signal loss |
| Weather | Wireless links are degraded by heavy rain, fog and physical obstacles |
| Physical barriers | Walls, buildings, vehicles and people absorb or reflect wireless signals |
| Crosstalk | Adjacent wires in a cable bleed signal into each other |
| Bad connectors | Loose plugs and corroded sockets introduce intermittent noise |
The three things that can go wrong with a transmitted byte:
Computers cannot recover from a corrupted transmission unless they can detect that the corruption has happened. That is what every method in this topic does: it gives the receiver a way to spot when something has gone wrong, so the data can be rejected or re-sent.
This topic covers five methods: parity check, parity blocks, checksum, echo check, and check digits. It also covers Automatic Repeat reQuest (ARQ), the protocol that handles re-transmission when an error is found.