Automatic repeat query (ARQ)

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Quick Questions

Shade in the A or B circle for each question.

1. What does ARQ stand for?

A Automatic repeat query

B Automatic receive question

2. In ARQ, who checks the received data for errors?

A The receiver, using a check such as parity or a checksum

B The sender, before it has even sent the data

3. Data arrives and the error check fails. What does the receiver do in ARQ?

A Sends a positive acknowledgement and keeps the bad data

B Sends a negative acknowledgement so the data is resent

4. The sender sends a packet and no reply comes back before the timeout ends. What does ARQ do?

A The sender automatically resends the packet

B The sender assumes it arrived and moves on

5. Which statement about ARQ is correct?

A The sender sends each packet once and never resends it

B The sender resends until the data is received correctly

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1ARQ		AIn ARQ the receiver must reply. If no reply arrives before the timeout, the sender resends rather than assuming success
2IF error_found THEN SEND NAK		BWHILE reply <> ACK SEND data ENDWHILE
3The sender waits for a reply after sending, and resends automatically if no reply arrives before the timeout		CThe receiver checks the data (e.g. with parity or a checksum) and, if it finds an error, replies with a negative acknowledgement so the sender resends
4The sender keeps resending until the receiver confirms the data was received correctly with a positive acknowledgement		DREPEAT SEND data WAIT timeout UNTIL reply_received
5Receiver gets bad data -> receiver says nothing -> sender assumes it was fine and stops		EAutomatic repeat query: the receiver replies to say if data arrived without error, and the sender automatically resends data that did not
6Receiver detects an error -> receiver fixes the data itself -> no resend needed		FARQ does not correct errors at the receiver. The receiver only detects the error and replies, and the sender resends the data

Definitions

ARQ

Fill in the Blanks

Complete each line of code by filling in the missing parts.

SEND packet ____ FOR reply IF reply = NAK THEN SEND packet

REPEAT SEND packet UNTIL reply = ___

IF errorFound = TRUE THEN SEND ___ ELSE SEND ACK

SEND packet IF no reply within _______ THEN SEND packet

SEND packet WHILE reply __ ACK RESEND packet

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A WHILE

B SEND packet

C WAIT for reply

D UNTIL reply = ACK

Question 2

A SEND packet

B WAIT for reply

C IF reply = NAK THEN

D DISCARD packet

Question 3

A RECEIVE packet

B IF parity is wrong THEN

C SEND ACK

D ELSE SEND NAK

Question 4

A SEND packet

B START timer

C IF ACK received THEN

D SEND packet again

Question 5

A RECEIVE data and checksum

B CALCULATE new parity bit

C IF new = received THEN

D SEND ACK

Answer Key

Automatic repeat query (ARQ)

Section A: Quick Questions

A — Automatic repeat query

A — The receiver, using a check such as parity or a checksum

B — Sends a negative acknowledgement so the data is resent

A — The sender automatically resends the packet

B — The sender resends until the data is received correctly

Section B: Match the Pairs

ARQ → Automatic repeat query: the receiver replies to say if data arrived without error, and the sender automatically resends data that did not

IF error_found THEN SEND NAK → The receiver checks the data (e.g. with parity or a checksum) and, if it finds an error, replies with a negative acknowledgement so the sender resends

The sender waits for a reply after sending, and resends automatically if no reply arrives before the timeout → REPEAT SEND data WAIT timeout UNTIL reply_received

The sender keeps resending until the receiver confirms the data was received correctly with a positive acknowledgement → WHILE reply <> ACK SEND data ENDWHILE

Receiver gets bad data -> receiver says nothing -> sender assumes it was fine and stops → In ARQ the receiver must reply. If no reply arrives before the timeout, the sender resends rather than assuming success

Receiver detects an error -> receiver fixes the data itself -> no resend needed → ARQ does not correct errors at the receiver. The receiver only detects the error and replies, and the sender resends the data

Section C: Definitions

ARQ — 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.

Section D: Fill in the Blanks

SEND packet
WAIT FOR reply
IF reply = NAK THEN SEND packet

REPEAT
   SEND packet
UNTIL reply = ACK

IF errorFound = TRUE THEN SEND NAK
ELSE SEND ACK

SEND packet
IF no reply within timeout THEN SEND packet

SEND packet
WHILE reply <> ACK
   RESEND packet

Section E: Spot the Error

A — The loop ends with UNTIL, so it must start with REPEAT. The sender keeps resending until an ACK arrives. Line 1 should be REPEAT.

D — A NAK tells the sender the data arrived with an error. ARQ means the sender automatically resends it, so line 4 should be SEND packet again.

B — The receiver should send an ACK only when the parity check passes. Line 2 should be IF parity is correct THEN. As written, bad data would be acknowledged.

C — The packet should be resent when no reply arrives before the timeout. Resending after an ACK is pointless. Line 3 should be IF no reply by timeout THEN.

B — The receiver must recalculate the checksum using the same method as the sender, so it can compare the two values. Line 2 should be CALCULATE new checksum.