IGCSE Computer Science Paper 1 Definitions Worksheet: 2.2 Methods of Error Detection (Modules 179–188)
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Key terms
Write what each term means, in your own words.
1
check digit
2
recalculate and compare
3
check digit limitation
4
ARQ
5
positive acknowledgement
6
negative acknowledgement
7
timeout
8
ARQ limits
9
error detection method
Answer Key
IGCSE Computer Science Paper 1 · Modules 179–188 · Student wording will vary for definitions
1check digit — The last digit of a bar code number, worked out from the other digits. The scanner recalculates it and rejects a misread if the digits do not match, so the item must be scanned again.
2recalculate and compare — Work out the check digit again from the other digits, then compare it with the one entered. If they differ, reject the number and ask the user to re-enter it.
3check digit limitation — A wrong number can still pass the check digit test, and a valid number may belong to the wrong item. The check digit tests the digits only, not what they mean.
4ARQ — Automatic repeat query: the receiver checks data for errors and replies. If the data is bad or no reply arrives, the sender automatically resends it until it is received correctly.
5positive acknowledgement — A message sent by the receiver to tell the sender that data arrived without error. Often written ACK. It is sent only after the error check passes, so the sender can send the next data.
6negative acknowledgement — A message (NAK) the receiver sends to the sender when the error check fails, telling it the data arrived with an error so the sender must resend the same data.
7timeout — The set time a sender waits for an acknowledgement after sending data. If none arrives in time, it assumes the data was lost and resends it.
8ARQ limits — ARQ resends data when a separate check finds an error, so it misses errors that check misses, and a noisy link causes many resends that slow the transfer.
9error detection method — Parity, parity block, checksum and echo check find errors in transmission; a check digit finds them at data entry; ARQ asks for a resend. Each sends different extra data.