QoS levels (0, 1, 2) in IOT Protocols - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does QoS level 0 mean in MQTT?

QoS level 0 means "At most once" delivery. The message is sent once without confirmation. It may be lost if the network fails.

beginner

Explain QoS level 1 in MQTT.

QoS level 1 means "At least once" delivery. The sender retries sending the message until it gets an acknowledgment, so duplicates may occur.

intermediate

What is the guarantee provided by QoS level 2 in MQTT?

QoS level 2 guarantees "Exactly once" delivery. It uses a four-step handshake to ensure the message is received only once.

beginner

Which QoS level is the fastest but least reliable?

QoS level 0 is the fastest because it sends messages without waiting for acknowledgment, but it is the least reliable.

intermediate

Why might you choose QoS level 1 over QoS level 2?

QoS level 1 balances reliability and performance. It ensures messages arrive at least once but with less overhead than QoS 2, which is slower due to extra steps.

Which QoS level in MQTT ensures a message is delivered exactly once?

A QoS 2

B QoS 0

C QoS 1

D QoS 3

QoS 2 uses a four-step handshake to guarantee exactly once delivery.

What happens if a message sent with QoS 0 is lost?

A It is resent until acknowledged

B It is lost and not resent

C It triggers an error

D It is stored for later delivery

QoS 0 sends the message once without confirmation, so lost messages are not resent.

Which QoS level may cause duplicate messages to be received?

A QoS 1

B QoS 0

C QoS 2

D None

QoS 1 retries sending until acknowledged, which can cause duplicates.

Why is QoS 2 slower than QoS 1?

A It uses encryption

B It sends messages multiple times without confirmation

C It requires multiple message exchanges for confirmation

D It compresses messages

QoS 2 uses a four-step handshake to ensure exactly once delivery, adding overhead.

Which QoS level is best when message loss is acceptable?

A All QoS levels guarantee no loss

B QoS 1

C QoS 2

D QoS 0

QoS 0 does not guarantee delivery, so it is suitable when occasional loss is acceptable.

Describe the differences between QoS levels 0, 1, and 2 in MQTT.

Think about delivery guarantees and message confirmation steps.

When would you choose each QoS level in a real IoT application?

Consider speed, reliability, and resource use.

Practice

1. What does QoS level 0 mean in IoT messaging?

easy

A. The message is stored indefinitely until acknowledged.

B. The message is delivered at least once, with confirmation.

C. The message is delivered exactly once, with a handshake.

D. The message is delivered at most once, with no confirmation.

Solution

1. Step 1: Understand QoS 0 delivery

   QoS 0 means the message is sent once without waiting for any acknowledgment.

2. Step 2: Compare with other QoS levels

   QoS 1 and 2 involve confirmations; QoS 0 does not guarantee delivery.

3. Final Answer:

   The message is delivered at most once, with no confirmation. -> Option D

4. Quick Check:

   QoS 0 = no confirmation, at most once delivery [OK]

Hint: QoS 0 means no delivery guarantee, just send once [OK]

Common Mistakes:

• Confusing QoS 0 with guaranteed delivery
• Thinking QoS 0 waits for acknowledgment
• Mixing QoS 0 with QoS 1 or 2 features

2. Which QoS level requires a four-step handshake to ensure message delivery?

easy

A. QoS 0

B. QoS 2

C. QoS 1

D. none of the above

Solution

1. Step 1: Recall handshake details for QoS levels

   QoS 2 uses a four-step handshake to guarantee exactly once delivery.

2. Step 2: Differentiate from other QoS levels

   QoS 0 has no handshake; QoS 1 uses a simpler acknowledgment.

3. Final Answer:

   QoS 2 -> Option B

4. Quick Check:

   Four-step handshake = QoS 2 [OK]

Hint: Four-step handshake means QoS 2 for exactly once delivery [OK]

Common Mistakes:

• Confusing QoS 1's single acknowledgment with four-step handshake
• Assuming QoS 0 has any handshake
• Choosing 'none of the above' incorrectly

3. Given a device publishes a message with QoS 1, what is the expected behavior?

medium

A. The message is sent at least once, with acknowledgment.

B. The message is sent once without acknowledgment.

C. The message is sent exactly once, with a four-step handshake.

D. The message is queued indefinitely until delivered.

Solution

1. Step 1: Understand QoS 1 delivery

   QoS 1 ensures the message is delivered at least once by requiring acknowledgment.

2. Step 2: Compare with QoS 0 and 2

   QoS 0 sends once without acknowledgment; QoS 2 ensures exactly once with handshake.

3. Final Answer:

   The message is sent at least once, with acknowledgment. -> Option A

4. Quick Check:

   QoS 1 = at least once with ack [OK]

Hint: QoS 1 means message sent at least once, confirmed by ack [OK]

Common Mistakes:

• Mixing QoS 1 with QoS 0's no acknowledgment
• Confusing QoS 1 with QoS 2's exactly once guarantee
• Assuming indefinite queuing for QoS 1

4. A developer sets QoS 2 but notices duplicate messages received. What is the likely cause?

medium

A. The handshake process was interrupted or not completed.

B. QoS 2 does not guarantee exactly once delivery.

C. QoS 0 was actually set by mistake.

D. The broker does not support QoS levels.

Solution

1. Step 1: Understand QoS 2 guarantees

   QoS 2 guarantees exactly once delivery only if handshake completes successfully.

2. Step 2: Analyze duplicate message cause

   If handshake is interrupted, duplicates can occur due to retries.

3. Final Answer:

   The handshake process was interrupted or not completed. -> Option A

4. Quick Check:

   Incomplete handshake causes duplicates in QoS 2 [OK]

Hint: Incomplete handshake causes duplicates in QoS 2 [OK]

Common Mistakes:

• Believing QoS 2 never allows duplicates
• Assuming wrong QoS level was set without checking
• Ignoring broker capabilities

5. An IoT system must send critical sensor data exactly once without loss or duplication. Which QoS level should be used and why?

hard

A. QoS 0, because it is fastest and sufficient for critical data.

B. QoS 1, because it guarantees delivery but may duplicate messages.

C. QoS 2, because it guarantees exactly once delivery with handshake.

D. None, critical data should be sent manually.

Solution

1. Step 1: Identify critical data requirements

   Critical data needs no loss and no duplication, so exactly once delivery is required.

2. Step 2: Match QoS level to requirement

   QoS 2 provides exactly once delivery using a four-step handshake, ensuring no duplicates or loss.

3. Final Answer:

   QoS 2, because it guarantees exactly once delivery with handshake. -> Option C

4. Quick Check:

   Critical data needs QoS 2 for exactly once delivery [OK]

Hint: Critical data needs QoS 2 for no loss or duplicates [OK]

Common Mistakes:

• Choosing QoS 0 for critical data due to speed
• Confusing QoS 1's at least once with exactly once
• Ignoring handshake importance