## 3 Related Work  
This section provides a detailed review of the literature in which blockchain technology and IOTA have been adopted in the domain of environmental monitoring and other IoT applications. [PERSON] et al. [30] present the use of a DAG-based blockchain structure called IOTA for the secure sharing of sensor data by integrating two technologies. The paper describes the work and the features of IOTA that can enable seamlessly integrating IoT devices with IOTA to safely transmit IoT data into the Tangle. An architecture was presented in the paper that included the use of Raspberry Pi devices to aggregate and send sensor data to the IOTA network. However, the architecture proposed in the paper was highly generic and lacked a working methodology. Moreover, there was no experimental evaluation of the architecture, especially in terms of performance. [PERSON] et al. [31] analyzed the stereotypical privacy and security issues in IoT and developed a framework that utilized Ethereum blockchain with an IoT system. A four-layered architecture was proposed where blockchain was used at the database layer to adapt to the IoT system. A good theoretical description of how the proposed framework tackles IoT security and privacy issues were provided. However, a proper working model to practically address these IoT security and privacy issues was missing. Furthermore, there was no performance or latency evaluation for the proposed framework. [PERSON] et al. [32] proposed a sensor node system that was distributed and utilized the IOTA distributed ledger to exchange data with IoT devices. In this paper, a distributed wireless sensor node system was proposed that ensured integrity of the data across the entire pipeline. The proposed system consisted of three entities: super nodes (SNs) that aggregated the data, full nodes (FNs), which are the IOTA nodes that perform the proof-of-work (PoW), and a back-end server. However, their proposed method did not show how  
Figure 6: The flow of data in restricted mode. Figure 7: Generation of channel key, using one-way hash functions. the three entities are connected and how the data is flowing. Additionally, there was no implemented architecture and an experimental evaluation of their proposed system. [PERSON] et al. [33] proposed an environmental monitoring system that uses blockchain to provide integrity to the environmental data and prevent falsification. Additionally, a three-dimensional architecture using intelligent trusted devices was presented for environmental monitoring, to ensure the integrity and originality of the data collected by the IoT devices. The raw data from the sensors were transmitted to a whole node that could send data to the blockchain and could synchronize all the data within the blockchain nodes. However, an extensive performance evaluation of the proposed model was missing, which would provide some details on the latency of blockchain operations. Moreover, the traditional blockchain setup cannot meet the scalability demands of the IoT system; hence, a scalable blockchain or distributed ledger technology is required. [PERSON] et al. [34] presented an approach using the IOTA distributed ledger technology and IoT devices to demonstrate two IoT applications on the Tangle, such as a \"smart utility meter system\" and a \"smart car transaction system\". These proposed applications were connected to the internet using low power wide area networks (LPWAN). A DAG-based blockchain IOTA was used by the researchers to overcome the scalability and transactional cost of the conventional blockchain. Although the research paper gave detailed steps to implement the proposed applications, a well-defined architecture was missing and only a proof-of-concept (PoC) was presented. [PERSON] et al. [35] proposed an implementation in the cloud that uses IoT-enabled blockchain to address some existing issues in smart cities. The research focuses on the use of \"chaincodes\", which are also known as smart contracts, for monitoring air quality systems in smart cities.