The ARQ process step by step

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Quick Questions

Shade in the A or B circle for each question.

1. The data arrives and passes the error check. What does the receiver send back?

A An ACK (positive acknowledgement)

B A NAK (negative acknowledgement)

2. The data arrives but the error check fails. What happens next in ARQ?

A The receiver sends a NAK and the sender resends the data

B The receiver sends an ACK and the sender sends the next data

3. When does the sender start its timer in ARQ?

A When it sends the data

B When it receives an ACK

4. The timer ends and the sender has received no reply. What does the sender do?

A Sends the next data

B Resends the data

5. Which message order shows the ACK path correctly?

A Send data, check passes, NAK, send next data

B Send data, check passes, ACK, send next data

Match the Pairs

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

Left column		Right column
1Sender -> Receiver: DATA (timer starts)		AThe check failed, so the receiver asks the sender to send the data again
2Receiver -> Sender: NAK		BA failed check must get a NAK, not an ACK. An ACK would make the sender think the data arrived correctly, so the error would never be fixed.
3If no reply arrives before the timer ends, the sender assumes the data or reply was lost and resends		CThe sender transmits the data and starts a timer while it waits for a reply
4When the receiver's check passes it replies ACK, and the sender moves on to the next data		DThe sender should not resend after an ACK. An ACK means the data arrived correctly, so the sender sends the next data. Resending happens only on a NAK or a timeout.
5Receiver gets DATA, check fails -> Receiver sends ACK		EIF timer ends AND no reply THEN resend DATA
6Sender gets ACK -> Sender resends DATA until timer ends		FReceiver checks OK -> ACK -> send next DATA

Definitions

ARQ

Fill in the Blanks

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

SEND packet START _____ WAIT for reply

IF check passes THEN SEND ___ ELSE SEND NAK

IF reply = ___ THEN SEND packet again

IF no reply BEFORE _______ THEN RESEND packet

REPEAT SEND packet _____ reply = ACK

Spot the Error

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

Question 1

A SEND data

B STOP timer

C WAIT for reply

Question 2

A RECEIVE data

B CHECK data for errors

C IF check passes SEND NAK

D ELSE SEND NAK

Question 3

A CHECK data

B IF check fails THEN

C SEND ACK

Question 4

A SEND ACK

B WAIT for reply

C IF reply is NAK THEN

D SEND data again

Question 5

A SEND data

B WAIT for data

C IF ACK arrives THEN

D SEND next data

Answer Key

The ARQ process step by step

Section A: Quick Questions

A — An ACK (positive acknowledgement)

A — The receiver sends a NAK and the sender resends the data

A — When it sends the data

B — Resends the data

B — Send data, check passes, ACK, send next data

Section B: Match the Pairs

Sender -> Receiver: DATA (timer starts) → The sender transmits the data and starts a timer while it waits for a reply

Receiver -> Sender: NAK → The check failed, so the receiver asks the sender to send the data again

If no reply arrives before the timer ends, the sender assumes the data or reply was lost and resends → IF timer ends AND no reply THEN resend DATA

When the receiver's check passes it replies ACK, and the sender moves on to the next data → Receiver checks OK -> ACK -> send next DATA

Receiver gets DATA, check fails -> Receiver sends ACK → A failed check must get a NAK, not an ACK. An ACK would make the sender think the data arrived correctly, so the error would never be fixed.

Sender gets ACK -> Sender resends DATA until timer ends → The sender should not resend after an ACK. An ACK means the data arrived correctly, so the sender sends the next data. Resending happens only on a NAK or a timeout.

Section C: Definitions

ARQ — Sender sends data and starts a timer. Receiver checks it and replies ACK or NAK. Sender resends on a NAK or a timeout until an ACK arrives.

Section D: Fill in the Blanks

SEND packet
START timer
WAIT for reply

IF check passes THEN
    SEND ACK
ELSE SEND NAK

IF reply = NAK THEN
    SEND packet again

IF no reply BEFORE timeout THEN
    RESEND packet

REPEAT
    SEND packet
UNTIL reply = ACK

Section E: Spot the Error

B — After sending the data the sender must START the timer so it can detect a missing reply. Line 2 should be START timer.

C — When the check passes the receiver sends a positive acknowledgement (ACK), not a NAK. Line 3 should be IF check passes SEND ACK.

C — If the check fails the receiver replies with a negative acknowledgement (NAK) so the data is resent. Line 3 should be SEND NAK.

A — The sender transmits the data. ACK is a reply sent by the receiver. Line 1 should be SEND data.

B — After sending, the sender waits for a reply (ACK or NAK) from the receiver, not for data. Line 2 should be WAIT for reply.