# Blockchain Technology for Intelligent Environments
## 6 Conclusions  
The Internet of Things (IoT) and Distributed Ledger Technology (DLT) are two distinct new technologies that have evolved along separate paths motivated by different challenges. IoT was born by the need to build streamlined management frameworks for myriads of diverse sensors and devices that gained networking capabilities in the last couple of decades. DLT was motivated by the desire to provide provable trust in online transactions without depending on any centralized service or powerful institution. Interestingly, the two technologies have proven to blend strikingly well with each other, enabling the development of IoT ecosystems where smart devices of multiple parties, of possibly conflicting interests, collaborate seamlessly enjoying complete trust and accountability with respect to their interaction.  
A number of distributed ledger designs have been proposed and deployed, targeting different application domains. We have identified four main characteristics that distinguish distributed ledgers into broad categories. First, the ledgers' access model, determining who can submit transactions, who can propose blocks, and who can read the blockchain. Second, the token model, that is, whether the blockchain encompasses a token for implementing a cryptocurrency and for allowing value representation and transfer. Third, the consensus protocol employed to let nodes validate transactions and reach global agreement on building and accepting blocks. Finally, the smart-contract functionality offered by a blockchain, ranging from simple scripting to more powerful smart contract programming languages that support Turing-complete computation.  
Reviewing IoT technologies, one realizes immediately that there is a large number of IoT platforms, many of them being proprietary and either fully or partially closed. There have been various attempts for defining or describing in their generality IoT architectures and systems using a layered and modular approach for facilitating interoperability. There are also proposals that aim to achieve interoperability at a higher, semantic layer, compared to technical (connectivity-oriented) and syntactic interoperability, in order to enable different entities to exchange information, data, and knowledge in a meaningful way. Some IoT systems have already adopted blockchain technology.  
These systems include industrial solutions, but also blockchain-community originating proposals that specifically target IoT data handling. More specifically, we have identified four general categories of frameworks merging IoT with blockchains: first, all-in-one systems, originating from organizations established in the IoT world; second, systems focusing on fast yet trusted and reliable data delivery and storage based on blockchains; third, systems oriented towards marketplace creation, enabling automated trading among IoT devices for various types of resources; and, finally, timestamping services, allowing an entity to timestamp their data, creating an indisputable proof of data ownership at a given point in time.  
Acquiring an elaborate picture of the IoT and blockchain interplay landscape is a non-trivial task, given the plethora of solutions proposed and the diverse target application domains. This work aimed at unfolding a number of representative blockchain designs and blockchain-based IoT systems, serving as a starting point for exploring this promising blend of novel technologies.  
This paper is the result of collaborative work of all authors. [PERSON], [PERSON], [PERSON], and [PERSON] contributed to the entire paper, focusing on blockchain elements, blockchain implementations, oracles, and some of the IoT systems. [PERSON] and [PERSON] contributed mainly on detailing certain IoT systems and on the oracles section.  
This research has been undertaken in the context of project SOFIE (Secure Open Federation for Internet Everywhere), which has received funding from EU's Horizon 2020 programme, under grant agreement No. 779984.  
The authors declare no conflict of interest.