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Sensor nodes need lightweight communication protocols that enable connectivity to a network from which commands can be retrieved and data can be uploaded. These systems need to ensure that constrained devices will function just as well as non-constrained devices. The Constrained Application Protocol (COAP) and Message Queuing Telemetry Transport (MQTT) are the two most common protocols. Two systems were built to test these with regards to communication delay and network traffic. It was found that COAP has on average smaller packet sizes and no keepalive messages. MQTT has lower communication delay and is simpler to implement. COAP is well designed to interface with systems that work on the HyperText Transfer Protocol (HTTP) while MQTT is designed to work well with automatically forward messages to multiple clients without any additional configuration. MQTT also has Quality of Service (QoS) built into the design to ensure message delivery.
Westhuizen et al. (Fri,) studied this question.