Timeout

Name:

Date:

Class:

Quick Questions

Shade in the A or B circle for each question.

1. In ARQ, when does the sender start the timeout timer?

A When it sends the data

B When it receives an acknowledgement

2. The timeout ends and no acknowledgement has arrived. What does the sender do?

A Resends the data

B Assumes the data arrived safely

3. Which best describes a timeout?

A The time taken to type a message

B The set time a sender waits for an acknowledgement

4. The data arrives safely, but the acknowledgement is lost on the way back. What happens when the timeout ends?

A The sender resends the data

B The sender knows the data arrived and does nothing

5. An acknowledgement arrives before the timeout ends. What does the sender do?

A Waits for the timeout to end, then resends

B Stops waiting and does not resend

Match the Pairs

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

Left column		Right column
1timeout = 5		Asend(packet) start_timer()
2IF no ACK within timeout THEN resend		BA timeout also happens when the acknowledgement is lost. The sender cannot tell the difference, so it resends.
3The timer starts at the moment the data is sent		CIf no reply arrives in time, the sender sends the data again
4The sender can stop resending after a set number of tries		DIF tries = 3 THEN stop
5The sender resends the data as soon as it has sent it, without waiting for the timeout		EThe sender will wait 5 seconds for an acknowledgement before it assumes the data was lost
6A timeout only happens when the data is lost, never when an acknowledgement is lost		FThe sender must wait for the whole timeout period first. It only resends if no acknowledgement arrives in that time.

Definitions

timeout

Fill in the Blanks

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

SEND packet START _____

IF timer > timeout THEN ______ packet ENDIF

IF ack = FALSE ___ timer > timeout THEN RESEND packet ENDIF

tries ← tries + 1 IF _____ = max_tries THEN STOP

WAIT _______ seconds IF ack = FALSE THEN RESEND packet

Spot the Error

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

Question 1

A SEND packet

B START timer

C WAIT for acknowledgement

D IF ack arrived THEN RESEND

Question 2

A timeout ← 5

B SEND packet

C WAIT forever for ACK

D IF no ACK THEN RESEND packet

Question 3

A tries ← 0

B SEND packet

C tries ← tries - 1

D IF no ACK AND tries<3 RESEND

Question 4

A Sender sends packet 7

B Receiver gets it, sends ACK

C The ACK is lost on the way

D Timeout ends, sender sends ACK

Question 5

A SEND packet AND STOP timer

B WAIT for ACK

C IF timer ends with no ACK

D RESEND packet

Answer Key

Timeout

Section A: Quick Questions

A — When it sends the data

A — Resends the data

B — The set time a sender waits for an acknowledgement

A — The sender resends the data

B — Stops waiting and does not resend

Section B: Match the Pairs

timeout = 5 → The sender will wait 5 seconds for an acknowledgement before it assumes the data was lost

IF no ACK within timeout THEN resend → If no reply arrives in time, the sender sends the data again

The timer starts at the moment the data is sent → send(packet) start_timer()

The sender can stop resending after a set number of tries → IF tries = 3 THEN stop

The sender resends the data as soon as it has sent it, without waiting for the timeout → The sender must wait for the whole timeout period first. It only resends if no acknowledgement arrives in that time.

A timeout only happens when the data is lost, never when an acknowledgement is lost → A timeout also happens when the acknowledgement is lost. The sender cannot tell the difference, so it resends.

Section C: Definitions

timeout — 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.

Section D: Fill in the Blanks

SEND packet
START timer

IF timer > timeout THEN
  RESEND packet
ENDIF

IF ack = FALSE AND timer > timeout THEN
  RESEND packet
ENDIF

tries ← tries + 1
IF tries = max_tries THEN
  STOP

WAIT timeout seconds
IF ack = FALSE THEN
  RESEND packet

Section E: Spot the Error

D — The sender only resends when the timer runs out with no acknowledgement. Resending after an ACK arrived is wrong. It should be IF timer expired THEN RESEND.

C — A timeout limits how long the sender waits. Waiting forever means the sender would never decide the data was lost. It should be WAIT up to timeout seconds.

C — The sender counts each attempt so it can stop after a set number of tries. Subtracting means tries never reaches the limit. It should be tries ← tries + 1.

D — The receiver sends the ACK, not the sender. When the ACK is lost, the timeout ends and the sender resends the packet. It should be Timeout ends, sender resends.

A — The timeout timer must start at the moment the data is sent. Stopping it means no waiting time is measured. It should be SEND packet AND START timer.