MQTT-SN Protocol

MQTT-SN (MQTT for Sensor Networks) is a lightweight publish/subscribe protocol designed for constrained devices and low-bandwidth networks. It runs over UDP, making it ideal for 2G/GPRS, NB-IoT, and other limited connectivity scenarios.

When to Use MQTT-SN

Use MQTT-SN when your device:

• Has limited memory or processing power
• Connects over 2G/GPRS, NB-IoT, or Zigbee
• Cannot maintain a persistent TCP connection
• Needs to minimize bandwidth usage

For WiFi or Ethernet devices with sufficient resources, standard MQTT over TLS is recommended.

Connection Details

Parameter	Value
Broker Host	mqtt.siliconwit.io
Port	1884 (UDP)
Authentication	Device ID (username) + Access Token (password)
Publish Topic	d/{device_id}/t
Subscribe Topic	d/{device_id}/c

How It Works

MQTT-SN uses a gateway that translates between MQTT-SN (UDP) and standard MQTT (TCP). Your device connects to the gateway, and the gateway handles communication with the MQTT broker.

Device (MQTT-SN/UDP) --> Gateway (port 1884) --> MQTT Broker --> SiliconWit Platform

Key differences from standard MQTT:

• UDP instead of TCP: No connection setup overhead
• Topic IDs: Short numeric topic IDs replace long topic strings to save bandwidth
• Sleeping clients: Built-in support for devices that sleep between transmissions
• Discovery: Devices can discover the gateway automatically

Topic Registration

Before publishing, MQTT-SN clients must register their topic string to get a numeric topic ID. The gateway handles this mapping:

1. Client sends REGISTER with topic string d/{device_id}/t
2. Gateway assigns a topic ID (e.g., 1)
3. Client uses topic ID 1 for all subsequent publishes

Most MQTT-SN client libraries handle registration automatically.

Authentication

MQTT-SN uses the same credentials as standard MQTT:

• Username: Your Device ID (from the SiliconWit dashboard)
• Password: Your Access Token (from the SiliconWit dashboard)

Credentials are sent during the CONNECT phase.

Example: Arduino with MQTT-SN

#include <WiFi.h>
#include <WiFiUdp.h>

// MQTT-SN broker details
#define BROKER_HOST "mqtt.siliconwit.io"
#define BROKER_PORT 1884

// Your SiliconWit device credentials
const char* deviceId = "YOUR_DEVICE_ID";
const char* accessToken = "YOUR_ACCESS_TOKEN";

WiFiUDP udp;

void setup() {
    Serial.begin(115200);

    // Connect to WiFi
    WiFi.begin("YOUR_SSID", "YOUR_PASSWORD");
    while (WiFi.status() != WL_CONNECTED) {
        delay(500);
        Serial.print(".");
    }
    Serial.println("\nWiFi connected");

    // Initialize MQTT-SN connection
    // (Implementation depends on your MQTT-SN library)
    // See: https://github.com/ty4tw/mqtt-sn-arduino
}

void loop() {
    // Publish telemetry as JSON
    String payload = "{\"temperature\": 25.5, \"humidity\": 60}";
    // mqttsn.publish(topicId, payload);
    delay(60000);
}

Note

MQTT-SN library support varies by platform. For ESP32 and Arduino, consider mqtt-sn-arduino. For constrained MCUs, check your SDK for built-in MQTT-SN support.

Payload Format

MQTT-SN payloads follow the same JSON format as standard MQTT:

{
    "temperature": 25.5,
    "humidity": 60,
    "battery": 3.7
}

See Topics & Payloads for detailed formatting guidelines.

Limitations

• No encryption: MQTT-SN runs over plain UDP without TLS. For security-sensitive deployments, use standard MQTT with TLS on port 8883
• QoS: Supports QoS 0 (at most once) and QoS 1 (at least once). QoS 2 is not supported
• Payload size: Keep payloads small (under 1 KB recommended) for reliable UDP delivery

Protocol Comparison

Feature	MQTT (TLS)	MQTT-SN	CoAP
Transport	TCP + TLS	UDP	UDP
Port	8883	1884	5683
Encryption	TLS	None	None
Model	Pub/Sub	Pub/Sub	Request/Response
Best for	WiFi, Ethernet	2G/GPRS, NB-IoT	NB-IoT, LPWAN

Next Steps

• CoAP Protocol - Alternative UDP protocol for constrained devices
• MQTT Basics - Standard MQTT guide
• Topics & Payloads - Message formatting
• Authentication - Credential management