Why Transmission Errors Happen — Error Detection | IGCSE Computer Science

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable

Rare

Stable5%

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:

Data loss: some bits never arrive.
Data gain: extra bits appear that were never sent.
Data change: bits arrive flipped (0s become 1s and vice versa).

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.