Echo Check — 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.

An detects errors by having the receiver re-transmit what it got to the original sender, who then compares the echo against what was originally transmitted.

How it works:

The sender transmits the data.
The receiver echoes the data straight back to the sender.
The sender compares the echo with what it originally sent.
If the two match, the transmission is treated as correct.
If they differ, the sender re-transmits.

Strengths and weaknesses

Strength: catches almost any error because the sender has the original data to compare against, bit for bit.
Weakness: it is impossible to tell whether the error happened in the forward direction (sender → receiver) or in the return direction (receiver → sender). The data may have arrived correctly and only the echo corrupted, but the sender will still re-transmit.
Cost: doubles the amount of data on the network because every byte travels twice. Slow and bandwidth-hungry.

Echo checks are mostly used in short or low-traffic links where reliability matters more than efficiency.

Exam tip

Explain how an echo check detects errors

What comes up: a 3-mark "explain how" question on the echo check process.

Write (three marks): (1) After the receiver receives the data, it sends a copy of that data back to the original sender. (2) The sender compares the copy it received back with the data it originally transmitted. (3) If the two do not match, an error has occurred and the data must be re-sent.

Watch out: make sure you state all three steps — the echo back, the comparison, and the conclusion. Stopping at "the data is sent back" earns only one mark.