MQTT, one of the most popular protocols for the IoT, works according to a publish/subscribe pattern in which multiple clients connect to a single broker, generally hosted in the cloud. However, such a centralised approach does not scale well considering the massive numbers of IoT devices forecasted in the next future, thus calling for distributed solutions in which multiple brokers cooperate together. Indeed, distributed brokers can be moved from traditional cloud-based infrastructure to the edge of the network (as it is envisioned by the upcoming MEC technology of 5G cellular networks), with clear improvements in terms of latency, for example. This paper proposes MQTT-ST, a protocol able to create such a distributed architecture of brokers, organized through a spanning tree. The protocol uses in-band signalling (i.e., reuses MQTT primitives for the control messages) and allows for full message replication among brokers, as well as robustness against failures. We tested MQTT-ST in different experimental scenarios and we released it as open-source project to allow for reproducible research.

Future university campuses will be characterized by a series of novel services enabled by the vision of Internet of Things, such as smart parking and smart libraries. In this paper, we propose a complete solution for a smart waste management system with the purpose of increasing the recycling rate in the campus and provide better management of the entire waste cycle. The system is based on a prototype of a smart waste bin, able to accurately classify pieces of trash typically produced in the campus premises with a hybrid sensor/image classification algorithm, as well as automatically segregate the different waste materials. We discuss the entire design of the system prototype, from the analysis of requirements to the implementation details and we evaluate its performance in different scenarios. Finally, we discuss advanced application functionalities built around the smart waste bin, such as optimized maintenance scheduling.

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The Publish/Subscribe communication pattern has proved to be particularly tailored to the IoT world, with the MQTT protocol being the nowadays standard de-facto for IoT applications. Request/response protocols explicitly designed for the IoT, such as CoAP, have been revised to support also Publish/Subscribe. The purpose of this paper is to perform a comparison between two protocols: MQTT-SN, the version of MQTT thought specifically for sensor networks, and CoAP in its Pub/Sub version, defined in a recent IETF draft. Both protocols are Pub/Sub in nature and based on UDP at the transport layer, allowing therefore a fair comparison of their functionalities. We propose a open-source implementation of the CoAP Pub/Sub version and we compare the two protocols: first from a theoretical perspective and, then, in a simulated environment characterized by varying number of clients and network conditions. Results show that CoAP represents a valid alternative to MQTT-SN for publish-subscribe environments; in particular, CoAP results being the best choice for highly dynamic networks.

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