MQTT Basics

MQTT Basics

MQTT (Message Queuing Telemetry Transport) is a lightweight messaging protocol ideal for IoT devices.

Why MQTT?

• Lightweight - Minimal overhead, perfect for constrained devices
• Reliable - Quality of Service levels ensure message delivery
• Bi-directional - Devices can send and receive messages
• Low bandwidth - Works well on slow 2G/GPRS connections

Core Concepts

Broker

The MQTT broker is the central server that receives and routes all messages. SiliconWit.IO provides a managed broker at:

mqtt.siliconwit.io

Client

Any device or application that connects to the broker. Each client has a unique Client ID (your Device ID in SiliconWit.IO).

Topics

Topics are hierarchical strings that categorize messages:

devices/abc123/telemetry
devices/abc123/commands
sensors/temperature/office

Publish/Subscribe

• Publish - Send a message to a topic
• Subscribe - Receive messages from a topic

Connection Details

Standard Connection

Setting	Value
Broker	mqtt.siliconwit.io
Port	1883
Username	Your Device ID
Password	Your Access Token
Client ID	Your Device ID

Secure Connection (TLS)

Setting	Value
Broker	mqtt.siliconwit.io
Port	8883
TLS	Enabled
Username	Your Device ID
Password	Your Access Token

WebSocket Connection

For browser-based clients:

Setting	Value
URL	wss://mqtt.siliconwit.io:8084/mqtt
Username	Your Device ID
Password	Your Access Token

Quality of Service (QoS)

MQTT offers three QoS levels:

QoS	Name	Description	Use Case
0	At most once	Fire and forget	Frequent sensor data
1	At least once	Guaranteed delivery, may duplicate	Important data
2	Exactly once	Guaranteed single delivery	Critical commands

Tip

For most IoT telemetry, QoS 0 is sufficient. Use QoS 1 for important alerts.

Keep Alive

The keep-alive interval tells the broker how often to expect a heartbeat:

client.setKeepAlive(60); // 60 seconds

If the broker doesn’t receive a message or ping within 1.5x the keep-alive time, it marks the device as offline.

Last Will and Testament (LWT)

LWT lets you specify a message to publish if your device disconnects unexpectedly:

client.connect(
  device_id,
  device_id,
  access_token,
  "devices/abc123/status",  // LWT topic
  0,                         // LWT QoS
  true,                      // LWT retain
  "{\"status\":\"offline\"}" // LWT message
);

Retained Messages

Retained messages are stored by the broker and sent to new subscribers immediately:

client.publish(topic, payload, true); // true = retained

Use for:

• Device status
• Configuration
• Last known values

Clean Session

• Clean session = true - Start fresh, no stored subscriptions
• Clean session = false - Resume previous session, receive missed messages

client.connect(device_id, user, pass, NULL, 0, false, NULL, false);
//                                                          ^ clean session

Example: Complete Connection

#include <PubSubClient.h>

WiFiClient espClient;
PubSubClient client(espClient);

void setup() {
  client.setServer("mqtt.siliconwit.io", 1883);
  client.setKeepAlive(60);
  client.setCallback(messageReceived);

  // Connect with LWT
  client.connect(
    "device123",           // Client ID
    "device123",           // Username
    "access_token_here",   // Password
    "devices/device123/status", // LWT topic
    1,                     // LWT QoS
    true,                  // LWT retain
    "{\"online\":false}"   // LWT payload
  );

  // Publish online status
  client.publish("devices/device123/status", "{\"online\":true}", true);

  // Subscribe to commands
  client.subscribe("devices/device123/commands");
}

void messageReceived(char* topic, byte* payload, unsigned int length) {
  // Handle incoming messages
}

Next Steps

• Topics & Payloads - Message format guide
• MQTT Authentication - Security details