QoS levels (0, 1, 2) in IOT Protocols - Full Explanation

Introduction

Imagine sending important messages over a network where some might get lost or arrive multiple times. The challenge is to control how messages are delivered to make sure they reach safely and correctly without wasting resources.

Explanation

QoS Level 0: At Most Once

This level sends a message only once without asking for confirmation. If the message gets lost, it won't be resent. It is the fastest and uses the least resources but does not guarantee delivery.

QoS 0 means messages are sent once with no guarantee they arrive.

QoS Level 1: At Least Once

Here, the sender keeps sending the message until it gets an acknowledgment from the receiver. This ensures the message arrives at least once but might be received multiple times if acknowledgments are delayed.

QoS 1 guarantees delivery but may cause duplicate messages.

QoS Level 2: Exactly Once

This level uses a handshake process between sender and receiver to make sure the message is delivered only once. It is the safest but requires more communication and time.

QoS 2 ensures each message is received exactly once with no duplicates.

Real World Analogy

Imagine mailing letters with different levels of care: dropping a postcard in the mailbox without tracking, sending a registered letter that requires a signature, and sending a courier package that confirms delivery and prevents duplicates.

QoS Level 0: At Most Once → Dropping a postcard in the mailbox without tracking or confirmation

QoS Level 1: At Least Once → Sending a registered letter that requires a signature to confirm delivery

QoS Level 2: Exactly Once → Sending a courier package with a detailed delivery process ensuring no duplicates

Diagram

Diagram

┌───────────────┐       ┌───────────────┐       ┌───────────────┐
│Sender         │       │Network        │       │Receiver       │
├───────────────┤       ├───────────────┤       ├───────────────┤
│QoS 0: Send →─┐│──────▶│               │──────▶│ Receive once  │
│No ack needed ││       │               │       │or lost        │
├───────────────┤       │               │       ├───────────────┤
│QoS 1: Send →─┐│──────▶│               │       │Send ack ◀─────┤
│Wait for ack  ││◀──────│               │       │May receive    │
│Resend if no  ││       │               │       │duplicates    │
│ack          ││       │               │       ├───────────────┤
│QoS 2: Send →─┐│──────▶│               │       │Send ack 1 ◀───┤
│2-step handshake│      │               │       │Send ack 2 ◀───┤
│Ensures once  ││◀──────│               │       │Receive once   │
└───────────────┘       └───────────────┘       └───────────────┘

Diagram showing how messages flow from sender to receiver with different QoS levels and acknowledgments.

Key Facts

QoS 0 → Messages are sent once without acknowledgment and may be lost.

QoS 1 → Messages are sent until acknowledged, ensuring delivery but allowing duplicates.

QoS 2 → Messages are delivered exactly once using a two-step handshake.

Acknowledgment → A signal sent by the receiver to confirm message receipt.

Duplicate Message → A message received more than once due to retransmission.

Common Confusions

Believing QoS 0 guarantees message delivery.

Believing QoS 0 guarantees message delivery. QoS 0 does not guarantee delivery; messages can be lost without retries.

Assuming QoS 1 prevents duplicate messages.

Assuming QoS 1 prevents duplicate messages. QoS 1 ensures delivery but may cause duplicates if acknowledgments are delayed.

Thinking QoS 2 is always faster than other levels.

Thinking QoS 2 is always faster than other levels. QoS 2 is slower due to extra steps but ensures no duplicates and guaranteed delivery.

Summary

QoS levels control how messages are delivered to balance speed, reliability, and duplicates.

QoS 0 sends messages once without confirmation, risking loss.

QoS 1 ensures delivery but may cause duplicates, while QoS 2 guarantees exactly one delivery with more overhead.

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