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.