IoT Lab (IT-705) - Important Questions

Write firmware for a microcontroller-based IoT node to read a temperature sensor and publish the readings to an MQTT broker. Your answer should include:

• Pseudocode or flowchart for sensor sampling, ADC read (if required), data formatting, and MQTT client lifecycle (CONNECT, PUBLISH, SUBSCRIBE if needed, DISCONNECT).
• A clear MQTT topic hierarchy for a deployment having multiple sites, multiple gateways and multiple sensor types. Show examples of topic strings.
• Choice of MQTT QoS level for periodic sensor telemetry and justification for the chosen QoS.
• Handling of network interruptions (reconnect logic, message buffering or persistence, retained messages, Last Will and Testament).

Assume typical constrained node resources and justify design decisions with respect to bandwidth, reliability and energy.

Compare MQTT and HTTP for an IoT telemetry application that transmits small, frequent sensor readings from battery-powered devices to a cloud backend. Discuss the differences in connection model, overhead, power consumption, reliability mechanisms and suitability for constrained devices. Conclude which protocol you would choose and why for the given application.

Explain the MQTT protocol in detail. Your answer must cover:

• MQTT architecture and roles (publisher, subscriber, broker, client).
• MQTT control packet types and their purpose (CONNECT, CONNACK, PUBLISH, PUBACK, PUBREC, PUBREL, PUBCOMP, SUBSCRIBE, SUBACK, UNSUBSCRIBE, UNSUBACK, PINGREQ, PINGRESP, DISCONNECT).
• MQTT Quality of Service levels and the packet flow for QoS 0, QoS 1 and QoS 2.
• Concepts of retained messages, Last Will and Testament, clean session vs persistent session, keepalive and session state.
• Typical use-cases where MQTT is preferred over HTTP.

Describe the Constrained Application Protocol (CoAP) and its suitability for constrained IoT devices. Include in your answer:

• CoAP message types and methods (CON, NON, ACK, RST; GET, POST, PUT, DELETE).
• The observe option and its use for asynchronous updates from a resource.
• Confirmable vs non-confirmable messaging and retransmission behavior.
• Block-wise transfer and payload fragmentation.
• Security mechanisms used with CoAP, including DTLS and trade-offs compared to TLS in MQTT/HTTP.

Design an MQTT topic hierarchy for a smart-building deployment that contains multiple buildings, floors and sensor types (temperature, humidity, motion). Show how publishers and different types of subscribers (global monitor, per-building dashboard, per-floor maintenance) would subscribe using wildcards. Explain best practices for topic naming, use of retained messages and how to manage access control at topic level.

Explain how to secure MQTT-based telemetry from end devices to cloud. Discuss the use of TLS, client authentication (username/password and client certificates), authorization at broker level, and additional measures (payload encryption, token-based authentication). Also contrast the equivalent security approach for CoAP using DTLS.

What is MQTT-SN and how does it differ from MQTT? Describe a gateway architecture that connects a group of low-power sensor nodes using MQTT-SN over a wireless mesh to a cloud MQTT broker. Explain the gateway responsibilities (message translation, topic mapping, addressing) and advantages of using MQTT-SN in such a deployment.