# 5G Support for Industrial IoT Applications--Challenges, Solutions, and Research Gaps
## 9 IoT and Smart Contracts on Distributed Ledgers
### A Brief Survey on Using Blockchain Technologies for IIoT  
There are already several survey papers available on the utilization of blockchain technologies for IoT [98; 101], describing use-cases and current research trends for blockchains concepts that may fulfill IoT requirements. Turning towards more industrial applications, the crucial blockchain platform requirements for IIoT are analyzed by [102], addressing issues of security, fault tolerance, durability, Public Access possibility, consensus algorithms and smart contract types. The same authors describe various actual blockchain implementations in [103], which are differing in block creation, consensus mechanisms and smart contracts support. Regarding IIoT and blockchain integration they propose that a new module should be created that connects the two technologies. From the Industry 4.0 and IIoT application aspect, the authors of [104] survey various case studies, covering the power industry, production/manufacturing, supply chain, logistics, agriculture, healthcare and the retail industry. They compare the proposed solutions through platforms and consensus algorithms used, data transferred, and whether it utilizes smart contracts or not. Related to 5G-enabled automation IoT, the authors of [99] provide a systematic review on solutions and challenges for blockchain technology utilization. The discussion within this paper is divided into three parts, firstly the background of blockchain, IoT, and 5G are covered, briefly followed by respective industrial applications, then lastly, as the main points, open issues and challenges are covered mainly for industrial applications. The paper covers blockchain applications at the domains smart homes, smart cities, healthcare, agriculture, Industry 4.0, supply chain management, as well as autonomous vehicles, unmanned aerial vehicles, and multimedia and digital rights management. The authors of [105] focus specifically on electric vehicle cloud and edge (EVCE) applications of blockchain technologies, whereas [106] provides a joint cloud study on smart travelling. Without aiming for another comprehensive survey of the field, let us merely highlight two typical approaches for lightweight blockchain proposals intending to fit IIoT needs. The BPIIoT platform [107] targets to to support the development of decentralized, end-to-end manufacturing applications, and uses a blockchain network powered with smart contracts serve as an agreement between service consumers and manufacturing resources to provide on-demand manufacturing services. In order to reduce the load and delay of the network, the BPIIoT platform is designed as a light-weighted network architecture consisting of an on-chain network and an off-chain network. All transactions are carried out on the on-chain network, such as digital signature based on admission control, programmable licenses, etc., the off-chain network handles problems that cannot be solved by blockchain technology, such as storage, complex data processing, and so on. Another typical approach is to decrease the computational burden of resource-constrained nodes, as the authors of [108] propose. They selected three lightweight hash functions (QUARK, PHOTON, and SPONGENT), which show better performance in perspective of implementing area, throughput, and power consumption. These hash functions can ensure cryptographic security and be implemented with minimal efforts for resource-constrained devices. Then, using these hash functions, they connected each data block by flexible hashchain. This approach can reduce the computational burden and latency. In order to improve scalability of the network, this solution introduces area control nodes that control all nodes in a given area or cell. Motivated by increased information security needs for IIoT-systems organized in wireless sensor networks, mechanisms from blockchains and distributed ledger technologies were derived within [109], and adopted to microcontrollers, with a small energy budget and low calculation capabilities.