Automatic Repeat Query

Courses iGCSE Computer Science (2023 - 2025) Data Representation Automatic Repeat Query

Resend Until It Arrives

Automatic Repeat Query (ARQ) uses acknowledgements and a timeout. After sending a packet, the sender waits. An ACK means send the next packet. A NAK, or silence before the timeout, means send the same packet again.

ARQ does not just detect an error; it tries to fix delivery by repeating the data. Cambridge wants ACK, NAK and timeout as the three controls that make the loop stop or continue.

ARQ resends data when

{"answers": {"A": "It receives a NAK or no ACK before the timeout", "B": "A Huffman tree is unbalanced", "C": "The user clicks a CSS button", "D": "A primary key is underlined"}, "question_text": "ARQ resends data when", "correct_answer": "A"}

ARQ versus Parity

Parity

Detects that a byte looks wrong. It does not, by itself, ask for a new copy. The receiver may only know “this byte failed”.

ARQ

Uses that failure (or silence) to request a repeat. ACK means success, NAK means send again, timeout means assume the ACK was lost and resend anyway.

ARQ uses [[ACK]] for success, [[NAK]] for failure, and a timeout if nothing returns.

{"cloze_text": "ARQ uses [[ACK]] for success, [[NAK]] for failure, and a timeout if nothing returns."}

{"keyword": "ARQ", "definition": "Automatic Repeat Query: resending data if an acknowledgement is missing or negative"}

Question Type: fill in the blanks

{"paragraph": "If no ACK arrives before the [[timeout]], the sender [[resends]] the packet."}

Question Type: short answer

{"answer": "The wait after which the sender assumes the acknowledgement was lost and resends.", "rubric": {"0": "Incorrect or no response", "1": "Partial credit", "2": "The wait after which the sender assumes the acknowledgement was lost and resends."}, "question": "What is a timeout in ARQ?"}