QoS levels (0, 1, 2) in IOT Protocols - Mini Project: Build & Apply

Understanding MQTT QoS Levels (0, 1, 2)

📖 Scenario: You are working with IoT devices that send messages to a central server using MQTT protocol. MQTT supports three Quality of Service (QoS) levels: 0, 1, and 2. Each level defines how messages are delivered and acknowledged to ensure reliability.Understanding these QoS levels helps you configure your devices to balance between message delivery assurance and network usage.

🎯 Goal: Build a simple Python dictionary that represents MQTT messages with their QoS levels, then filter and display messages by a chosen QoS level.

📋 What You'll Learn

Create a dictionary named mqtt_messages with message IDs as keys and their QoS levels as values.

Create a variable named selected_qos to hold the QoS level to filter messages.

Use a dictionary comprehension to create a new dictionary filtered_messages containing only messages with the selected_qos.

Print the filtered_messages dictionary.

💡 Why This Matters

🌍 Real World

MQTT is widely used in IoT devices to send sensor data or commands. Understanding QoS levels helps ensure messages are delivered reliably according to the needs of the application.

💼 Career

Knowing how to handle MQTT QoS levels is important for IoT developers, DevOps engineers managing IoT infrastructure, and anyone working with message brokers and device communication.

1

Create MQTT messages dictionary

Create a dictionary called mqtt_messages with these exact entries: 101: 0, 102: 1, 103: 2, 104: 1, 105: 0 representing message IDs and their QoS levels.

IOT Protocols

# Create the mqtt_messages dictionary with message IDs and QoS levels
# Your code here

Hint

Use curly braces {} to create a dictionary with keys as message IDs and values as QoS levels.

2

Set the QoS level to filter

Create a variable called selected_qos and set it to 1 to filter messages with QoS level 1.

IOT Protocols

mqtt_messages = {101: 0, 102: 1, 103: 2, 104: 1, 105: 0}
# Set selected_qos to 1
# Your code here

Hint

Assign the integer 1 to the variable selected_qos.

3

Filter messages by selected QoS

Use a dictionary comprehension to create a new dictionary called filtered_messages that includes only the entries from mqtt_messages where the QoS level equals selected_qos.

IOT Protocols

mqtt_messages = {101: 0, 102: 1, 103: 2, 104: 1, 105: 0}
selected_qos = 1
# Create filtered_messages dictionary with messages having QoS == selected_qos
# Your code here

Hint

Use {key: value for key, value in dict.items() if condition} syntax for dictionary comprehension.

4

Display filtered messages

Write a print statement to display the filtered_messages dictionary.

IOT Protocols

mqtt_messages = {101: 0, 102: 1, 103: 2, 104: 1, 105: 0}
selected_qos = 1
filtered_messages = {msg_id: qos for msg_id, qos in mqtt_messages.items() if qos == selected_qos}
# Print the filtered_messages dictionary
# Your code here

Hint

Use print(filtered_messages) to show the filtered dictionary.

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