MQTT, how it is different from HTTP and SOMEIP, Where and Why it is used
What is MQTT?
Definition:
MQTT is a lightweight, publish-subscribe messaging protocol designed for devices with:
- Low bandwidth
- Limited CPU and memory
- Unreliable or intermittent network connections
Unlike HTTP, devices do not communicate directly with each other. Instead, they communicate through a broker.
MQTT Architecture
Publisher | | Publish Message | +--------------+ | MQTT Broker | +--------------+ / | \ / | \ Subscriber Subscriber Subscriber
Example:
Temperature Sensor | Publish: 32°C | MQTT Broker / \ Mobile App Cloud Dashboard
The publisher doesn't know who receives the message.
Main Components
1. Broker
The central server that:
- Receives messages
- Filters them by topic
- Sends them to subscribers
Examples:
- Eclipse Mosquitto
- EMQX
- HiveMQ
- AWS IoT Core
- Azure IoT Hub
2. Publisher
A device that sends data.
Example:
- Temperature sensor
- Vehicle ECU
- Battery Management System (BMS)
3. Subscriber
Receives messages from topics.
Example:
- Mobile app
- Cloud analytics
- Dashboard
- Service center
4. Topic
Messages are organized using hierarchical topics.
Examples:
car/speed car/engine/rpm car/battery/soc factory/line1/motor/temp
Subscribers receive only the topics they subscribe to.
Example
Vehicle publishes:
Topic vehicle/engine/temp Message 95°C
Subscribers:
Cloud Dashboard ↓ Receives 95°C
Mobile App ↓ Receives 95°C
Analytics Engine ↓ Receives 95°C
The ECU publishes once; the broker distributes to all subscribers.
How MQTT Works
Step 1
Client connects to Broker.
CONNECT
↓
Broker replies
CONNACK
Step 2
Client subscribes
SUBSCRIBE vehicle/speed
↓
Broker replies
SUBACK
Step 3
Publisher sends
PUBLISH vehicle/speed 80 km/h
↓
Broker forwards to all subscribers.
MQTT Packet Types
| Packet | Purpose |
|---|---|
| CONNECT | Connect client |
| CONNACK | Connection acknowledgement |
| PUBLISH | Send data |
| SUBSCRIBE | Subscribe to topic |
| SUBACK | Subscription acknowledgement |
| UNSUBSCRIBE | Remove subscription |
| PINGREQ | Keep-alive request |
| PINGRESP | Keep-alive response |
| DISCONNECT | Close connection |
Quality of Service (QoS)
One of MQTT's biggest strengths.
QoS 0 — At Most Once
Send once No acknowledgement
Fastest, but messages may be lost.
Example:
- Live temperature updates
QoS 1 — At Least Once
Send ↓ ACK? ↓ If No Resend
Guaranteed delivery, but duplicates are possible.
Example:
- Sensor readings
- Diagnostics
QoS 2 — Exactly Once
Four-step handshake
Guarantees no duplicates.
Used when duplicate messages are unacceptable, such as financial transactions.
MQTT vs HTTP
| Feature | MQTT | HTTP |
|---|---|---|
| Communication | Publish-Subscribe | Request-Response |
| Header Size | Very small | Larger |
| Bandwidth | Low | Higher |
| Power Consumption | Low | Higher |
| Connection | Persistent TCP | Typically new request/response cycles |
| Latency | Very low | Higher |
| Real-Time Updates | Excellent | Requires polling or additional technologies |
| IoT Friendly | Yes | Less suitable |
MQTT vs SOME/IP (Automotive)
| Feature | MQTT | SOME/IP |
|---|---|---|
| Domain | IoT, Cloud | In-vehicle Ethernet |
| Communication | Publish-Subscribe | Client-Server + Publish-Subscribe |
| Transport | TCP (optionally WebSockets) | TCP and UDP |
| Internet Friendly | Yes | No (primarily in-vehicle) |
| Cloud Integration | Excellent | Limited |
| Automotive ECU Communication | Usually indirect | Primary use |
Example:
Inside vehicle:
ADAS ECU ↓ SOME/IP ↓ Body ECU
Vehicle to Cloud:
Telematics ECU ↓ MQTT ↓ Cloud
MQTT vs CAN
| CAN | MQTT |
|---|---|
| Vehicle bus | Application protocol |
| 8–64 byte payload (depending on CAN/CAN FD) | Larger application messages |
| Real-time | Near real-time |
| Local network | Internet capable |
| ECU communication | Cloud communication |
Why MQTT?
MQTT was designed for:
- Low bandwidth networks
- High latency links
- Intermittent connectivity
- Low-power devices
- Battery-operated systems
It minimizes overhead while maintaining reliable messaging.
Automotive Use Cases
Modern connected vehicles use MQTT for:
- Remote diagnostics
- OTA software updates (status/control)
- Vehicle health monitoring
- Battery monitoring
- Fleet management
- Vehicle tracking
- Predictive maintenance
- Telemetry
Example:
Vehicle ↓ MQTT ↓ Cloud ↓ AI predicts battery failure
Industrial IoT Use Cases
- Smart factories
- PLC communication
- SCADA integration
- Smart meters
- Energy monitoring
- Building automation
- Agriculture sensors
Security
MQTT supports:
- TLS/SSL encryption
- Username/password authentication
- Client certificates
- Access control lists (ACLs)
- Topic-level authorization
Advantages
- Very lightweight protocol
- Low bandwidth consumption
- Low power usage
- Supports one-to-many communication
- Scales to millions of connected devices
- Reliable delivery with configurable QoS
- Ideal for cloud and IoT ecosystems
Limitations
- Requires a broker
- Not a hard real-time protocol
- Depends on TCP, which adds some overhead
- Topic hierarchy must be designed carefully in large systems
Real-World Automotive Example
Consider a connected electric vehicle:
-
The Battery Management System (BMS) measures:
- State of Charge (SOC)
- Battery temperature
- Voltage
-
The Telematics Control Unit (TCU) publishes these values to MQTT topics such as:
vehicle/123/battery/soc vehicle/123/battery/temp -
The MQTT broker distributes the data to:
- Fleet dashboard
- Mobile app
- AI analytics platform
- Service center
- An AI model analyzes trends and predicts battery degradation or maintenance needs.
Interview Answer (2–3 Minutes)
**MQTT (Message Queuing Telemetry Transport) is a lightweight publish-subscribe messaging protocol designed for constrained devices and unreliable networks. Unlike HTTP's request-response model, MQTT uses a broker that receives messages from publishers and distributes them to subscribers based on topics. It supports three Quality of Service levels, enabling applications to balance performance and reliability. MQTT is widely used in IoT, industrial automation, and connected vehicles because it has low bandwidth overhead, maintains persistent connections, and scales efficiently. In automotive systems, MQTT is commonly used between the telematics unit and cloud services for telemetry, remote diagnostics, fleet management, and predictive maintenance, while protocols like SOME/IP are used for communication between ECUs inside the vehicle.
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