MQTT-SN for sensor networks in IOT Protocols - Step-by-Step Execution

Process Flow - MQTT-SN for sensor networks

Sensor Node wakes up

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Sends MQTT-SN CONNECT

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Gateway receives CONNECT

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Gateway sends CONNACK

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Sensor Node sends PUBLISH

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Gateway forwards PUBLISH to MQTT Broker

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Sensor Node sleeps or waits for ACK

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Repeat or Disconnect

This flow shows how a sensor node connects using MQTT-SN, publishes data via a gateway, and then sleeps to save power.

Execution Sample

IOT Protocols

CONNECT -> CONNACK -> PUBLISH -> PUBACK -> SLEEP

A sensor node connects to the gateway, publishes a message, receives acknowledgment, then goes to sleep.

Process Table

Step	Action	Message Sent	Message Received	Node State	Gateway State
1	Sensor wakes and sends CONNECT	CONNECT	-	Connecting	Waiting for CONNECT
2	Gateway receives CONNECT	-	CONNECT	Connecting	Received CONNECT
3	Gateway sends CONNACK	CONNACK	-	Waiting CONNACK	Sending CONNACK
4	Sensor receives CONNACK	-	CONNACK	Connected	Connected
5	Sensor sends PUBLISH	PUBLISH	-	Publishing	Waiting PUBLISH
6	Gateway receives PUBLISH	-	PUBLISH	Publishing	Forwarding to Broker
7	Gateway sends PUBACK	PUBACK	-	Waiting PUBACK	Sent PUBACK
8	Sensor receives PUBACK	-	PUBACK	Published	Connected
9	Sensor goes to sleep	-	-	Sleeping	Connected
10	Cycle repeats or disconnects	-	-	Idle or Disconnect	Idle or Disconnect

💡 Sensor node sleeps or disconnects after publishing; cycle ends or repeats on next wake-up.

Status Tracker

Variable	Start	After Step 4	After Step 8	After Step 9	Final
Node State	Idle	Connected	Published	Sleeping	Idle or Disconnect
Gateway State	Idle	Connected	Connected	Connected	Idle or Disconnect
Message Sent	-	CONNACK	PUBACK	-	-
Message Received	-	CONNECT	PUBLISH	-	-

Key Moments - 3 Insights

Why does the sensor node go to sleep after publishing?

To save battery power in sensor networks, the node sleeps after sending data, as shown in step 9 of the execution_table.

What is the role of the gateway in MQTT-SN?

The gateway translates MQTT-SN messages from sensor nodes to MQTT messages for the broker, as seen in steps 6 and 7.

Why does the sensor node wait for CONNACK after CONNECT?

CONNACK confirms the gateway accepted the connection, so the node waits before sending data, shown between steps 3 and 4.

Visual Quiz - 3 Questions

Look at the execution_table, what is the Node State after step 4?

A Publishing

B Connected

C Sleeping

D Idle

💡 Hint

Check the 'Node State' column at step 4 in the execution_table.

At which step does the gateway send PUBACK?

A Step 7

B Step 5

C Step 3

D Step 9

💡 Hint

Look for 'Gateway sends PUBACK' in the Action column of execution_table.

If the sensor node did not receive CONNACK, what would happen next?

A Node would send PUBLISH anyway

B Gateway would send PUBACK

C Node would retry CONNECT or sleep

D Node would disconnect immediately

💡 Hint

Refer to the flow where node waits for CONNACK before publishing (steps 3-4).

Concept Snapshot

MQTT-SN is a lightweight protocol for sensor nodes.
Sensor nodes connect via CONNECT message.
Gateway replies with CONNACK to confirm.
Nodes publish data with PUBLISH and get PUBACK.
Nodes sleep after publishing to save power.
Gateway bridges MQTT-SN and MQTT broker.

Full Transcript

MQTT-SN is designed for sensor networks where devices have limited power and bandwidth. The sensor node wakes up and sends a CONNECT message to the gateway. The gateway replies with CONNACK to confirm the connection. Then the sensor node sends PUBLISH messages with sensor data. The gateway forwards these messages to the MQTT broker and sends PUBACK back to the node. After publishing, the sensor node goes to sleep to save battery. This cycle repeats when the node wakes again. The gateway acts as a bridge between MQTT-SN and MQTT, enabling sensor nodes to communicate efficiently with the broker.