2.2 Methods of error detection

By the end of this section you should be able to

• 2.2.1 Explain how errors occur during transmission
• 2.2.2 Describe parity checks (odd and even, byte and block)
• 2.2.3 Describe checksums and echo checks
• 2.2.4 Explain how check digits detect data entry errors
• 2.2.5 Describe automatic repeat query (ARQ)

Lessons

1. Errors and parity checks
2. Checksums
3. Echo checks
4. Check digits
5. Automatic repeat query (ARQ)

Key vocabulary

Parity check

Checks that the number of 1s in a byte is odd or even, as agreed before sending.

Parity block check

Parity is checked across rows and columns to locate the bit in error.

Checksum

A value calculated from the data before sending and recalculated on arrival to compare.

Echo check

The receiver sends the data back so the sender can compare it with the original.

Check digit

An extra digit calculated from the others, used to catch data entry errors, e.g. in ISBNs and barcodes.

Automatic repeat query (ARQ)

Uses acknowledgements and a timeout so data is resent if it is not received correctly.

Timeout

The time the sender waits for an acknowledgement before resending.