Source: IoT Communication Protocols

Data gathered from sensors and sent to the cloud = telemetry

HTTP (not ideal for IoT)

Characteristics that make HTTP problematic for IoT:

Request-response: client must always pull — no server-push
Stateless: no persistent connection, each request independent
Large headers: uncompressed overhead (~8 bytes header)
DNS lookup: 3-way handshake overhead
Constant polling = slow OR power/data intensive

Pub/Sub Pattern

Two actors:

Broker: server that accepts messages, routes them to subscribers
Clients: publish messages to topics OR subscribe to receive them

A cloud service is also a client — connects to broker, subscribes to telemetry topics.

MQTT

Message Queuing Telemetry Transport — lightweight pub/sub protocol.

Originally designed for oil pipeline monitoring (unreliable connectivity). Facebook Messenger is the world’s largest MQTT network.

Manages messages asynchronously over a network socket
Messages can be 2 bytes to 256 MB
UTF-8 encoded headers/topics; payload encoding is app-defined (JSON, binary, etc.)
Can be open/public or encrypted (username/password or certificates)

Topics & Wildcards

Topic hierarchy example: sensors/<FLOOR>/temperature/<HVAC_UNIT_ID>

Subscription	What you get
`sensors/floor3/temperature/+`	All temp sensors on floor 3
`sensors/+/temperature/+`	All temp data, all floors
`sensors/#`	Everything on all floors
`sensors/floor10/#`	All topics on floor 10

# = multi-level wildcard (replaces remaining levels)
+ = single-level wildcard (replaces one level)

QoS Levels

QoS	Name	Delivery guarantee
0	At most once (Fire & Forget)	Sent once, no acknowledgement
1	At least once	Retried until ACK received
2	Exactly once	Two-level handshake, no duplicates

Default QoS = 0. If subscriber defines lower QoS than publisher, broker uses subscriber’s lower value.

Sessions

Session type	Behavior
Clean (`CleanSession=true`)	Broker stores nothing, purges previous subscriptions
Persistent (`CleanSession=false`)	Broker stores subscriptions and missed QoS 1/2 messages

KeepAlive

Time interval (seconds) — client and broker ping each other at this interval to detect dead connections.

Last Will & Testament (LWT)

Client provides LWT message+topic on connect. If client disconnects unexpectedly, broker sends the LWT on its behalf — notifies other clients of disconnection.

Retained Messages

Message with retained=true. Broker stores the last retained message per topic. New subscribers receive it immediately upon subscribing (instant status update).

HTTP vs MQTT Comparison

Criteria	MQTT	HTTP
Complexity	Simple, lightweight	More complex
Energy	More efficient	Less efficient
Direction	Bi-directional	Uni-directional (client initiates)
Architecture	Pub/Sub	Request/Response
Connection	Can stay open	Closes after every request
Delivery assurance	QoS 0/1/2	None
Header size	~2 bytes	~8 bytes

paho-mqtt Python Code

Install

pip install paho-mqtt

Publisher

import paho.mqtt.client as mqtt
import json, time, random
 
rnd = random.Random()
common_key = 'd5a4d5e6-d597-4bd4-8196-5f51d12345'
topic_name = common_key + '/temperature'
client_name = common_key + 'publisher'
 
mqtt_client = mqtt.Client(client_name)
mqtt_client.connect('test.mosquitto.org')
print("MQTT connected!")
 
while True:
    temp = rnd.randint(-10, 30)
    payload = json.dumps({'temperature': temp})
    print("Sending telemetry", payload)
    mqtt_client.publish(topic_name, payload)   # QoS=0, retain=False
    mqtt_client.loop()
    time.sleep(3)

Subscriber

import paho.mqtt.client as mqtt
import time
 
common_key = 'd5a4d5e6-d597-4bd4-8196-5f51d12345'
client_name = common_key + 'subscriber'
topic_name = common_key + '/temperature'
 
mqtt_client = mqtt.Client(client_name)
mqtt_client.connect('test.mosquitto.org')
print("MQTT connected!")
 
def message_received(mqtt_client, userdata, message):
    print(f'Received message: {message.payload}')
 
mqtt_client.on_message = message_received
mqtt_client.subscribe(topic_name)
 
while True:
    mqtt_client.loop()
    time.sleep(1)

Public brokers

test.mosquitto.org (Eclipse Foundation)
broker.emqx.io (EMQX)

co-test

Explorer

iot-communication-protocols