DT80 MQTT demo and description

MQTT is a Client Server publish/subscribe messaging transport protocol. It is light weight, open, simple, and designed to be easy to implement. These characteristics make it ideal for use in many situations, including constrained environments such as for communication in Machine to Machine (M2M) and Internet of Things (IoT) contexts where a small code footprint is required and/or network bandwidth is at a premium.

MQTT, or Message Queuing Telemetry Transport, is a …

-publish (send a message)

-subscribe (wait for a message)
  … protocol.

Publishing means that data is uploaded to a well-defined place (called topic) on a central server (called message-broker). Subscribing means that data is downloaded from a topic of the message broker, once it was published there. MQTT hence is an event-driven communication protocol, notifying a subscriber once a data message has arrived on its topic.

What is MQTT used for?

MQTT is a lightweight publish/subscribe messaging protocol designed for M2M (machine to machine) telemetry in low bandwidth environments.

MQTT is fast becoming one of the main protocols for IOT (internet of things) deployments.

• TCP is on layer 4
• MQTT is on layer 5-7
• MQTT protocol is on top of a TCP connection
• MQTT benefits of TCP layer, due to its:
  - reliable
  - ordered
  - error checked

MQTT clients

MQTT clients publish a message to an MQTT broker and other MQTT clients subscribe to messages they want to receive. Implementations of MQTT clients typically require a minimal footprint so are well suited for deployment on small constrained devices and are very efficient in their bandwidth requirements.

MQTT Brokers

MQTT brokers receive published messages and dispatch the message to the subscribing MQTT clients. An MQTT message contains a message topic that MQTT clients subscribe to and MQTT brokers use these subscription lists for determining the MQTT clients to receive the message.

Presistent Sessions

MQTT allows for a persistent session between the client and the broker. This allows for sessions to persist even if the network is disconnected. Once the network is reconnected, the information to reconnect the client to the broker still exists. This is one of the key features that makes the MQTT protocol more efficient than HTTP for use over unreliable cellular networks.

Quality of Service Levels

MQTT implements 3 quality of service levels for agreement between the sender and receiver: 1) At most once, 2) At least once, and 3) Exactly once. These QoS levels allow for more reliable IoT applications since the underlying messaging infrastructure and adapt to unreliable network conditions.

Retained Messages

MQTT clients that subscribe to a new topic have no insight into when to expect the first message they will receive. However, an MQTT broker can store a retained message that can be sent immediately upon a new MQTT subscription. In this case, the MQTT client will receive at least one message upon subscribing to the topic.

Last Will and Testament

An MQTT client can specify to an MQTT broker a message, called the last will, that will be sent if the MQTT client ungracefully disconnects. This allows for a more graceful system wide notification that a client has been disconnected.